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The outbreak of the COVID-19 epidemic has led to a large number of waste disposable medical face masks (DMFMs) worldwide, which seriously pollute the environment and threaten human health. In this paper, waste DMFMs were recycled as mask fiber and mixed into self-compacting recycled aggregates concrete (SCRAC) to form a mask fiber reinforced self-compacting recycled aggregates concrete (FRSCRAC). The effects of recycled coarse aggregate (RCA) replacement ratio, DMFM fiber content, and length on the working and mechanical properties of FRSCRAC were investigated. The results show that the specimens meet the requirements of self-compacting concrete (SCC) in terms of working and mechanical performance, but the RCA and DMFM fiber could reduce the workability of the FRSCRAC, and the higher the amount of RCA and DMFM fiber, the more obvious the reduction. At the same time, the mechanical properties of FRSCRAC increased as the decreasing RCA replacement ratio and growing DMFM fiber content. Compared with specimens without DMFM fiber reinforcement, the compressive strength, split tensile strength, flexural strength, and elastic modulus were increased by 1.3%-9.6%, 1.46%-8.6%, 24.4%-54.69%, and 0.65%-4.73%, respectively. Moreover, the reduction of the DMFM fiber length is favorable to the mechanical properties of FRSCRAC as well. Furthermore, the X-ray CT and scanning electron microscope (SEM-EDS) techniques were applied to analyze the microstructure of typical specimens. The results showed that the DMFM fiber and surrounding mortar could form a multiphase composite, which reduces the porosity and improves interfacial stress transfer efficiency, thus enhancing the mechanical properties of FRSCRAC. In addition, based on the experimental results, the mechanical strength index of FRSCRAC and the calculation formula of its mutual conversion relationship were proposed. The research conclusions of this paper can provide a reference for the application of waste DMFM fibers in FRSCRAC. • Waste disposable medical face masks were recycled as fiber and mixed into self-compacting recycled aggregate concrete. • Parameters were investigated to make clear the influential mechanisms of waste fiber on recycled concrete performance. • The mechanical strength index of FRSCRAC and the calculation formula of its mutual conversion relationship were proposed. [ABSTRACT FROM AUTHOR]

The objective of this paper is to investigate the microstructure of high-viscosity modified asphalt binder (HVA) containing high-content polymers and its effect on fatigue performance, as well as the applicability of different analytical methods and failure criteria of linear amplitude sweep (LAS) test to HVA. In this paper, two types of HVAs with different polymer contents, a commercial HVA, a conventional polymer-modified asphalt binder, and a base asphalt binder were first prepared or selected. Fluorescence microscopy tests, time sweep tests, and LAS fatigue tests were then performed on these binders. Finally, the fatigue properties of the different binders were analyzed in depth using different analytical methods and failure criteria. The results show that the polymers in HVA crosslink with each other to form a strong cross-linking structure, and the phase inversion of the polymers renders them into a continuous phase, which significantly improves the fatigue damage resistance of the HVA; moreover, the evolutions of stress with shear strain for the fatigue process of HVAs is related to the polymer network. Besides, the two-dimensional damage characteristic curve (DCC) cannot accurately reflect the evolution of the fatigue performance of HVA, and this paper suggests the use of a three-dimensional DCC curve with strain information. Furthermore, the 35% reduction in C 0 in the dissipative energy method is not a suitable failure criterion, a more accurate fatigue performance ranking can be obtained by using the yield stress as the failure criterion. Although the fatigue resistance of HVA after yield stress is theoretically considered using the pseudo-strain energy approach, HVA does not show a peak of stored pseudo-strain energy even when the strain is increased to 60% in the LAS test. Therefore, the maximum strain value (30%) in the standard LAS test is not sufficient in characterizing high-polymer HVAs because there is not enough fatigue accumulation of HVAs in this range, consequently, the maximum shear strain parameter in the LAS experiment needs to be reconsidered. • Polymer morphology and its crosslinking networks in HVA with high-content polymers were analyzed. • The fatigue properties of HVAs based on LAS test and different analytical methods are compared. • The fatigue crack length and its evolution of HVA are characterized. • The deficiencies of the LAS test to evaluate the fatigue resistance of HVA and suggestions for modifications are presented. [ABSTRACT FROM AUTHOR]

