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Risks associated with carbonation are a key limitation to greater replacement levels of ordinary portland cement (OPC) by supplementary cementitious materials (SCMs). The addition of pozzolanic SCMs in OPC alters the hydrate assemblage by forming phases like calcium-(alumina)-silicate-hydrate (C-(A)-S-H). The objective of the present study was to elucidate how such changes in hydrate assemblage influence the chemical mechanisms of carbonation in a realistic OPC system. Here, we show that synthetic zeolite Y (faujasite) is a highly reactive pozzolan in OPC that reduces the calcium content of hydration products via prompt consumption of calcium hydroxide from the evolving phase assemblage prior to CO 2 exposure. Suppression of portlandite at moderate to high zeolite Y content led to a more damaging mechanism of carbonation by disrupting the formation of a passivating carbonate layer. Without this layer, carbonation depth and CO 2 uptake are increased. Binders containing 12–18% zeolite Y by volume consumed all the calcium hydroxide from OPC during hydration and reduced the Ca/(Si+Al) ratio of the amorphous products to near 0.67. In these cases, higher carbonation depths were observed after exposure to ambient air with decalcification of C-(A)-S-H as the main source of CO 2 buffering. Binders with either 0% or 4% zeolite Y contained calcium hydroxide in the hydrated microstructure, had higher Ca/(Si+Al) ratios, and formed a calcite-rich passivation layer that halted deep carbonation. Although the carbonated layer in the samples with 12% and 18% zeolite Y contained 70% and 76% less calcite than the OPC respectively, their higher carbonation depths resulted in total CO 2 uptakes that were 12x greater than the OPC sample. Passivation layer formation in samples with calcium hydroxide explains this finding and was further supported by thermodynamic modeling. High Si/Al zeolite additives to OPC should be balanced with the calcium content for optimal carbonation resistance. [Display omitted] • Portland cement is chemically modified by Synthetic Zeolite Y additions. • Synthetic Zeolite Y reacts promptly as a pozzolan to consume calcium hydroxide. • Calcium hydroxide is required to form a passivating carbonation layer. • The Ca/(Si+Al) ratio of hydrates influences the carbonation mechanisms. • Synthetic Zeolite Y indirectly affected CO 2 uptake in portland cement. [ABSTRACT FROM AUTHOR]

Utilizing nano-SiO 2 (NS) to reinforce the old mortar and the weak interfacial transition zone (ITZ) in recycled coarse aggregate (RCA) is a productive approach to enhance the quality of RCA. Throughout this paper, RCA was immersed in NS solutions with different concentrations (2, 5, 10 and 30 wt%). Subsequently, a systematic evaluation of the crushing index and water absorption of the treated NS modified recycled coarse aggregate (SRCA) was conducted. In addition, the calcium hydroxide (CH) content of RCA before and after modification was analyzed by Thermogravimetric and Differential Thermal Analysis (TG-DTA) curves. This study finds that the performance of SRCA modified with 2% concentration achieves comparable modification effects to other concentrations. Additionally, this paper delves into the impact of NS powder on the slump characteristics and compressive strength of concrete. Considering the above research and economic costs, this study adopts 2% concentration of NS for RCA modification and further investigates the stress-strain relationship of NS modified recycled aggregate concrete (SRAC) with various SRCA replacement ratios. Moreover, this paper establishes mesoscopic models of SRAC for various replacement rates and proposes constitutive models corresponding to these rates. The results indicate that when the SRCA replacement rate is within 60%, the stress-strain curve shape of SRAC closely resembles that of natural aggregate concrete (NAC); when the replacement rate exceeds 60%, the descending segment of the SRAC stress-strain curve becomes more precipitous, showing a higher tendency towards fragility in the concrete. ● Nano-SiO 2 can be used to improve the properties of recycled coarse aggregates. ● NS powder affects the slump and compressive strength of recycled concrete. ● As the concentration of NS solution increases, the compressive strength of concrete increases. ● Mesoscopic models of modified recycled concrete were established and constitutive models were proposed. [ABSTRACT FROM AUTHOR]

In this work, two PC pastes were used to carry out accelerated carbonation experiments, and phase assemblages of paste samples collected at different sampling depths were analyzed. The source of CaCO 3 originating from the carbonation reaction of hydrates in cement was quantitatively determined by a novel method based on the transformation of hydrates. Experimental results indicated that the phase assemblages in the carbonated zone of two PC pastes showed minor differences. C(A)SH, ettringite, AFm-CO 3 , and unhydrated cement particles in the complete carbonation zone of two cement pastes were carbonated and transformed into other phases. For the surface of two PC pastes (0–3 mm), calculations showed that the CaCO 3 originating from the carbonation of calcium hydroxide and C(A)SH accounted for 75.3% and 72.6% of the total CaCO 3 , respectively. A concentration of 20% CO 2 was not beneficial for the diffusion of CO 2 and the subsequent precipitation of CaCO 3 at the later period of carbonation. • The change in phase assemblages of two carbonated PC pastes collected at different depths was analyzed. • A calculation method was proposed to analyze the source of CaCO 3 originating from the carbonation of hydrates in PC. • For the surface, the CaCO 3 originating from the carbonation of portlandite and C(A)SH accounted for 75.3% of the total CaCO 3. [ABSTRACT FROM AUTHOR]

Alkali Silica Reactivity (ASR) is a prominent issue concerning the durability of concrete structures. To mitigate this deleterious reaction, a widely accepted practice involves replacing significant cement portions with supplementary cementitious materials (SCMs). Given the scarcity of high-quality SCMs, nanoSiO 2 exhibits considerable promise, given its minimal dosage requirement for ASR mitigation, whether applied independently or in conjunction with other SCMs. This research investigates the efficacy and underlying mechanisms of colloidal nanoSiO 2 (CNS) in accelerated mortar bar tests according to the ASTM C1260 method. The experimental methodology incorporated two distinct forms of CNSes, CNS1 and CNS2, with average particle sizes of 14 and 67 nanometers, respectively. The outcomes are subsequently compared to the ASR performance of a sample containing silica fume (SF). The results demonstrate that the CNS size is pivotal in governing the pozzolanic reaction rate. The CNS2 and SF exhibited analogous pozzolanic reactions, displaying superior effectiveness in promoting pozzolanic activity compared to the CNS1. At a 4.75% cement replacement level, the CNS2 achieved a 78% reduction in mortar expansion over the control sample. The CNS1, when utilized at 4.75% and 7% cement replacement levels, yielded expansion reductions of 38% and 43%, respectively. It was found that the inclusion of the CNS can increase the porosity of mortar and contribute to silica dissolution from the reactive aggregates. Meanwhile, the addition of CNS improved the bulk resistivity of mortar by 200%-600% and flexural strength by 25%-62% based on the replacement levels and the CNS types. Moreover, improvements in calcium hydroxide consumption and alkali binding in the ASR gel were observed during the test, indicating the underlying mechanisms for expansion mitigation. These research outcomes can accelerate the CNS utilization to mitigate the ASR in practical field applications. • Replacing PLC with 4.75% nanoSiO 2 can mitigate ASR, where nanoSiO 2 with 67 nm showed highest mitigation. • Using nanoSiO 2 showed improved bulk electric resistivity of mortar, even though water-accessible porosity was higher. • Lower CH contents are found in mortars due to the presence of nanoSiO 2. • Using nanoSiO 2 has the potential to increase silica dissolution from reactive aggregates. • Expansion due to ASR gel showed dependency on the (Na+K)/Si and Ca/Si ratios of the ASR gel. [ABSTRACT FROM AUTHOR]

A new type of self-curing cement-based material was proposed for the first time by simultaneously using internal superabsorbent polymers (SAP) and external waterborne epoxy coatings (WEC). The mechanical properties and drying shrinkage of 8 groups of mortar were investigated. The hydration products, phase compositions, and micro-mechanism of SAP and WEC on the properties of self-curing paste were revealed through thermo-gravimetric/differential scanning calorimetric, Fourier transform infrared spectroscopy, and X-ray diffraction analysis. The results showed the synergistic effects of SAP and WEC contribute to the strength enhancement and shrinkage mitigation significantly. The addition of SAP induced no new crystalline phase, whereas sprayed WECs introduced a new product which containing C H and Si–H in the paste. The concurrently use of SAP and WEC had impact on crystal size of calcium hydroxide (CH) phase, and the enhancement of flexural strength was related to the crystal sizes for the (1 1 0) reflection. The drying shrinkage was negatively correlated with crystal size for the (0 0 1) reflection and positively correlated with crystal size for the (1 1 0) reflection at 60 days. Moreover, the crystal size of CH phase provided insights into the paste's micro-properties development, which also affected the development of mechanical strength and shrinkage performance of mortar samples. • The synergistic effects of SAP and WEC promote the strength enhancement and shrinkage mitigation significantly. • The concurrently use of SAP and WEC has impact on crystal size of calcium hydroxide phase. • The micro-properties affect the development of mechanical strength and shrinkage performance of mortar samples. [ABSTRACT FROM AUTHOR]

This research assessed the mechanical performance, gamma-ray shielding, and interfacial transition zone (ITZ) microstructural properties of concrete mixes that incorporate barite coarse aggregates and nanosilica. To achieve this goal, several concrete mixes containing normal coarse aggregate (NC), barite aggregate (BC), and nano-silica/barite aggregates (NSBC) were fabricated. Several properties such as workability, mechanical strength, electrical resistivity (ER), ultrasonic pulse velocity (UPV), chloride migration (RCPT), and gamma shielding were comparatively assessed for these concrete mixtures. In addition, analytical characterizations viz., x-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electronic microscopy (SEM), and energy dispersive x-ray (EDX) were employed to study the ITZ microstructure at mortar/aggregate interface of concrete mixtures. The results showed that barite concrete incorporating nanosilica (NSBC) exhibited higher density and improved mechanical properties compared to both normal concrete (NC) and barite concrete (BC). Specifically, concrete mixture containing barite aggregate and 3% nanosilica (3NSBC) content exhibited improved impermeability and reduced concrete porosity. The gamma-ray shielding properties for 3NSBC were superior among all concrete mixtures. After 28 days, 3NSBC exhibits higher compressive strength (18.26% and 14.76%) and greater shielding capacity (12.82% and 6.66%) compared to NC and BC, respectively. The analytical characterizations results have demonstrated that nanosilica helps prevent the accumulation of calcium hydroxide in the ITZ by utilizing it to form calcium silicate hydrate. This leads to the development of a more compact and denser microstructure in the ITZ of nano-silica-based barite concrete (NSBC). Furthermore, it was revealed that, the incorporation of nanosilica exceeding 3% led to agglomeration, resulting in a weaker ITZ within the concrete leading to overall reduction in concrete properties. • Fabricated normal (NC) and heavyweight concrete with/without nanosilica (NSBC and BC). • ITZ microstructure analyzed using SEM/EDX, XRD, and FTIR. • NSBC showed improved physical and mechanical properties. • Exceptional nuclear shielding performance observed for NSBC. [ABSTRACT FROM AUTHOR]