Modified bitumen is a critical material to ensure the durability of asphalt pavements. Traditional polymer-modified bitumen (PMB) has been widely researched and applied in the past two to three decades. Recently, a high-content SBS (Styrene-Butadiene-Styrene) polymer-modified bitumen (HSBSMB) with a higher SBS content of up to 7–9% by weight has started attracting interest from researchers. There are, however, still few specifications or technical guidelines for HSBSMB. To gain a better understanding of HSBSMB, the composition and structure of typical polymer SBS and base bitumen, the micromorphology and micromechanics of HSBSMB, its rheological and aging characteristics, as well as its asphalt mixture and field pavement performance were reviewed comprehensively in this paper. The content covered in this paper is primarily derived from 210 pertinent papers published between 1990 and 2023 in two databases, Web of Science and Google Scholar. It appears from our review that HSBSMB has a stronger internal polymer network than conventional SBSMB and thus exhibits better resistance to rutting, fatigue, and cracking as well as better aging resistance. Furthermore, findings from both laboratory scale and field pavement scale indicate that HSBSMB not only offers higher performance and durability but also leads to reduced pavement structural layer thicknesses in ensuring performance. However, many issues and challenges still remain before HSBSMB will become a viable option for large-scale application in road infrastructure, aspects such as the mechanism of its internal structural changes, the optimization of methods for evaluating its rheological properties, the life cycle assessment of its long-term benefits, as well as the storage stability and workability of HSBSMB. It is hoped that this review will contribute to a better understanding of HSBSMB and promote its application. • The chemical properties of SBS polymer and base bitumen as well as their interactions were comprehensively summarized. • The mechanisms of SBS in bitumen and polymer phase inversion were discussed under different microscopy techniques. • The current standard rheological evaluation methods may need to be improved for high-content SBS polymer-modified bitumen. • High-content SBS polymer-modified bitumen exhibits better resistance to rutting, fatigue, cracking, and aging. [ABSTRACT FROM AUTHOR]

This paper focuses on the study of the reaction between limestone and sodium hydroxide solution. This reaction is used to produce alkali-activated mortars after curing and in the presence of unreactive siliceous sand. The kinetics of this reaction is investigated by semi-adiabatic calorimetry experiments. The formulation parameters are fixed but the curing parameters are tested: temperature (38 and 45°C) and time of drying (14, 25, 27 and 28 days). Their impact on reaction products (identified by FTIR and thermal analysis), on microstructure (SEM observations and porosity measurements) and on mechanical strength (flexural and compressive tests) are determined. The reaction was fast and a temperature drying was required to obtain stabilized hydration products in mortars. The products of the alkali-activation of limestone are double sodium/calcium carbonate (mainly pirssonite and with specific conditions of curing gaylussite) and calcium hydroxide. The microstructure and the proportions of reaction products, impacted by drying conditions, influence the mechanical strength of mortars. With an appropriate time (25 days) and temperature of drying (45°C), the compressive strength reached, after 28 days, almost 25 MPa. • The reaction between limestone and sodium hydroxide solution take place in two stages. • FTIR measurements allow the identification of stages of the reaction process. • Drying in the curing process is necessary to end the crystallization stage. • Curing temperature plays an important part in the nature of reaction products. • The microstructure and the strength of alkali-activated mortars are linked to the curing process (time and temperature). [ABSTRACT FROM AUTHOR]

Magnesium ammonium phosphate cement (MAPC), commonly used in emergency engineering applications, represents a novel type of rapid setting and early hardening cement. However, prolonged exposure to water causes the hydration product of MAPC, struvite, to partially dissolve into ions and leach out from the matrix, leading to poor water resistance. Polyvinyl alcohol (PVA) powder, which can adsorb onto the surface of cement particles and form a film upon dehydration, has the potential to encapsulate hydration products and improve the water resistance of MAPC. In this paper, the workability, rheological properties, mechanical strength, water resistance, hydration characteristics, hydration products and microstructure of MAPC modified by PVA powder were investigated. The results revealed that the addition of PVA powder prolonged the setting time of MAPC while enhancing the flexural strength, bonding strength, and water resistance. This improvement could be attributed to the adsorption of PVA powder onto the surface of magnesium oxide during the early hydration stages, which delayed hydration heat release and weakened the transformation of struvite into porous dittmarite. Furthermore, PVA powder cross-linked and formed a film between hydration products, resulting in a denser microstructure. • PVA powder effectively improves the water resistance of MAPC. • PVA powder can reducing the transformation of struvite into porous dittmarite. • The network formed by PVA film and hydration products densified the microstructure. [ABSTRACT FROM AUTHOR]

Reusing of iron ore tailings (IOT), particularly as recycled aggregate to prepare concrete, plays an important role in reducing dependence on natural resources and solving the problem of waste accumulation. In this paper, the effects of IOT as aggregate on the flowability, compressive strength of ultra-high performance geopolymer concrete (UHPGC) were studied, and the enhancement mechanism of IOT on compressive strength was elucidated through microscopic characterizations. The results show that with the increase of IOT content, the flowability of UHPGC decreases, and the compressive strength increases first and then decreases. 25 % IOT replacement yielded the best compressive strength, and 50 % substitution of IOT was comparable to the without IOT regarding compressive strength. The microstructure analysis shows that the crystalline components in the IOT do not participate in the hydration reaction and have the effect of micro-aggregate filling. The amorphous components on the IOT dissolve rapidly in the early stage, participate in geological polymerization reaction, and produce synergistic effect with slag to promote the formation of hydration products. However, excessive IOT incorporation will inhibit the dissolution of slag active ions and hinder the formation of hydration products. In addition, the enhanced interfacial transition zone (ITZ) and mechanical interlocking effect formed by IOT can effectively inhibit the crack development of UHPGC. • Proper use of IOT as aggregate can increase compressive strength of UHPGC. • The enhancement mechanism of IOT was elucidated through microscopic characterizations. • This study provided a new route to improving the comprehensive utilization of IOT. [ABSTRACT FROM AUTHOR]

Glass fiber reinforced plastics (GFRP) have been widely used in the field of civil engineering infrastructure, while it also generating a large amount of waste glass fibers simultaneously. This paper uses waste glass fiber in self compacting recycled concrete (SCRC), forming a kind of waste glass fiber reinforced self-compacting recycled concrete (RGF-SCRC). The effects of recycled coarse aggregate (RCA) replacement ratio, water to binder (W/B) ratio, the content and type of glass fiber on the working and mechanical properties of RGF-SCRC were investigated. The results showed that the specimens could meet the requirements of self-compacting concrete (SCC) in terms of working performance, but the addition of RCA and glass fiber could reduce the workability of RGF-SCRC, when RCA is replaced with 100 % and the fiber content reaches 10 kg/m3, the slump expansion is reduced by 9.5 % and 12.4 %, respectively. The mechanical properties of RGF-SCRC decreased as the growing value of RCA replacement ratio and the W/B ratio, among them, the elastic modulus decreased the most, by 40.4 % and 30.0 %, respectively. Compared with the results of the control specimen, the properties could be improved significantly under an appropriate content of waste glass fiber: the cube compressive strength, splitting tensile strength, flexural strength, and elastic modulus of RGF-SCRC were increased by 1.3∼9.6 %, 13.5∼59.4 %, 17.8∼40.9 %, and 3.3∼13.4 %, respectively. However, excessive fiber content showed a negative effect on the mechanical properties. Specimens with the 7.5 kg/m3 alkali resistant glass fiber content showed the best mechanical properties. Moreover, the X-ray CT and scanning electron microscope (SEM) techniques were applied to explore the influencing mechanism of waste glass fibers on RGF-SCRC. The results showed that after adding an appropriate content of waste glass fiber, the porosity and the number of large pores decreased, and the pore size and distribution became more reasonable, which enhanced the compactness of RGF-SCRC to some extent. The mechanical force and friction force anchored at the fiber end at the interface with the concrete matrix increase the bonding strength, thereby enhancing the mechanical properties of RGF-SCRC. This study achieve the resource utilization of waste glass fiber and improve the mechanical properties of concrete and the research results can provide a reference for the practical application of RGF-SCRC in civil engineering. • The waste glass fiber was applied in self compacting recycled concrete (SCRC). • Various parameters were investigated to make clear the influential mechanisms of glass fiber on the working and mechanical properties of waste glass fiber reinforced self-compacting recycled concrete. • The enhanced mechanism of fiber addition on SCRC was illustrated by the microstructure method and analysis. [ABSTRACT FROM AUTHOR]