Superabsorbent polymers (SAPs) have emerged as promising additives to regulate cement hydration via their intrinsic water absorption/desorption characteristics. This paper aims to investigate the influence of SAPs with different functional groups on the development of cement hydration and growth of calcium hydroxide (CH). The current experiment firstly uncovered the active control mechanism of CH in cement paste by SAPs, especially for the nucleation site and crystallization orientation. Specifically, the pores left behind SAPs after water release usually processed higher w/c ratio, Ca2+ concentration and smaller crystallization pressure, which jointly triggered the in-situ nucleation and crystallization of CH along (0 0 1). However, the hydration degree of cement paste was reduced by 0.1∼4.4% with addition of SAPs, especially in the early stage, which was attributed to that less free water can be used for cement hydration due to the strong water absorption property of SAPs. These insights provide a valuable foundation for deploying SAPs in an active control strategy for CH growth position and crystal form in cement-based materials. • The nucleation site and crystallization orientation of CH can be controlled by SAPs. • Higher w/c ratio, Ca2+ concentration triggers the formation of CH around SAPs. • The addition of SAPs reduced the hydration degree of cement paste by 0.1 ∼ 4.4%. • The addition of SAPs primarily affects hydration during the early stage. [ABSTRACT FROM AUTHOR]

The frequent dry-wet cycles in the Loess Plateau can lead to the deterioration of fiber-reinforced loess (FR loess), which in turn can cause collapse when exposed to rainfall erosion or immersion. In recent years, geopolymers have received considerable attention due to their advantages, such as solid waste disposal and low carbon dioxide emissions. In this paper, the effects of geopolymer, consisting of steel slag (SS), ground granulated blast furnace slag (GGBS), desulfurization gypsum (DG), and sodium silicate, on the durability and water stability of FR loess were studied by conducting dry-wet cycle tests, immersion tests, and unconfined compression tests. The results show that the unconfined compressive strength (UCS) of fiber-reinforced and geopolymer-stabilized loess (FRG loess) decreases exponentially with the increase of dry-wet cycles while increasing approximately linearly with geopolymer dosage. The water stability coefficient of FRG loess shows a logarithmic increase with dry-wet cycles and increases linearly with geopolymer dosage, and the higher the dosage, the greater the water stability. The stabilization and dry-wet deterioration mechanisms of FRG loess were investigated by XRD and SEM-EDS tests. It reveals that the main mineral phases of FRG loess are calcium silicate hydrate, calcium aluminate hydrate, calcium aluminate silicate hydrate gel, and ettringite crystal. The microstructure of FRG loess is quite dense, but after the dry-wet cycles, it appears some cracking and loosening. The pores and microcracks of the low-dosage samples develop distinctly under dry-wet cycles, whereas the microstructure of high-dosage samples shows significant overall stability. The study on the durability and water stability of FRG loess under dry-wet cycles suggests that the optimal geopolymer dosage should be 15%. • The dry-wet durability and water stability of fiber-reinforced and geopolymer-stabilized loess were investigated. • The influence of the dry-wet cycles and the geopolymer dosage on the water stability coefficient was discussed. • The XRD and SEM-EDS tests were used to reveal the mechanism of stabilization and deterioration. [ABSTRACT FROM AUTHOR]

On the basis of hydration by amorphous SiO 2 and Al 2 O 3 on the surface of ultrafine iron tailings powder (UITP) and its synergistic hydration with slag powder, we used a UITP-based clinker-free full-tailings consolidation agent to prepare a low-carbon full-tailings backfill material. The consolidation effect of this agent in full-tailings backfill materials and the consolidation mechanism was studied. The UITP-based consolidation agent had a higher strength consolidation effect on the full-tailings backfill specimen. At the same slurry concentration, the compressive strength of cement specimens with a cement-sand ratio of 1:8 and UITP-based consolidation agent specimens with a consolidation agent-sand ratio of 1:12 were equivalent. At the slurry concentration from 73% to 79% and the consolidation agent-sand ratio of 1:24–1:12, the strength of the backfill specimen at 28 d can reach 1–5 MPa. Increasing the ratio of consolidation agent to tailings in the low dosage range of 1:24–1:12 improved the rheological properties of the slurry. Microscale tests such as X-ray diffraction (XRD), thermogravimetric differential scanning calorimetry (TG-DSC), and scanning electron microscopy (SEM) indicated that calcium aluminosilicate hydrate (C-(A)-S-H) and ettringite (AFt) were the two main hydration products in the consolidated body. During hydration process by the consolidation agent, the surface of UTIP with a particle size of less than 1 µm produced many hydration products, which produced strength by cementing the original iron tailings particles and functioned as 'active node' in the backfill specimen. This paper revealed that the multigene regulatory effect of integrating the hydration and physical backfill effect of UITP and the nucleation effect of broken bonds on the surface of the primary tailings sand were the main reason why UITP-based clinker-free consolidation agents had better consolidation effects than cement. The synergistic hydration effect of UITP and slag powder can achieve low carbonization of full tailings backfill. • A clinker-free cementitious material containing 50% of iron tailings powder was developed. • A full tailings filling material was prepared with clinker-free cementitious material. • The mechanism of ultrafine iron tailings powder to consolidate full iron tailings was studied. [ABSTRACT FROM AUTHOR]

The present study investigated the effect of accelerated carbonation curing (ACC) at early age on the thermal evolution of hydrates in calcium sulfoaluminate (CSA) cement. CSA paste samples with a molar ratio of calcium sulfate to ye'elimite of 1 were cured under 10% atmospheric CO 2 concentration until 28 days of curing. The CSA saFfigmples were subjected to 200, 400, 600, and 800 °C, thereafter the thermal evolution of the hydrates in the CSA samples was investigated. The main carbonation outcomes were aragonite and vaterite, and were recrystallized to calcite at exposed temperature between 400 and 600 °C, mitigating strength loss in the CSA cement. However, a significant strength loss was observed after exposure to 800 °C due to the decalcification of the calcium carbonates in the samples, showing a reduced strength of 65.7% and 53.0% compared to the carbonation-cured sample exposed to 600 °C and non carbonation-cured sample exposed to 800 °C. In addition, the both carbonation- and non carbonation-cured CSA samples showed similar 27Al and 29Si MAS NMR spectral shapes upon exposure to 800 °C, implying that the Si- and Al- environments became similar at high temperatures regardless of the ACC process. • Aragonite and vaterite were the main carbonation products of CSA cement. • Aragonite and vaerite were undergone the phase transformation to calcite between 400 and 600 °C. • Accelerated carbonation curing mitigated the compressive strength loss at temperatures below 600 °C. • Si- and Al- environments of CSA cement became similar at the temperature of 800 °C regardless of the accelerated carbonation curing process. [ABSTRACT FROM AUTHOR]

Utilization of aluminosilicate-rich solid waste via geopolymerization, such as mine tailings (MTs), possesses a dual advantage of reducing CO 2 release from cement manufacturing and increasing tailings utilization. However, a major challenge for this technology is the low reactivity of MTs. In this study, an alkali-hydrothermal method at 200 °C was developed for MTs activation, and MTs were converted into an active alkaline aluminosilicate precursor that could play an alkaline activator-like role. Geopolymers were subsequently prepared from activated MTs and slag by simply adding water, and to improve the performance of geopolymers, steel slag (SS) was added as a supplementary alkali source. Results showed that the advantage of SS as a calcium-based alkaline material could be fully exerted in this system. The Ca(OH) 2 phase derived from SS hydration increased the pH of the pore solution, on the one hand, and complexation reactions between Ca2+ ionized from Ca(OH) 2 and Si and Al monomers dissolved from activated MTs might occur and generate calcium aluminosilicate hydrate gel, on the other hand. As a result, the addition of SS accelerated the hydration process, prompted the production of hydration products, and thus enhanced the strength of geopolymers. The 3-day and 28-day strengths of geopolymers achieved as high as 31.10 and 44.79 MPa, respectively, when SS content was kept at 10 wt%. In addition, SS-doped geopolymers had satisfactory soundness because the fast consumption of Ca2+ prevented the growth of Ca(OH) 2 crystals and the resulting expansion effect. • Alkali-hydrothermal activation converted tailings into an active alkaline aluminosilicate precursor. • The advantage of steel slag as a calcium-based alkaline material was fully exerted. • Both the low reactivity of tailings and the poor volume stability of steel slag were overcame. • This study provided a new route to improving the comprehensive utilization of tailings and steel slag. [ABSTRACT FROM AUTHOR]

Alkali-Activated Slag Cement (AASC) activated by weak alkaline components has good workability and safety. Sodium carbonate (Na 2 CO 3 , SC), sodium sulfate (Na 2 SO 4 , SS), reactive magnesium oxide (R-MgO), and calcium hydroxide (Ca(OH) 2 , CH) were used to prepare the AASC. The hydration process, phase composition and hydration kinetics of AASC were studied. The results showed that the pH value of AASC slurry activated by weak alkaline components ranging from 10.8 to 13.2. CH could significantly affect the pH of AASC slurry. When the dosage reached more than 5%, the pH value of AASC slurry reached more than 12.0. The hydration heat release of AASC activated by weak alkaline components was mainly concentrated in 1 day. Two hydration heat release peaks appeared. The initial hydration peak appeared within 0.5 h-1 h, and the acceleration hydration peak appeared within 1 h-8 h. When the content of CH was 5%, and the content of SS and SC was 7% and 4%, the acceleration peak of AASC was much higher than that of others. At the same time, the Krstulovic-Dabic model was used to study the hydration kinetics of AASC. The increase of SS content prolonged the NG-I and I-D processes. CH accelerated the hydration rate of AASC and shortened the hydration process of AASC. The main hydration products of AASC activated by weakly alkaline components were C-(A)-S-H gel, calcite, ettringite, hydrotalcite and gaylussite. R-MgO was favorable for the formation of hydrotalcite, while SS was favorable for the formation of ettringite. • Weakly alkaline salt can make Alkali-Activated Slag Cement (AASC) have a good alkaline environment. • The nucleation and crystal growth (NG)- phase boundary reaction (I) - diffusion (D) process of AASC is affected by weak alkaline salt. • There are more crystals in hydration products. [ABSTRACT FROM AUTHOR]