Limestone of the purity mandated by the standards for use in cement manufacture (as raw material or as performance improver) is only a fraction of the mined material. Sometimes, limestone contains impurities like silica, clay, dolomite etc. beyond the levels specified for use as an additive or for cement production. This limestone, commonly known as low-grade limestone, also may have a potential to be used as a supplementary cementitious material. This paper reports a study conducted to understand the hydration behaviour of ordinary Portland cement in combination with low-grade limestone. Low-grade limestone samples with dolomite, silica and clay impurities were collected from various limestone mines in India and used in cementitious blends at various replacement levels after suitable processing. Reaction kinetics was studied using calorimetry and phase assemblage using techniques like X-ray diffraction and thermogravimetry. Mercury intrusion porosimetry was employed to study the microstructure of cementitious blends. Dilution effect of low-grade limestone was studied by measuring compressive strength of cement mortar cubes for various replacement levels. Shortening of induction period, increased peak heat rate and reduced time to peak were observed in the blends of ordinary Portland cement with low-grade limestone. Carboaluminate formation, ettringite stabilisation and higher bound water content were also seen. The studies showed that low-grade limestone also can be used as a partial replacement for ordinary Portland cement without compromising the strength up to 15% replacement. Higher replacement levels, although showing a reduction in strength due to dilution effect, could still be considered for non-structural applications. • A systematic assessment of the effect of low-grade limestone with different types of impurities on the hydration of ordinary Portland cement is done. • Similarities and differences in the behaviour of OPC blended with cement-grade and low-grade limestone are brought out. • Non-cement grade limestone, or low grade limestone, can efficiently be utilised as SCM with up to 15% replacement of OPC. [ABSTRACT FROM AUTHOR]

Some research has been conducted on ultra-high performance seawater sea sand concrete (UHPSSC) in response to the shortage of freshwater and river sand in coastal regions. However, utilizing seawater and sea sand presents a multifaceted challenge. This paper reviews the properties of seawater and sea sand, elucidating their impact on hydration, microstructure, and properties of UHPSSC. The results indicate that salt ions in seawater and sea sand expedite the early-stage hydration, refining the pore structure and reducing the thickness of the ITZ of UHPSSC. It also exacerbates the self-shrinkage and dry shrinkage. Compared to ultra-high performance concrete (UHPC), UHPSSC exhibits early strength, especially at 1 d. However, the comparative conclusions for strength at 28 d are inconsistent, which may be related to the factors such as SCMs, w/b ratio, and s/b ratio. UHPSSC demonstrates excellent durability including resistance to carbonation, frost, chloride attack, and sulfate exposure. Limited literature suggests that the steel fibers within the UHPSSC are in a corrosion-free steady state for a short time (0–1080d). The low permeability of the UHPSSC reduces the diffusion of water, oxygen, and chloride ions, effectively mitigating chemical corrosion caused by sea salt. However, steel fiber corrosion in cracked UHPSSC deserves more attention. • The hydration, microstructure, and properties of UHPSSC are critically reviewed. • Effects of salt ions on the hydration process, hydration product, and microstructure of UHPSSC are studied. • The durability of UHPSSC is reviewed and compared to UHPC. • The corrosion of steel fibers inside the UHPSSC is illustrated. [ABSTRACT FROM AUTHOR]