In order to meet function requirements of slope protection and ecology for grass-planting concrete with a random porous structure, this study proposes two typical alcohol amines, namely, water-reducing agent modified triethanolamine (TEA) and diethanol monoisopropanolamine (DEIPA), as admixtures for the production of grass-planting concrete. The crystal structures, elemental ratios, and hydration products of cementitious materials with modified alcohol amines were investigated to explore their mechanism of action, reinforcement, and appropriate dosage. The compressive strength of cementitious materials and the spread of mortar made from modified alcohol amines were investigated to evaluate their impact on the compressive strength and workability of grass-planting concrete. The results show that modified TEA can more uniformly reduce the calcium-silicon ratio of the hydration products, leading to enhanced crystallization and densification at a dosage of 0.08%. At a dosage of 0.16%, modified DEIPA can effectively promote the hydration of tricalcium silicate and the secondary hydration of generated calcium hydroxide. The reduction in calcium hydroxide content resulted in an increase in strength. Both these amines enhance the bonding ability of cementitious materials. The spread of the modified cementitious materials is further improved, and suitable cementitious materials with a low water-cement ratio can be obtained for making grass-planting concrete to create uniformly wrapping coarse aggregates with the cementitious material. Modified TEA and modified DEIPA significantly increased the 28-day compressive strength of cementitious material by more than 104%, and the improvement effect of modified DEIPA was significantly better than that of modified TEA. The modified alcohol amines show great potential for practical application in production. [Display omitted] • Introduction of alcohol amines modified with water-reducing agent improve their performance. • Modified alcohol amines can improve the fluidity of mortar. • Modified alcohol amines can increase crystallization degree and density and reduce CH content. • The application of modified alcohol amine in grass-planting concrete effectively improves the strength and workability. [ABSTRACT FROM AUTHOR]

This paper investigates the influence of temperature on autogenous deformation and early-age stress (EAS) evolution in ordinary Portland cement paste using a recently developed Mini Temperature Stress Testing Machine (Mini-TSTM) and Mini Autogenous Deformation Testing Machine (Mini-ADTM). In the Mini-TSTM/ ADTM, CEM I 42.5 N paste with a water-cement ratio of 0.30 was tested under a curing temperature of 10, 15, 20, 25, 30, and 40 °C. X-Ray diffraction (XRD) tests were conducted to measure the amount of ettringite and calcium hydroxide, which reveals the micro-scale mechanisms of autogenous expansion. The applicability of the Maturity Concept (MC) for the prediction of autogenous deformation and relaxation modulus under different temperatures was also examined by the experimental data and the viscoelastic model. This paper leads to the following findings: 1) The autogenous deformation of ordinary Portland cement paste is a four-stage process comprising the initial shrinkage, autogenous expansion, plateau, and autogenous shrinkage; 2) Higher temperature leads to higher early-age cracking (EAC) risk because it accelerates the transitions through the first three stages and causes the autogenous shrinkage stage to start earlier. Moreover, higher temperatures also result in increased rates of autogenous shrinkage and EAS in the autogenous shrinkage stage; 3) Autogenous expansion and plateau are attributed to the crystallization pressure induced by CH. Temperature-dependent CH formation rates determine the duration of the plateau stage; 4) Low-temperature curing can delay but not completely prevent the EAC induced by autogenous deformation; 5) The MC cannot predict the autogenous deformation at different temperatures but can be used to calculate the relaxation modulus, which in turn aids in EAS prediction based on autogenous deformation data. • The effects of temperature on early-age cracking risk induced by autogenous deformation is systematically analysed. • The autogenous deformation and the stress evolution were tested by a mini–Temperature Stress Testing Machine. • The effects of calcium hydroxide on autogenous expansion were found. • A viscoelastic model combined with maturity concept was developed to predict the stress evolution. [ABSTRACT FROM AUTHOR]

A novel method for reusing construction waste and simultaneously sequestering CO 2 in the form of mineral carbon is presented in this study. The method involves the production of construction bricks using waste concrete fines treated with carbonation. Initially, waste concrete is crushed and ground into recycled concrete fines (RCF), which are then subjected to a carbonation process to obtain carbonated recycled concrete fines (CRCF). Comprehensive characterization using XRD, TGA, FTIR, and zeta potential measurements reveals that CRCF exhibits a microstructure featuring silica-coated calcium carbonate particles on its surface, imparting significant reactivity to CRCF. In this investigation, RCF and CRCF are separately mixed with Ca(OH) 2 , molded, and compressed into bricks, with performance testing conducted under various material ratios and manufacturing conditions. The results indicate that bricks made with CRCF exhibit mechanical properties exceeding those of RCF-based bricks by more than two-fold. This improvement can be attributed to the refined pore structure observed in CRCF bricks. The highly reactive silica gel present on the surface of CRCF reacts with Ca(OH) 2 to form C-S-H, providing a rationale for the superior strength displayed by CRCF bricks. • Novel method combines construction waste reuse with CO 2 sequestration in brick production. • Carbonated recycled concrete fines (CRCF) exhibit superior reactivity and microstructure. • CRCF bricks surpass RCF-based bricks by over two-fold in mechanical properties. • Optimal conditions for water-solid ratios, curing temperature, and forming pressure identified. • CRCF-bricks demonstrate higher strength and refined pore structure, offering sustainable construction. [ABSTRACT FROM AUTHOR]

The hydration of cement-based materials, especially at early ages, directly affects the development of matrix strength and the later durability. In this paper, in order to explore the hydration behavior and microstructure of cement-based composites modified by low melting point Field's Metal Particles (FMP), inverted cone method, standard Vicatometer method, isothermal calorimetry, and compressive strength test, TGA, SEM, and XRD are used to investigate the fresh properties, heat of hydration, matrix strength, hydration products and transition zone between FMP and cement matrix. The results show that the incorporation of FMP can improve the fluidity of cementitious composites by 31.6%, due to the hydrophilicity of FMP. Meanwhile, due to the small heat capacity and large thermal conductivity of low-melting FMP, the heat stored in FMP is naturally released and used to promote hydration reaction when the hydration rate slows down. Additionally, FMP does not affect the types of hydration products in cement-based composites, but the total amount of hydration products increased at 7 d, especially calcium hydroxide (CH). Unexpectedly, FMP can reduce the compressive strength of the hardened paste, due to the large particle size of FMP than cement particles and the presence of directionally growing CH crystals in the interface transition zone between FMP and cement matrix. • Field's Metal (FMP) can promote the hydration and the generation of products at early age. • FMP can improve the fluidity of cement pastes. • Directional growth of calcium hydroxide is the main reason for strength reduction in cement pastes with FMP. [ABSTRACT FROM AUTHOR]

• The hydration of glass is heterogeneous due to phase-separation structure; • The heterogeneous hydration of glass impact the formation of calcium hydroxide; • The hydration rate advantage of calcium-rich and silicon-rich phase can be reversed; • The heterogeneous hydration is suitable for thermodynamic simulation of cement paste; Supplementary cementitious materials (SCMs) affect the hydration process and long-term properties of cementitious systems. Since a large percentage of SCMs belong to alkaline-earth aluminosilicate glasses that possess a phase-separated structure, further research into their hydration characteristics is essential. This study investigated the effects of the hydration characteristics of synthetic glasses with various phase-separated structure on the phase evolution of cementitious systems by combining simulations with experiments. The results showed that with the increasing degree of phase separation, the hydration of synthetic glass transitions from homogeneous to heterogeneous hydration. The phase evolution of hardened paste, particularly for calcium hydroxide, is influenced by the difference in hydration rates among each separated glassy phase. Compared to homogeneous hydration, the rapid hydration of the Ca-rich phase reduced the consumption of calcium hydroxide, while the rapid hydration of the Si-rich phase accelerated calcium hydroxide consumption. For glasses exhibiting phase separation characteristics, the consistency between thermodynamic simulations employing a heterogeneous hydration model and experimental results exhibits a notable enhancement. [ABSTRACT FROM AUTHOR]

The objective of this study is to reduce the hydrogen embrittlement risk of rebar caused by the continuous constant current (CC) in the bidirectional electromigration rehabilitation (BIEM). Three types of square wave current (SWC) were applied in the BIEM treatment on a Cl--contaminated concrete, in the saturated calcium hydroxide containing 1 mol/L triethylenetetramine (TETA). The sample experienced the BIEM with a constant current density of 3 A/m2 was chosen as the control sample with the same electricity consumption. After 4 cycles of treatment, the concentration profiles of Cl- and TETA were measured in the protective layer. The corrosion current densities of the rebar were analyzed by various electrochemical tests. The stress-strain curve of the rehabilitated rebar was tested, and its hydrogen embrittlement risk was estimated. The results suggested that the SWC more effectively removed Cl- and migrated TETA than the CC did, with the same electricity consumption and treatment duration. The rebar experienced the former BIEM exhibited a higher fracture energy ratio and a lower corrosion rate than the one experienced the latter treatment. In general, the SWC was more effective and lower hydrogen embrittlement risk than CC in the BIEM rehabilitation treatment. • SWC more effectively removed Cl- and immigrated TETA than CC did with the same electricity consumption and treatment duration. • Fracture energy ratio and corrosion rate of the SWC-treated rebar were higher and lower than those of the CC-treated one. • Hydrogen embrittlement of rebar can be reduced by using SCW instead of CC in the BIEM treatment. [ABSTRACT FROM AUTHOR]