Seashells are abundantly available as waste products from the seafood industry and from coastal areas. Accordingly, processed shells can be used to partially replace aggregates in traditional concrete. This paper investigates the effects of different proportions of seashell particles on the mechanical and microstructural properties of 3D-printed cementitious composites. The seashells were crushed and milled to be transformed into seashell particles, replacing the river sand at 15 wt% and 30 wt%. It was found that when the seashell particle content increased to 15 wt% and 30 wt%, the mechanical strengths all decreased significantly, which could be attributed to the increasing volumetric proportion of voids and the lower elastic modulus of seashell particles compared to river sand. However, when the seashell particle proportion further rose from 15 wt% to 30 wt%, the mechanical strengths did not continue decreasing substantially. Instead, the compressive and indirect tensile strengths nearly levelled off with minor fluctuations. Based on the microstructural analysis results, it was inferred that the fine seashell powders as part of the seashell particles, which were comprised of aragonite-based calcium carbonate, participated in the cement hydration process that could help with microstructure densification of the cement matrix. This positive effect brought by the fine seashell powders was thought to prevent the further weakening of mechanical properties. Overall, this study aims to understand the role of seashell particles on the mechanical performance of 3D-printed cementitious composites from the perspectives of microstructural characteristics, further promoting the utilisation of waste seashells in 3D concrete printing applications. • Examines seashell particle effects on 3D-printed composite mechanics and microstructures. • Finds increased seashell content significantly reduces mechanical strength due to voids. • Mechanical strengths plateau with higher seashell percentages; fine powders aid cement hydration. • Uses X-ray μCT and SEM/EDX for structural analysis; nanoindentation tests interfacial properties. • Suggests future research on seashell powders as cement substitutes, impacting 3D printing properties. [ABSTRACT FROM AUTHOR]

To comprehensively utilize the abundant solid wastes of superfine river sand and fly ash, a mixed sand of superfine river sand with coarse manufactured sand and high-flowability concrete made of mixed sand and high-content fly ash were developed. To address the critical question about the long-term strength of this new concrete, this paper presents an experimental study on the long-term development of cubic compressive and tensile strengths of the concrete over a curing age from 7 days to 360 days. In the study, the mixed sand was a hybrid of 40 % superfine river sand and 60 % coarse manufactured sand, and the fly ash content varied from 30 % to 75 % of the total binders by weight. The law of strength development of concrete with fly ash content was analyzed, combined with explanations of the mechanism and observations of hydration exotherms and microstructures of the concrete. Equations were proposed for predicting the long-term development of cubic compressive and tensile strengths, incorporating coefficients to reflect the effects of fly ash on the binder's compressive strength and the development tendency of concrete strength. The validity of the proposed prediction equations was verified against the test results and compared with the extended application of the equation specified in CEB-FIP MC 2010. • Mixed sand of superfine river sand with coarse manufactured sand was developed. • High-flowability concrete made of mixed sand and high-content fly ash was produced. • Long-term strength of the concrete was measured within the curing age of 360 days. • Formulas for predicting strength development were proposed. • Coefficients reflecting the effect of fly ash on strength development were suggested. [ABSTRACT FROM AUTHOR]

To further enhance our understanding of the microstructure of SRM and its intrinsic relationship with macroscopic properties, this paper conducted indoor freeze-thaw cycles, EIS and uniaxial compression tests. The results indicated that the number of freeze-thaw cycles has a significant exponential relationship with R CPP , R CPP 1 and C DSRP. As the number of cycles increased, R CPP and R CPP 1 exhibited a decreasing trend, whereas C DSRP showed an increasing pattern. The freeze-thaw cycles led to the expansion and connection of different pores, resulting in the widening or multiplication of channels in CPP , leading to a decrease in both R CPP and R CPP 1. However, in DSRPP , the liquid-filled pores underwent radial expansion during freeze-thaw cycles, connecting with gas-filled pores around them. This transition led the conductive path to transform into CPP , reducing the accumulated thickness of non-continuous points. Consequently, C DSRP exhibited an increasing trend. Furthermore, the increase in porosity weakened the deformation resistance, increasing the compaction stage of pores and the peak strain, while reducing its peak strength and secant modulus. The peak strength, strain and secant modulus also exhibited significant exponential relationships with different cycles. There was a good exponential correlation between ∆ R CPP of CPP and the uniaxial strength, and the freeze-thaw deterioration model constructed with it as an influence factor could better assess its peak mechanical strength after freezing and thawing. • An electrochemical circuit model for different conductive paths was proposed. • The microstructure was characterized by EIS parameters of the equivalent model. • The influence and mechanism of freeze-thaw cycles on EIS parameters were analyzed. • A degradation model for the uniaxial strength was established with EIS parameters. [ABSTRACT FROM AUTHOR]

The use of supplementary cementitious materials (SCMs) is an effective way to reduce CO 2 emissions within the cement industry. In this paper, the Cement-Metakaolin (MK)-Sugarcane Bagasse ash (SCBA) ternary composite was designed based on the simplex centroid design method. The influence of MK and SCBA on mechanical properties of ternary composite was investigated. In addition, the hydration products and microstructure of ternary composite were analyzed by Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD), low-field nuclear magnetic resonance (LF-NMR). At the same time, the life cycle assessment (LCA) method was used to quantitatively analyze the environmental impact and economic cost of the ternary composite system. A multi-performance indicator was used to optimize the mix ratio of the ternary composite based on the simplex centroid design method, and it was concluded that the Cement: MK: SCBA ratio was 75%: 20%: 5% (by weight) to achieve the optimal synergistic effect. The findings of this study can offer a design methodology that considers both performance and environmental factors, while also providing a theoretical foundation for the utilization of SCBA in cement-based materials. • The proportion design of Cement-MK-SCBA cementitious material was carried out by using the simplex-centroid design method. • The macroscopic mechanical properties and hydration reaction of Cement-MK-SCBA ternary composites were analyzed. • The life cycle assessment (LCA) method was used to consider the CO 2 emissions and economic benefits of MK and SCBA. • The synergistic effect of both SCBA and MK can achieve the performance of cement gel and reduce the economic cost. [ABSTRACT FROM AUTHOR]

The low water-cement ratio has a significant effect on increasing the compacted density of the concrete, thereby enhancing its mechanical properties and microstructures. This paper introduces an innovative approach of utilizing the cold isostatic pressing method to prepare reactive powder concrete (RPC) specimens with an ultra-low water-cement ratio which not exceeds 0.16. And it is compared with the specimens prepared by the traditional vibrational method which conversely requires the minimum water-cement ratio of 0.21. The results show that the RPC specimens under cold isostatic pressing exhibit enhanced compressive strength and microhardness, as well as improved matrix compactness. Conversely, the RPC matrix produced under the vibrational method not only contains a small number of air-entrained bubbles that can influence the strength of matrix but also exhibits a significantly larger cumulative volume of harmful pores that affect the performance of RPC compared to cold isostatic pressing. Therefore, cold isostatic pressing technology based on all-directional hydroforming is a powerful tool for the preparation of high-strength RPC, which significantly reduces the internal defects of the specimen, and the prepared RPC has superior mechanical properties and matrix structure. • Significant increase in the compressive strength of RPC has been achieved by cold isostatic pressing. • The molding method has a significant influence on the Vickers hardness of RPC. • The hydration reaction of cold isostatic pressing specimens after autoclave curing is still incomplete. • Specimens prepared by cold isostatic pressing exhibit dense matrix without air-entrained bubble observed. [ABSTRACT FROM AUTHOR]