[Display omitted] • NNPs activated with Ms of 1.25-1.5 and Na 2 O% of 9.25-13% can result in compressive strengths greater than 25 MPa. • The best compressive strength-giving mixtures had a workability of at least 50%. • Calcium hydroxide is a compressive strength enhancer as well as a set controller in NNP geopolymer systems. • Shrinkage-reducing admixtures (SRA) can be used as set retarders in NNP geopolymer systems. • The contour plots and multiple linear regression models are applicable in predicting the compressive strength of NNP geopolymers. The recent shortage of traditional fly ash and slag, commonly used precursors in alkali-activated binders, prompted a rigorous search for alternative materials. This study evaluated four groups of currently underused supplementary cementitious materials (SCMs) as precursors for producing alkali-activated mortars. The materials evaluated include three calcined clays, three ground bottom ashes, three volcanic ashes, and two fluidized bed combustion ashes, collectively named non-traditional and natural pozzolans (NNPs). Firstly, the compositions of the NNPs were optimized using trial-and-error methods to minimize the concentration of the activating solution. Then, the two solution parameters i.e., silica modulus and Na 2 O% were optimized for compressive strength using a central composite design. The compressive strength (ASTM C109), the workability (ASTM C1437), and the setting time (ASTM C191) of the mortar were determined. The results show that all 11 materials could produce mortar mixtures with compressive strength in the 20–40 MPa range at 28 days, satisfactory workability of at least 50 % (flow value), and at least 30 min of initial setting time without heat curing and without blending with conventional SCMs. Most of the materials were activated with lower solution parameters than those used in the literature. The binder optimization suggested Si/Al range 1.0–3.0 and a minimum Ca/Al of 0.25 as favorable molar ratios for potentially reactive aluminosilicate precursors. Multiple linear regression models validated by higher R2 and minimum RMSE and MAE could accurately predict the compressive strength of NNP mortar. Furthermore, calcium hydroxide and shrinkage-reducing admixtures were identified as set regulators in alkali-activated NNP mortars. [ABSTRACT FROM AUTHOR]

Cementitious composites are widely used in architecture and construction field, but usually show quasi-brittle failure at macroscale under tensile or bending loads. In contrast, the calcium silicate hydrates (C-S-H) exhibits noticeable ductile response at nanoscale. In this article, we focus on the interfacial transition zone between cement paste and aggregate, which consists of the C-S-H/quartz and the calcium hydroxide (CH)/quartz interface structures. To comprehend how a nanoscale design might improve concrete's macro mechanical qualities, molecular dynamics and finite element simulations are performed at the nanoscale and macroscale, respectively. The elastic properties, modulus, fracture toughness and critical energy release rate of C-S-H/CH/graphene oxide (GO)/quartz composites are calculated using the molecular dynamics method at nanoscale. These parameters then are used in macro-simulation and calculating mechanical properties of macro concrete structures. The incorporation of GO sheets into the interface results in a 38.40% to 58.06% increase in the fracture toughness of interfacial transition zone (ITZ) systems and an 80% and 9.81% improvement in the tensile strength and tensile modulus of the concrete model, respectively. This fundamental insight into nanoscale interfaces notably allows for more accurate predictions of macroscopic mechanical behavior, paving the way for innovative and enhanced cement-based materials in the future. [Display omitted] • Fracture properties of cementitious composites at nanoscale are calculated. • Crack expansion in concrete is simulated by finite element method. • GO enhances fracture and elastic properties of C-S-H/CH/quartz interface. [ABSTRACT FROM AUTHOR]

• The three-dimensional vector hydration model of cement paste is built. • The first-passage cube is innovatively proposed to solve the diffusion coefficient. • The validity of Brownian motion algorithm is verified by experimental results. • Sensitivity analysis is made to evaluate the effect of main factors on diffusivity. In view of the important role of cement profile in hydration process, pore structure characteristic, porosity and chloride ingression of cement paste, this paper aims to propose a numerical method for predicting the chloride diffusion coefficient of cement paste by considering the particle shape. By introducing periodic boundary conditions and contact function, the three-dimensional fresh cement paste with ellipsoidal particles is firstly constructed. By taking into account the interferences induced by overlapping of hydration layers between adjacent hydrated cement particles and calcium hydroxide precipitation, the vector hydration model is proposed. Each hydrated cement particles is composed of inner and outer hydration layers, unhydrated cement particle and partial calcium hydroxide embedded in the hydration layer. After the validity of hydration model is verified by the experimental data of hydration degree and the content of hydration products, the effects of aspect ratio on hydration degree, pore size distribution and porosity are evaluated. Secondly, based on first-passage theory, the Brownian motion algorithm applicable to voxel model is proposed. In order to improve the simulation efficiency, the first-passage radius and Brownian number are determined. The accurancy of this approach is fully verified by comparing with sufficient experimental data. Finally, resorting to this numerical algorithm, the effects of the aspect ratio and the diffusion coefficients of inner and outer hydration layer on chloride diffusion coefficient are analyzed. [ABSTRACT FROM AUTHOR]

• The effects of triethanolamine, diethanol-isopropanol amine on GGBFS-cement binder at curing temperatures of 5 and 20 °C were investigated. • The effect of Ca(NO 2) 2 is sensitivity to the curing temperatures, the performance of Ca(NO 2) 2 is better at a curing temperature of 20 °C. • TEA and DEIPA at 0.02 % dosage did not improve strength at 5 °C due to their complex interaction with Ca2+, which hinders calcium hydrate crystal formation. • Among the four admixtures, Na 2 SO 4 showed the best promoting effect on the slag-blended cement at 3 days. The initial reactivity and strength development of blast furnace slag cement (BFSC) are crucial factors that increase its utilization. Therefore, it is important to select the most suitable accelerator. In this study, the impact of four accelerators, namely sodium sulfate (Na 2 SO 4), calcium nitrite (Ca(NO 2) 2), triethanolamine (TEA), and diethanolisopropanolamine (DEIPA), on the hydration and strength development of BFSC at curing temperatures of 5 and 20 °C was investigated. Isothermal calorimetry, X-ray diffraction (XRD), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) were conducted. The results revealed that Na 2 SO 4 effectively enhanced early age strength by accelerating the hydration of the clinker and slag. Ca(NO 2) 2 is sensitive to temperature changes. At 20 °C, Ca(NO 2) 2 promoted the reaction of the aluminates and slag in the initial stage. However, the presence of slag inhibited clinker hydration, and the addition of Ca(NO 2) 2 exacerbated this effect, resulting in reduced strength at 5 °C. Indeed, the use of TEA and DEIPA at 0.02 % at 5 °C is not recommended. Complex interactions between TEA and Ca2+ may inhibit the formation of calcium hydroxide crystals, further inhibiting aluminate hydration. This may lead to low strength development at low curing temperatures such as 5 °C. [ABSTRACT FROM AUTHOR]

• Alkali-activated binders (AAB) were produced with açai seed ash (ASA) as alkaline activator. • Physicochemical characterization of ASA showed soluble K 2 O and P 2 O 5 content. • ASA was combined with calcium hydroxide (CH) to remove harmful soluble phosphate. • Applications of ASA, CH and ASA/CH in AAB based on blast-furnace slag were carried out. • Results showed that the combination of ASA/CH led to a proper alkaline activator. Açai seed ash (ASA) was assessed as a new alkaline activator in alkali-activated binders (AAB) based on blast-furnace slag (BFS). ASA was produced by calcination of açai seed in the laboratory, and it is mainly composed of K 2 O (33.75 wt%) and P 2 O 5 (22.59 wt%), with a pH of 12.3 and solubility in water of over 35 wt%. Due to the harmful soluble phosphate, ASA was combined with Ca(OH) 2 (CH) to form insoluble calcium phosphate. Four AAB mixtures were assessed after 1 day of curing at 60 °C: BFS/water, BFS/CH/water, BFS/ASA/water, and BFS/ASA/CH/water. Microstructural studies of pastes showed that the BFS/ASA/CH/water mixture presented the formation of insoluble hydroxylapatite by X-ray diffraction, the highest reaction product content (26.52 %) by Fourier transform infrared spectroscopy, and densest microstructure by scanning electron microscopy. The compressive strength of mortars showed that BFS/ASA/CH/water mixture presented the highest value of 8.7 MPa. [ABSTRACT FROM AUTHOR]

• Na 2 CO 3 was used to enhance the reactivity of substituted ground bottom ash in OPC system. • Enhanced C 4 AF reaction and slight C 3 S acceleration led to more ettringite formation. • A small amount of 0.025% was significantly effective in strength improvement. • Compared to control, it increased the strength 35.4 % at 7 days and 70.6 % at 28 days. In this study, the use of sodium carbonate anhydrous (Na 2 CO 3) to enhance the reactivity of bottom ash (BA) in the ordinary Portland cement (OPC) system was investigated. The presence of Na 2 CO 3 exerted an interactive effect on the hydration mechanism of the BA-OPC composite, leading to a significant enhancement in the pozzolanic reactions between BA and calcium hydroxide, following the improved silicate reactions. Consequently, remarkable improvements were observed in the mechanical properties of the BA-OPC composite with an extremely small amount of the activator. Specifically, the sample containing 0.025 % Na 2 CO 3 exhibited the highest increase in compressive strength compared to the sample without Na 2 CO 3 , showing an improvement of 35.4 % at 7 days and 70.6 % at 28 days. Based on the proposed method, the reactivity of BA and silicate phases can be enhanced which contributes to the sustainable development of BA-OPC composite. [ABSTRACT FROM AUTHOR]

• Distribution characteristics of waste rock particles in the mixed aggregate cemented backfill and its settlement delamination stratification mechanism. • Mechanism of the effect of waste rock contents on the strength of cemented backfill. • Effect of waste rock contents on deformation and failure characteristics of cemented backfill. • Acoustic emission characteristics of mixed aggregate cemented backfill during uniaxial compression. • Hydration products and microstructural characteristics of cemented backfill with different waste rock contents. To study the effects of waste rock (WR) contents on the delamination and mechanical characteristics of the cemented backfill, the backfill specimens with 40%-80% WR content were prepared based on the stacking density of mixed aggregate. The characteristics of the WR distribution inside the cemented backfill with various WR contents were examined by scanning the sections images of the specimens. The strength and damage properties of the cemented backfill were investigated by uniaxial compression acoustic emission (AE) tests, and the strength and damage evolution mechanisms were explained by microstructure testing experiments. The results show that the viscosity and yield stress of the filling slurry gradually decreased with the contents of WR increased, and the WR settlement and delamination more obvious. The uniaxial compressive strength (UCS) has the same trend as the stacking density of the mixed aggregate, showing the trend of "increased first and then decreased" with the content of WR increased. When the content of WR was 60%, it had the highest UCS. As the content of WR increased, the fracture area at the interface of WR and cementitious matrix gradually increased, and the extrusion between coarse aggregates became more obvious, leading to more complex crack extension and higher AE cumulative energy. The main products of hydration in the cemented backfill were calcium silicate hydrated (C-S-H gels), ettringite (AFt), and calcium hydroxide (Ca(OH) 2). The specimens with higher content of hydration products have better cementation among aggregate particles, denser microstructure, and higher compressive strength of cemented backfill. [ABSTRACT FROM AUTHOR]

• The bonding properties between FCNTs repair mortar and fire exposed old mortar substrate has been investigated. • The bonding strength and microscopic test were performed to determine the reinforcing mechanism of FCNTs in the ITZ. • An effective prediction model is proposed to quantify the effects of porosity and different FCNTs on the interfacial bonding strength of composite specimens. Reinforced concrete structures need to be repaired after exposure to fire, in which the weakness of the interfacial transition zone between the repair mortar and the old concrete substrate is a crucial problem. Carbon nanotube can be added into repair mortar to address the above-mentioned problem because of its bridging, pore filling and nucleation effects. However, the bonding mechanism of functionalized carbon nanotube repair mortar and the old concrete is not yet fully understood. Therefore, this study investigates the bonding performance between different functionalized carbon nanotube repair mortar and old mortar substrate exposed to high temperature. Firstly, to evaluate the roughness of old mortar exposed to high temperature, three-dimensional microscope and mercury intrusion porosimetry were performed in this paper. Secondly, thermogravimetric analysis and X-ray diffraction were carried out to further analyze the phase composition of the repair mortar. Thirdly, microstructure of the interfacial transition zone was investigated by electron backscatter diffraction characterization and microhardness measurements. Fourthly, the bonding mechanism of interface between new and old mortar was discussed. The results show that the surface roughness of the old mortar can gradually increase with increasing of temperature, which improves the interfacial mechanical interlocking. The functionalized carbon nanotube can reduce the content and orientation of calcium hydroxide and improve the microhardness of the interfacial transition zone. Finally, an interfacial bonding strength prediction model considering porosity and the reinforcing effects of different functionalized carbon nanotube is proposed. The findings of current study can benefit further research and application of nanomaterials in the field of structural repair and maintenance. [ABSTRACT FROM AUTHOR]

• Experiments coupling slow calcium leaching with dry/wet cycles were designed. • The coupling effect exists between dry/wet cycles and calcium leaching. • A quantitative prediction model for mechanical properties was established. The durability of marine concrete is significantly influenced by the coupled effect of calcium leaching (CL) and dry/wet cycles (DW). This study investigates their combined impact on the Vickers hardness and pore structure of hardened cement paste. The experimental setup includes two groups: Group A (CL followed by DW) and Group B (DW followed by CL), which superimpose the single-factor effects of each factor. Calcium leaching levels were set at 5 %, 10 %, and 15 % loss of sample mass, and dry/wet cycles were varied at 4, 8, and 12 times. A 4 mol/L ammonium nitrate solution was used to accelerate calcium leaching. The results demonstrate that the coupled effect of CL and DW reduces the Vickers hardness by increasing the porosity of the cement paste. Specifically, CL primarily increases the porosity of pores above 200 nm by dissolving calcium hydroxide, and it augments the porosity of 7 ∼ 50 nm pores by removing calcium from C-S-H gels. However, the combined effect of CL and DW on Vickers hardness is less pronounced than the total individual effects. Moreover, Group A exhibits a more substantial impact on the deterioration of mechanical properties in hardened cement paste compared to Group B. In addition, a model is applied for quantitatively predicting mechanical performance based on nominal porosity. [ABSTRACT FROM AUTHOR]

• The micromechanical properties of different phases in FNS particles are revealed. • The micromechanical property of N-A-S-H gel in AAFNS is studied. • The effects of FNS and activator type on the microstructural characteristics of AASF are investigated. In recent years, low-calcium and high-magnesium ferronickel slag (FNS) has been gradually used in the preparation of alkali-activated cements (AACs). An in-depth exploration of the microstructural characteristics is crucial for a thorough understanding and enhancement of the macroscopic behaviors of AACs. In this study, the microstructural composition and micromechanical properties of the alkali-activated ground granulated blast furnace slag (GGBS)-FNS cements (AASF) prepared with sodium silicate and sodium hydroxide as alkali activators were quantitatively studied. Backscattered electron image analysis (BSE-IA) and nanoindentation were selected to reveal the micromechanical properties of different phases in FNS particles and AASF pastes. The results show that the elastic modulus of sodium aluminosilicate hydrate (N-A-S-H) in alkali-activated FNS cement is about 17 GPa, which is similar to that of the gel phase in alkali-activated fly ash cement (AAFA) and alkali-activated metakaolin cement (AAMK), confirming that the micromechanical property of N-A-S-H gel is independent of the kind of precursor materials. The Ca/Si and Al/Si of the hydration products in AASF gradually decline as FNS is incorporated, whereas the Mg/Al steadily rises, increasing the elastic modulus of the inner hydration product (IP) phase in AASF. The reaction degree of GGBS is higher in the sodium silicate-activated AASF system, while more N-A-S-H gels are formed in the sodium hydroxide-activated AASF system. The incorporation of FNS is conducive to improving the reaction degree of GGBS, resulting in the increase of calcium silicoaluminate hydrate (C-A-S-H) content and a significant decrease in IP content in AASF. [ABSTRACT FROM AUTHOR]

[Display omitted] • DEC effectively improves the early strength of the Cement-Steel Slag Composite. • DEC effectively promotes the early hydration process of the Cement-Steel Slag Composite. Steel slag (SS) is a solid waste produced from the steelmaking process. The weak early hydration activity of steel slag limits its application in cementitious materials. This study attempts to improve the early-age performance of cement-SS composite by direct electric curing (DEC) to broaden the application of SS in cementitious materials. Specifically, The study involves continuous monitoring of performance changes of the specimens over time, with a particular focus on early-age characteristics. The change in strength and capillary water absorption of the hardened paste are evaluated. The pore size distribution and content are investigated by BET, and the effects of DEC on the hydration process of cement-SS composite are determined by XRD, TG, and BSE. The results demonstrate that DEC effectively improves the early-age strength of specimens. The 1d strength of the specimens increased by more than 50% at 50% steel slag dosage. Additionally, DEC effectively reduces the pore size. DEC does not change the type of hydration products of the paste but effectively accelerates the hydration process of the cement-SS composite. This accelerated reaction leads to a notable increase in calcium hydroxide and bound water content at an early age, consequently improving the degree of hydration reaction in steel slag. SEM-BSE analysis revealed changes in the atomic composition of the C-S-H gel near the SS, accompanied by blurred boundaries in the morphology of SS particles. Moreover, this study analyzes the economic and environmental benefits of using cement-steel slag composite with DEC. Considering the strength factor, the cost required to obtain a unit strength by DEC is only 55.88%-71.28% of that required with standard curing method. Simultaneously, carbon emission is reduced by 28.79%-44.16%. Thus, DEC is a sustainable and practical method for promoting the utilization of steel slag in cementitious materials, offering both economic and environmental benefits. [ABSTRACT FROM AUTHOR]

• The effect of nano-ZrO 2 addition on the efficiency of carbonation curing was investigated. • The mechanism of the impact of nano-ZrO 2 in carbonation and sealed curing conditions was compared. • The addition of nano-ZrO 2 in carbonation curing conditions resulted in improved late macroscopic properties. • Adding nano-ZrO 2 can change the microscopic morphology of the matrix and increase the CH reactivity. • Nano-ZrO 2 increased the production of calcium carbonate in carbonation curing conditions. Nanomaterials are characterized by large surface areas and ultrasmall particle sizes, which affect their micromorphologies. In this study, the impact of nano-ZrO 2 on the efficiency of carbonation curing was systematically investigated to compare the mechanism for nano-ZrO 2 in the hydration and carbonation reactions occurring during sealed and carbonation curing and the effects on the micro- and macroscopic properties. The experimental results showed that the hydration rate and degree of carbonation increased gradually with increasing nano-ZrO 2 content. The effect of nano-ZrO 2 on the hydration rate was more significant in the early period (0–72 h); the impact on the carbonation curing efficiency was more significant in the later period (28 days). After 28 days of carbonation curing, the compressive strengths of samples doped with 0.5% and 1% nano-ZrO 2 were higher by 7.5% and 4.9%, respectively, compared to those of the samples without nano-ZrO 2. Moreover, the resistivities of the samples doped with nano-ZrO 2 were higher than that of the control sample (NZ-0%–CC). The efficiency of carbonation curing was mainly attributed to optimization of the substrate micromorphology by the nano-ZrO 2. The addition of nano-ZrO 2 decreased the orientation of the calcium hydroxide (CH) crystals in the mixture and reduced their sizes, leading to an increase in the reactivity of the CH. Moreover, with increases in the amount of nano-ZrO 2 added, the calcium carbonate content of the carbonation product increased slightly. Thus, nano-ZrO 2 improved the efficiency of concrete carbonation curing; high carbonation, high strength and high durability were realized by adding a small amount of nano-ZrO 2 for microscopic morphology optimization. [ABSTRACT FROM AUTHOR]