In the variable water level area, hydraulic concrete structures are susceptible to variable temperature and alternating dry-wet cycles, especially for thin-walled structures. To elucidate alterations in the microstructure and strength of hydraulic concrete subjected to variable temperature dry-wet cycles, this paper firstly conducted a durability test on hydraulic concrete. Subsequently, CT scan images of the specimens are acquired during the variable temperature dry-wet cycle. Secondly, in view of the shortcomings of existing threshold methods for CT image pore segmentation, a new image segmentation method (RASLPSO-TV-Top-hat) is proposed by using Tensor voting and Top-hat technology, combined with a Reverse Adaptive Particle Swarm Optimization algorithm with Self-Learning ability. The evolution process of pore structure (microcracks) of hydraulic concrete under the variable temperature dry-wet cycles is captured. Thirdly, an in-depth analysis is conducted on the compressive strength and acoustic emission characteristics. The analysis shows that with the number of cycles increases, there is a discernible interconnection among the pore structures. This interconnection leads to a noteworthy reduction in the compressive strength of the concrete. Additionally, the cumulative number of Acoustic Emission ringing events exhibits an initial rise followed by a subsequent decline. The study successfully identifies the patterns governing the deterioration of strength and microstructure evolution in hydraulic concrete. The mechanism underlying the decline in performance of hydraulic concrete under variable temperature dry-wet cycles is elucidated. It provides a reliable and valuable basis for the safety assessment of hydraulic concrete structures in the variable water level area. • We conduct a durability test of concrete under variable temperature dry-wet cycles. • We propose a new image segmentation method. • Pore structure, compressive strength and acoustic emission character are analyzed. [ABSTRACT FROM AUTHOR]

This paper aims to examine the effect of coal gangue aggregate (CGA), fly ash (FA), and ground-granulated blast slag (GGBS) on the rheological, mechanical and microstructure properties of concrete. The rheological properties, compressive strength, splitting tensile strength, flexural strength, and uniaxial tensile behaviour of coal gangue concrete (CGC) were determined, and the microstructure of the concrete mixtures were investigated by use of XRD and SEM analyses. The results revealed the action mechanism of CGA and mineral admixtures on the microstructure and macroscopic properties of CGC. FA first increased and then decreased the yield stress and plastic viscosity, while GGBS considerably increased the yield stress and plastic viscosity. Furthermore, a constitutive model was established for CGC containing FA and GGBS under compression. The proposed stress–strain model for CGC is accurate and can be used for nonlinear analysis of structures made of this type of concrete. Moreover, the addition of FA and GGBS consumes calcium hydroxide, but increases products such as hydrated calcium silicate aluminate. These findings elucidate the understanding and optimisation of CGA use in concrete production, while contributing to lowering carbon emissions. • Concrete strength degrades with increasing coal gangue aggregate substitution rate. • Fly ash degrades coal gangue concrete mechanical properties, and slag enhances them. • Fly ash and slag combination provides a complementary superposition effect. • A precise coal gangue concrete uniaxial compression analysis model is proposed. [ABSTRACT FROM AUTHOR]