• The white deposits were newly formed carbonized products C 4 A C ¯ H 11 and CaCO 3. • SB had a good inhibiting effect on the early and long-term whitening of CAC mortar. • C 4 A C ¯ H 11 was no longer generated resulting in no visible whitening with 12% SB addition. • Capillary water absorption was closely related to the whitening of CAC mortar. • SB affects cement hydration, hydrophobicity and capillary water adsorption of CAC mortar. This paper was based on the results of the research group's previous research on early whitening of mortars. The inhibition of whitening and changes of white deposit on the surface of calcium aluminate cement (CAC) mortar with styrene-butadiene copolymer (SB) were studied using a long-term accelerated whitening method under conditions of (20 ± 2) ℃ and (60 ± 5)% RH. The whitening phenomenon and white deposits of CAC mortars were studied using digital processing of images, optical microscopy (OM) and scanning electron microscopy (SEM). The results indicate that the white deposit on the surface of CAC mortar primarily consists of newly formed carbonation products, calcium carboaluminate hydrate (C 4 A C ¯ H 11) and calcium carbonate (CaCO 3), and with the extension of this exposure time, C 4 A C ¯ H 11 will be carbonized to generate CaCO 3 and Al(OH) 3 (AH 3). It is interesting to note that SB reduces the production of C 4 A C ¯ H 11 and CaCO 3 and that at SB dosing levels above 12% the C 4 A C ¯ H 11 disappears and there is hardly whitening of mortar at this time. The inhibition mechanism of SB on the whitening of CAC mortar was investigated using X-ray diffraction (XRD), capillary water absorption and water contact angle tests. The analysis reveals that hydration product of CAC is mainly CAH 10 after curing at 20 ℃ for 1 d, and the addition of SB did not change the type of CAC hydration products. Furthermore, incorporating SB enhances the hydrophobicity and reduces the capillary water absorption in CAC mortar, and forms a polymer film in CAC mortar. They prevent the transfer of ions from the interior of CAC mortar to the exterior and the contact of CO 2 from the air with the cement hydrate on the surface of CAC mortar, thus reduces the production of white deposit on the mortar surface and inhibits the whitening of CAC mortar. [ABSTRACT FROM AUTHOR]

• Nano-SiO 2 was modified with silane coupling agents KH550 to resolve the agglomeration problem of nano-SiO 2. • The influences of modified nano-SiO 2 on the service performances such as workability, mechanical properties, shrinkage, and durability of HPCC were comprehensively investigated. • The hydration process, hydration products and microstructure of HPCC were investigated via different characterization methods. The effect of modified nano-SiO 2 (MNS) on the performance of high-performance cementitious composites (HPCC) was investigated and the underlying mechanism was revealed by various microscopic analysis and testing methods. Results indicated that the fluidity of HPCC initially increased and then decreased with the increase of MNS content. Meanwhile, The HPCC with 0.8% MNS exhibited a 25% increase in compressive strength at the curing age of 3 days, while the HPCC with 0.8% nano-SiO 2 showed a 10% increase. Compared with the reference mixture, the durability and autogenous shrinkage of HPCC with MNS were significantly improved, the chloride diffusion coefficient of HPCC with 0.8% MNS decreased by 25.9%. Furthermore, the quantitative analysis results of hydration process and hydration products show that the inclusion of nano-materials accelerated the hydration rate of cementitious composites, resulting in increased cumulative heat release. HPCC with MNS exhibited a higher generation of amorphous phases and greater consumption of calcium hydroxide compared with reference mixture. Moreover, the addition of MNS effectively refined the pore size of HPCC and converted some harmful pores into less harmful or harmless pores, enhancing the microstructure of HPCC with MNS and improving the interface transition zone. This optimization was mainly attributed to the effect of filling, nucleation of crystal and pozzolanic reactive of nano-materials, which further benefit from the good dispersion of MNS, resulting in excellent mechanical properties and durability of HPCC. [ABSTRACT FROM AUTHOR]

• Asphalt rubber pellets are made of a high amount of desulfurized crumb rubber, asphalt binder and calcium hydroxide. • The microstructure and storability of asphalt desulfurized rubber pellets were characterized. • The performance of asphalt desulfurized rubber pellets modified asphalt was evaluated. In this paper, two kinds of asphalt rubber modifier pellets (ARP) and asphalt desulfurized rubber modifier pellets (ADRP) that can be directly injected were prepared by a screw extruder. Scanning electron microscopy (SEM) was employed to examine the internal microstructure of the two types of modifier pellets, and the storability were further evaluated. The rotational viscosity, high and low temperature rheological behaviors and fatigue properties of asphalt with two modifier pellets were investigated by rotational viscometer, dynamic shear rheometer (DSR), and bending beam rheometer (BBR), respectively. Infrared spectroscopy was used to explore the modification mechanism. The findings reveal that the content of desulfurized crumb rubber in the ADRP can be elevated to 80%, compared with 50% of crumb rubber in ARP. The surface of crumb rubber (CR) is smooth with prominent edges and corners, while desulfurized crumb rubber (DCR) reflects the rough surface with unclear edges and corners. Micro morphology verifies that asphalt and rubber manifest a good fusion phenomenon in ADRP. Incorporating anti-sticking agent lifts the collapse height of ARP and ADRP to nearly 9 cm and prevents their complete agglomeration, endowing their excellent storability. 20% DCR-MA reveals a Nf(5%) of 106,839 and superior fatigue resistance than 20% CR-MA, while it presents lower viscosity, rutting resistance, crack resistance and anti-aging performance. ADRP-MA demonstrates higher viscosity, rutting resistance, fatigue resistance and almost equivalent low-temperature performance than that of ARP-MA under the same dosage. Action mechanism of modified asphalt with both types of modifier pellets is regarded as a physical process. The modifier pellets prepared in this study reveal the merits of easy transportation, convenient direct injection, and wide application field. [ABSTRACT FROM AUTHOR]

[Display omitted] • The FRCSB is proved to be difficult-flammable and less smoky. • Once lose corn straw fibers' supporting and connection, the FRCSB could not maintain its flame-retardant characters any more. • The sizes of crevices agree with the extent of the pyrolysis of corn straw fiber in FRCSB. • Ca(OH) 2 can effectively inhibit the thermal mass exchange between the FRCSB and the environment during heat treatment. The use of corn straw in brick is significant for the utilization of agricultural waste and the improvement of brick's properties. This research investigated the thermal stability, flammability, and the flame-retardant mechanism of flame-retardant corn straw brick (FRCSB). The results show that the FRCSB could stand the test of heat treatment, and has good flame retardant properties. The occurrence temperature of chemical reactions was rose, and the occurrence moment of chemical reactions was postponed due to the calcium carbonate barrier on the surface of FRCSB and the air untouchable corn straw fiber well covered with calcium hydroxide. The growth of crevices destroyed the FRCSB surface, leading to the pyrolysis of deeper corn straw fiber and calcium hydroxide, weight, dimensions, color changes, and promoting the thermal mass exchange between the FRCSB and the air. The strong alkali environment provided by calcium hydroxide enables the corn straw fiber to maintain its own characteristics for a long time, and then plays the role of support and connection, which promotes the performance of FRCSB during heat treatment. [ABSTRACT FROM AUTHOR]

• NH 4 HCO 3 possesses the capacity of extraction and precipitation towards calcium. • The regeneration of NH 4 HCO 3 realizes the catalyst effect and continuous supply. • The regeneration potential of NH 4 HCO 3 increased with the increase of NH 3 ·H 2 O. Repeatable self-healing at the same cracking site is one of the essential issues in self-healing concrete. The inorganic admixture ammonium bicarbonate (NH 4 HCO 3) was proposed as the primary healing agent in this study to achieve repeatable self-healing. Among ammonium carbamate (NH 2 COONH 4), NH 4 HCO 3 , ammonium acetate (CH 3 COONH 4) and calcium hydroxide (Ca(OH) 2), the molar ratio of precipitation reaction and the effect of calcium extraction were determined. Whether before or after the self-healing, ammonia water (NH 3 ·H 2 O) invariably exists in the products and will be carbonised to regenerate NH 4 HCO 3 , which was verified by Fourier transform infrared (FT-IR). The proposed self-healing solution achieves a two-in-one strategy for calcium extraction and carbon sequestration in response to the global strategy for carbon neutrality. Self-healing experiments have been performed. The results suggested that the healing efficiency of the crack area of the tri-component (NH 2 COONH 4 , NH 4 HCO 3 and CH 3 COONH 4) is up to five times higher than that of the control group. Three cycles of self-healing in the simulated system and actual crack environment concluded that the regeneration potential of NH 4 HCO 3 could be enhanced after the completion of more cyclic self-healing processes due to the increasing amount of NH 3 ·H 2 O at the same cracking site. [ABSTRACT FROM AUTHOR]

• Employs semi-empirical quantum mechanics method to reproduce the atomic process of sulfate attack on C-A-S-H. • Reveals the interacting mechanism of sulfate attack on C-A-S-H. • Suggests that the low bond dissociation energy of Al-O are responsible for facile dealumination and explains its origin. Calcium-aluminosilicate-hydrates (C-A-S-H) is the major hydration product of sustainable concrete that incorporates industrial waste as a partial substitute for cement. Although C-A-S-H is considered a durable material, it's susceptible to sulfate attack. To better understand this phenomenon, the atomic process of sulfate attack on C-A-S-H was studied using semi-empirical quantum chemical methods. The results showed that sulfate attacks can be broken down into several subprocesses: adsorption, vibration, and breaking. The underlying causes of these processes were revealed through electronic structures, in which the weak Al O bond and strong electronegativity are the main factors leading to Al-Si breaking. The Atom in Molecular theory revealed that the weak electron localization of the [AlO 4 ] tetrahedron leads to a lower water-assisted bond dissociated energy for breaking the Al O bond (18.3KJ/mol) than the Si O bond (109.4KJ/mol). This study provides valuable insights into the sulfate attack on C-A-S-H and its implications for improving the durability of sustainable concrete. [ABSTRACT FROM AUTHOR]

• Self-healing macroscopic properties of cement-based materials in different matrix were studied. • Composition and microstructure of the mineralized products were analyzed by XRD, SEM, EDS and TGA. • Self-healing mechanism of cracks in different matrix was systematically revealed. Self-healing properties of cement-based materials in different matrix based on microbial mineralization coupled with bimetallic hydroxide were systematically studied. Firstly, the self-healing macroscopic properties of cement-based materials in different matrix were studied. The area repair rate of cement paste early crack was better than that of mortar and concrete, which showed that self-repairing agent also had excellent repair properties in the early stage. With the extension of the curing time, the repair effect was significantly enhanced; The repair effect was basically the same when the curing time was 28 d. After curing for 28 d, the resistance of the three matrix to water permeability were basically restored to intact. At each curing time, the ultrasonic propagation velocity in concrete was the largest, and that in cement paste was the smallest. Secondly, the composition and microstructure of the mineralized products were analyzed by XRD, SEM, EDS and TGA. Additionally, the pore size of the product was also tested. The characteristic peaks of XRD were consistent with the products of the target design, corresponding to the formation of calcium-aluminum bimetallic hydroxide and calcium carbonate. Mineralized products were mainly composed of two forms of inorganic minerals, one was bimetallic hydroxide with lamellar structure, and the other was CaCO 3 ; Moreover, the quantity of calcite was much higher than that of bimetallic hydroxide. Finally, the principle that ultrasonic velocity differently after repair in different matrix was explained, and the mechanism of microbial mineralization coupled with bimetallic hydroxide to repair crack was revealed. The bimetallic hydroxide could fix the hydroxide ion, chloride ion and sulfate ion, which not only improved the repair efficiency, but also reduced the corrosion. [ABSTRACT FROM AUTHOR]

• Deep-sea exposure tests revealed the advantages of CAC with high alumina content. • Hydrate disintegration and Cl ingress were mitigated in the hardened CAC. • Both high pressure and low temperature affected property changes in the CAC. • A two-component injection tool using the CAC was developed for in situ connection. • The first-ever in situ connection of precast members was successfully achieved. The deep sea is the last frontier on Earth for humankind. Previous studies indicated that the use of Portland cement (PC) in deep seas requires extreme caution as the binding phase forming the skeleton is unstable. Owing to its low solubility and high Cl-binding ability, hydrated calcium aluminate cement (CAC) may be one of the most durable binders for deep-sea applications; however, no experimental data and findings discussing its potential benefits are yet available. Therefore, investigating the changes in the physicochemical properties of hardened CAC under deep-sea conditions is of great importance. The results of deep-sea exposure tests revealed that the CAC specimen retained its original shape owing to limited hydrate disintegration and showed expansion. Cl ingress did not reach the specimen center, penetrating to a depth of only 13 mm, and S ingress was localized after 295 d of exposure because anions from seawater were intercalated with the interlayers of calcium aluminate hydrates. In terms of mitigating disintegration and ion penetration from seawater, CAC presented greater advantages for deep-sea use compared with PC. Seawater penetration under deep-sea conditions was more pronounced than that under ambient-pressure conditions because of the high hydraulic pressure, which increased water saturation in pores and enhanced Cl and S ingress. Cl and S penetration transformed calcium aluminate hydrates within the dense binding phase of aluminum hydroxide, exerting a crystallization pressure and leading to specimen expansion. This work is the first to report the successful in situ connection of precast members using CAC mortar. [ABSTRACT FROM AUTHOR]

• Pressurized carbonation with presoaking process was used to improve RCA. • The fracture property of RCAC with RCA treated by carbonization was evaluated. • The microstructure changes of carbonated RCA and RCAC were discussed. Carbonation is effective in enhancing recycled coarse aggregate, but the shortage of carbonizable Ca2+ in recycled coarse aggregate and the low permeability efficiency of CO 2 at ambient pressure limit the carbonation efficiency and industrial application. To solve these issues, the Ca2+ content in recycled coarse aggregate and the carbonation effectiveness were increased by presoaking recycled coarse aggregate in calcium hydroxide and enhancing the carbonation pressure, respectively. The optimal carbonation condition was obtained based on the physical properties of the recycled coarse aggregate and the strength of recycled coarse aggregate concrete: the carbonation pressure was 4 bar, and the concentration of the calcium hydroxide solution was 0.04 mol/kg. The fracture behaviour and microstructure of the recycled coarse aggregate concrete under different carbonation conditions were evaluated. The recycled coarse aggregate concrete after pressurized carbonation with presoaking in calcium hydroxide solution (RCAC-C0.04-P4) has higher toughness and a more stable crack propagation process than untreated recycled coarse aggregate concrete. Pressurized carbonation with presoaking in calcium hydroxide solution significantly increased the initial cracking load, peak load, Young's modulus, initial toughness, unstable toughness, and fracture energy of the recycled coarse aggregate concrete by 67.35%, 2.83%, 12.19%, 60.77%, 2.31%, and 5.38%, respectively, and reduced the critical crack mouth (CMOD c) and tip opening displacement (CTOD c) by 9.13% and 10.96%. Simple pressurized carbonation made the toughness of the specimens worse than that of untreated recycled coarse aggregate concrete. Compared with untreated recycled coarse aggregate concrete, although the initial toughness, unstable toughness, and fracture energy of recycled coarse aggregate concrete after simple pressurized carbonation (RCAC-P4) were increased by 16.02%, 1.86%, and 0.28%, respectively, the Young's modulus was slightly reduced by 1.35%, and the CMOD c and CTOD c were increased by 5.37%. In addition, a microstructural analysis showed that the contents of Ca(OH) 2 and CaCO 3 of RCAC-C0.04-P4 were increased by 50.56% and 37.62% compared to untreated recycled coarse aggregate concrete, respectively, and the average indentation modulus of the old aggregate and old mortar interface of RCAC-C0.04-P4 was increased by 24.37% compared to untreated recycled coarse aggregate concrete, which confirmed that pressurized carbonation with presoaking in a calcium hydroxide solution made CaCO 3 fill the cracks in the interface and improved the macroperformance. These results can promote the application of recycled coarse aggregate in engineering. [ABSTRACT FROM AUTHOR]

[Display omitted] • CaCl 2 is more effective than CaSO 4 in improving compressive strength. • SO 4 2 - promotes ettringite formation, while Cl− promotes Friedel's salt formation. • Cl− dominates the competitive hydration in materials compared to SO 4 2 -. • When SO 4 2- and Cl- are added together, the product formed by CaCl 2 is more stable. This article presents a study on alkali-activated slag cementitious materials that were doped with varying levels of calcium chloride and calcium sulfate. The article delves into the hydration products and mechanisms of the cementitious material, utilizing pH values, X-ray diffraction (XRD), thermogravimetric (TG), and scanning electron microscopy (SEM). The results show that under the condition of Ca2+ dosage less than 2.5%, CaCl 2 is more advantageous than CaSO 4 in improving compressive strength. Under the condition of Ca2+ dosage of 2%, adding CaCl 2 to the cement paste increased the compressive strength of the test block by 230% at 3 curing days and by 190% at 28 curing days compared to the sample with calcium oxide. After calcium chloride replaced calcium sulfate, the compressive strength was improved. At a Ca2+(CaCl2)-Ca2+(CaSO4) ratio of 1:1, the strength was increased by 44% and 18% at 3 days and 28 days, respectively, compared with the addition of calcium sulfate alone. SO 4 2 - can promote the formation of ettringite (AFt), while Cl− can promote the formation of Friedel's salt (3CaO·Al 2 O 3 ·CaCl 2 ·10H 2 O, F's). The contribution of hydrated Friedel's salt to compressive strength is greater than that of AFt. As Cl− is added to the alkali-activated slag cementitious materials stimulated by SO 4 2 - , Cl− is dominant in the hydration process, and form Friedel's salt with hydrated calcium aluminate and calcium hydroxide, while promoting the increase of C-S-H gel quantity, thus improving the compressive strength of the material. [ABSTRACT FROM AUTHOR]

• Lime powder mortars present higher carbonation rate in relation to lime putty ones. • The pore distribution is unimodal for lime powder mortars and bimodal for putty ones. • Water capillary rise coefficient is higher for lime powder mortars. • Lime powder mortars present higher compressive strength compared to putty mortars. • The aggregates' mineralogy does not affect significantly the mortars' characteristics. The aim of this work is to study the influence of calcium hydrated lime (lime putty or hydrated lime powder) to the chemical and physico-mechanical characteristics of lime mortars. The mortars were designed with the same mixing ratio of lime/aggregates and evaluated till 18 months of curing. The carbonation rate is evaluated via DTA/TG, the pore structure by mercury intrusion porosimetry and the hygric properties via capillary rise and water immersion tests. Mortars' shrinkage and apparent density were also measured along with mechanical strength and static and dynamic modulus of elasticity that were evaluated by using conventional mechanical tests and ultrasonic technique. The obtained results revealed that lime powder mortars present a higher carbonation rate and higher values of compressive strength compared to lime putty mortars. Lime powder mortars also exhibit a unimodal pore size distribution (while lime putty mortars present a bimodal one) and higher capillary rise coefficient and porosity accessible to water in respect to lime putty mortars. [ABSTRACT FROM AUTHOR]

• Nanosilica provides physical barrier to prevent GO from aggregation. • GO-NS shows pozzolanic properties. • GO-NS ameliorate the interface between GO and cement via formation of C-S-H. • Chemical combined GO and NS show synergistic effect in cement composite. In this study, graphene oxide-nanosilica (GO-NS) was prepared by the reported sol–gel method, and the concept of nanocomposite was creatively introduced to prevent GO from aggregating. Results showed that the nanosilica particles on GO surface formed a physical barrier to maintain GO and NS disperse well in pore solution, respectively. The reaction in saturated Ca(OH) 2 solution was conducted to investigate the pozzolanic effect of GO-NS. When added into cement, GO and NS resulted in a cooperative improvement in the mechanical properties of cement composite, and the enhancing mechanisms were investigated by X-ray diffraction (XRD), Raman and SEM. It revealed that GO-NS with refined surface could provide more nucleation sites during hydration and decrease the orientation index of CH crystal. Moreover, some calcium hydroxide (CH) transformed into calcium silicate hydrate (C-S-H) due to the pozzolanic NS coating. The layered nanocomposites provided platforms for the deposition and growth of C-S-H. [ABSTRACT FROM AUTHOR]

• An ambient cured one-part geopolymer is developed. • Precursor type has significant influence on geopolymerisation development. • Type/amount of solid activators affect mechanical and microstructure properties. • Reasonable compressive strength of up to 38 MPa is achieved at 28 days. To improve the commercial viability of geopolymers, there is a need to develop one-part geopolymer mixes. In this research, 100% fly ash or a mixture of fly ash and slag was employed as the precursor. Meanwhile, the highly corrosive alkali solutions used as activators in conventional geopolymers were replaced by different solid activators (in powder form), such as sodium silicate, calcium hydroxide, sodium oxide, lithium hydroxide, potassium carbonate or their combinations. The mixes were subjected to both ambient and heat curing to check the influence of curing conditions. Effects of type/combination of the solid activators on the workability, early and final strength, hardened density and microstructure of these one-part geopolymer mixes were examined. The incorporation of slag in the binder increased the early and final strength of mixes but reduced the workability. Furthermore, the type/combination of solid alkaline activators affects the microstructure of the geopolymer gels and subsequently impacts the mechanical properties. Among the investigated ambient cured geopolymers, a relatively high compressive strength of 38 MPa at 28 days was obtained when the geopolymer precursor was composed of 50 wt% fly ash and 50 wt% slag activated by a combination of sodium silicate, calcium hydroxide and lithium hydroxide. [ABSTRACT FROM AUTHOR]

• Slag-based composites show different self-healing phenomena according to activator types. • CaCO 3 is the major healing material of the calcium hydroxide-based AAS composite. • C-(N)-A-S-H and other carbonates are the dominant healing products of sodium-based AAS composites. This paper presents an experimental investigation of the self-healing ability of fiber-reinforced slag-based composites activated by a single type of alkali activator at an early age. In order to investigate the effects of the type of alkali activator on the self-healing ability of fiber-reinforced slag-based composites, three mixtures reinforced by polyethylene fibers and with different alkali activators, in this case calcium hydroxide, sodium hydroxide, and sodium silicate, were designed and prepared. Compressive strength and uniaxial tension tests were conducted to measure the mechanical properties of each mixture. The self-healing ability in each case was evaluated by observing the crack width and by measuring the resonance frequency. Scanning electron microscopy and energy-dispersive X-ray spectroscopy were also utilized to analyze the morphology and chemical compositions of the healing materials. The results of these tests showed that autogenous healing of alkali-activated slag-based composites is accomplishable through structural restoration, specifically by reducing the crack opening and through mechanical recovery. The dominant healing materials of the calcium activator-based alkali-activated slag composite was CaCO 3 , in contract to the C-(N)-A-S-H, CaCO 3 , and some amount of Na 2 CO 3 which formed in the sodium activator-based alkali-activated slag composites. [ABSTRACT FROM AUTHOR]

• Getting relationships between the carbonation mechanism and dissolution properties. • Calcite contributes a lot to the good mechanical properties of carbonation products. • Compact stacks and strong mechanical bonds between calcite particles in carbonated β-C 2 S were observed. In-depth understanding regarding the carbonation properties of pure minerals including β-dicalcium silicate (β-C 2 S), calcium hydroxide (CH), and tetracalcium aluminoferrite (C 4 AF) was conducted to explore the types, microstructure, crystallinity of products produced during carbonation, thereby developing a tentative microscopic mechanism of the contribution of carbonation to macroscopic properties. An investigation of changes in pH value and Ca leaching of the pure minerals was conducted to explore the effect of dissolution properties to the carbonation. Results showed that the descending order of the ultimate carbonation degrees of minerals was: (i) CH, (ii) β-C 2 S, (iii) C 4 AF. The carbonation degrees and rates were not only controlled by the dissolution properties of the minerals, but also the distribution of the produced CaCO 3. The order of the contribution of minerals induced by carbonation to the macroscopic mechanical strength was, in descending order: (i) β-C 2 S, (ii) CH, (iii) C 4 AF. This difference was the combined effects of the microstructure, crystallinity, and particle size of calcite. Effects of the compact stack and the strong mechanical bond between the calcite particles were observed in carbonated β-C 2 S samples; In contrast, pronounced defects on the calcite particles were observed in carbonated C 4 AF samples. Carbonation is an effective way to reduce the density of which mainly contained phases of β-C 2 S and C 4 AF, and to store greenhouse gas CO 2 permanently. These findings are meaningful for the further in-depth researches of the carbonation mechanism of materials which primarily contain β-C 2 S, CH and C 4 AF phases. [ABSTRACT FROM AUTHOR]

• LiAl-LDHs was prepared using modified separate nucleation and aging step method. • Pastes with and without LiAl-LDHs were prepared and cured at temperature −10 °C. • Setting times of pastes with 1.5 % LiAl-LDHs is shortened to half of plain one. • Most hydrations in paste with 1 % LiAl-LDHs complete before 1 day. • Modified paste achieves 5.5 times compressive strength of plain one at 1 day. To reveal the effect of LiAl-layered double hydroxides (LiAl-LDHs) on hydration of calcium sulfoaluminate cement at low temperature, pastes with and without it were prepared and cured at temperature −10 °C. Setting times and compressive strength of pastes were measured. Mercury intrusion porosimetry (MIP) was used to reveal the evolution of pastes microstructure. X-ray diffraction (XRD), Thermogravimetric analysis (TG) and 27Al nuclear magnetic resonance (NMR) were adopted to qualify and quantify the hydration productions. The results show that, LiAl-LDHs is an effective hydration accelerator of CSA cement at low temperature. Initial and final setting times are reduced linearly with the increase of LiAl-LDHs content. However, peak of compressive strength emerges at the content of 1.0%. At this content, modified paste achieves 5.5 times compressive strength of plain one at 1 day and 1.4 times at 28 days. In presence of LiAl-LDHs, more ettringite crystallizes in modified pastes up to 28 days. Denser pastes obtained thereby. [ABSTRACT FROM AUTHOR]

• Solid synthetic C-S-H is produced from calcium hydroxide and nano-silica. • Shown to be crystallographically similar to the more commonly produced slurry. • Curing time studies show significant C-S-H is produced after 1 day for C/S ratio = 0.81. We present new results on solid (monolithic) synthetic C-S-H made at room temperature from easily obtainable calcium sources (calcium oxide, calcium hydroxide) and nanosilica (w/s = 2) with a 2 minute mixing time. Using synchrotron X-ray diffraction combined with TGA/DSC, 29Si NMR and synchrotron pair distribution function (PDF) data we show that the solid C-S-H is crystallographically similar to the more conventionally made synthetic C-S-H slurry. We show that C-S-H is present after 1 day curing (C/S = 0.81) and no X-ray visible portlandite is present at day 3. PDF data shows that the ordered domain size is 2.5 – 4.5 nm, depending on the fit chosen. [ABSTRACT FROM AUTHOR]

• Insulating geopolymer samples were prepared using industrial wastes raw materials. • Aluminum reacted with sodium and calcium hydroxides to produce hydrogen. • Pores were formed in the geopolymer matrix owing to the evolution of hydrogen. • Porosity reached 50% on using 5% aluminum trimmings and 15% dealuminated kaolin. • Compressive strength of bricks was 1.4 MPa and thermal conductivity 0.26 W.m−1.K−1. Insulating geopolymer samples were prepared from cheap sources, namely, the powder residue from the production of clay bricks (clay bricks waste), slaked lime waste from acetylene production and waste aluminum trimmings from workshops. 0.5% caustic soda was used, together with slaked lime as alkaline activator. De-aluminated kaolin, a waste from the alum industry, was also added as binding agent to substitute part of the clay bricks waste. Hydrogen produced from reaction between aluminum and alkalis induced the formation of pores inside the geopolymer matrix. It was found that using 5% (by weight) aluminum trimmings and substituting 15% of clay bricks waste by de-aluminated kaolin was enough to raise the porosity above 50%, thus producing light bricks of densities in the 1000 kg.m−3 range. The compressive strength of these bricks was about 1.4 MPa and their thermal conductivity as low as 0.26 W.m−1.K−1. [ABSTRACT FROM AUTHOR]

• Binding of admixed chloride in cement paste is significantly improved with aluminum compound addition. • Aluminum compound addition promotes the formation of AFm phases and destabilizes the ettringite. • Admixed chloride benefits delayed ettringite formation, but this effect can be suppressed with aluminum compound addition. • Higher amount of admixed chloride results in less calcium hydroxide content and free water in hardened cement blends. This study investigates the effects of aluminum compound on the binding of admixed chloride in cementitious materials. Three commonly available aluminum compounds, aluminum oxide, aluminum hydroxide and colloidal nano-alumina are applied. The bound chloride content, hydration products and calcium hydroxide content are analyzed. The results show that aluminum compounds transform to AFm phases such as carboaluminate, monosulfoaluminate if there is no available chloride. When the mixing water includes chloride, the above AFms transform to chloroaluminate, e.g. Kuzel's or Friedel's salt, depending on the sodium chloride and aluminum compound content. The bound chloride content is significantly improved with aluminum compound addition during the whole curing age, even though in an early age of 3 days. Since aluminum compounds react slower than alumina-containing clinker, the bound chloride content of aluminum compound blended pastes experience a considerable increase after 7 days, whereas that for plain paste is very minor. Generally, admixed chloride promotes the formation of delayed ettringite and this effect can be suppressed by aluminum compound addition due to higher amount of formed AFms. A higher dosage of admixed chloride increases the bound chloride content and reduces the calcium hydroxide as well as free water content in the system. [ABSTRACT FROM AUTHOR]

• Volcanic pumice powder (VPP) could contribute to the compressive strength of concrete. • Utilization of 10% of VPP produces the optimum high strength concrete (HSC). • VPP contributes to pore filling (HSC) thereby lowering the permeability of HSC. • Strength of 70 MPa or more could be achieved with VPP replacement of OPC in HSC. • Ionic and fluid transport reduces with increase in VPP-OPC replacement in HSC. In this study, the potency of volcanic pumice powder (VPP) as a supplementary cementitious material at varied levels (0%, 10%, 20%, and 30%) in high strength concrete (HSC) was investigated for its pozzolanic reactivity as a basis for strength and durability enhancement. Workability, transport properties and mechanical properties were tested in addition to the characterization of the products. X-ray florescence and strength activity index established the VPP oxides (SiO 2 + Al 2 O 3 + Fe 2 O 3 = 76.01%) and pozzolanicity while diffraction techniques revealed the presence of anorthite (Al 2 CaO 8 Si 2), and clinopyroxene (AlCaMgSiO 6). Despite the paucity of calcium oxide and the excess of silica and alumina in VPP in contrast to OPC, yet it contributes significantly to the microstructural density and tortuosity of HSC pores thereby improving the strengths and transport properties as evidenced in the reduction of rapid chloride ions penetration (RCPT), and low initial surface (water) absorption (ISA) values. The maximum 180-day tensile, flexural and compressive strengths of the HSC-VPP sample were 6.22, 10.5 and 78.9 MPa, respectively. [ABSTRACT FROM AUTHOR]