Unmodified thermal-activated recycled cement (RC) paste has unfavorable properties like fast setting. This paper intends to modify RC by adding phosphogypsum (PG) and ground-granulated-blast-furnace-slag (GGBS). A comprehensive experimental program combining isothermal calorimetry, mechanical tests, TG, XRD, MIP, 29Si NMR and SEM of modified RC (PG-GGBS-modified-recycled-cement) pastes was conducted. The results show that GGBS and PG not only effectively modified the setting time but also contributed to a more compact skeleton. The final setting time was extended from 12 mins to 84 mins. The 28d and 90d compressive strength reached 29.8 and 36.9 MPa, increaseing by 21.1 % and 39.9 %, respectively. It is shown that PG acted as a sulfate excitation to the active Al 2 O 3 in the GGBS and RC to generate AFt. Rehydrated C-S-H gel could both wrap and protect the CaSO 4 ·2 H 2 O crystals and crystalline contact points, and effectively filled the pores of the hardened paste, thus promoting the generation of C(-A)-S-H and Ca 5 (SiO 4) 2 (OH) 2. The rehydration of modified RC was associated with a remarkable amount of AFt and AFm intergrowing with the reformed C-S-H gel and generating a compact network skeleton. • Modification of recycled cement with phosphogypsum and GGBS was investigated. • GGBS and PG not only effectively modified the setting time but also contributed to a more compact skeleton. • Rehydrated C-S-H gel could effectively fill the pores, thus promoting the generation of C(-A)-S-H and Ca 5 (SiO 4) 2 (OH) 2. • The final setting time of recycled cement was extended from 12 mins to 84 mins. • The 28d and 90d compressive strength reached 29.8 and 36.9 MPa, increased by 21.1 % and 39.9 %. [ABSTRACT FROM AUTHOR]

The porous hydrophilic nature of cement-based material is susceptible to water and corrosive media, severely reducing the durability. In this paper, methyl siloxane-based hydrophobic powder was prepared, and its effect on the physicochemical and anti-corrosion properties of mortar with different water-cement ratios were investigated. The results show that the addition of hydrophobic powder significantly improved the hydrophobic properties of mortars. Especially for the mortars with w/c ratios of 0.2 and 0.4, it showed superhydrophobicity with a water contact angle up to 156.5 ° and 152.3 ° respectively. Lower w/c ratio mortar has a denser microstructure, which reduces the water absorption rate of the mortar, and the hydrophobic powder enhances the hydrophobic effect of low w/c ratio mortar more significantly. The methyl siloxane groups in the hydrophobic powders will bind to the C-S-H gel in the mortar through Si-O bonding, preventing the C-S-H gel connections from forming a continuous network structure. This increased the pore volume and reduced the compressive strength by 14.6%-22.7%, but the total hydration degree was increased, and significant corrosion protection of the steel rebar was demonstrated. [Display omitted] • Super hydrophobicity of mortar was achieved by modification of hydrophobic powder. • Hydrophobic mortar with lower w/c ratio exhibits better anti-corrosion performance. • Methylsiloxane groups combine with C-S-H gel by Si-O enhancing hydrophobic properties of mortar. [ABSTRACT FROM AUTHOR]

The mechanical properties and durability of concrete are significantly affected by early frozen, during the early curing phase, concrete has a relatively low initial strength, and its hydration reaction rate is impeded by the early frost stage, leading to internal water freezing and subsequent expansive stress. The freezing and expansion deformations induce crack formation in the concrete structure. Even if positive-temperature curing is implemented for a specific period, some deformations remain irreparable, significantly compromising the mechanical properties and durability of concrete. Research on early frozen concrete mainly focus on water-cement ratio, freezing temperature, curing age, and additive dosage, which have different effects on the mechanical properties, durability, and microstructure of concrete. This paper systematically summarizes and analyzes the effects of early freezing on the mechanical properties and durability of concrete subjected to early frozen at home and abroad, and predicts and analyzes its future research directions and trends in this field. • Influences of early frost on the mechanical properties, durability, and microstructure of concrete are reviewed. • Effects of different admixtures and dosages on early frozen concrete are discussed. • The directions for further research on the effect of early frost on concrete are proposed. [ABSTRACT FROM AUTHOR]

Supersulfated cement (SSC) incorporates a significant amount of ground granulated blast-furnace slag (GGBS) and phosphogypsum (PG), which contributes to low carbon emissions and offers for extensive application. This paper investigated the evolution characteristics of performance and microstructure of SSC and Portland slag cement (PSC) pastes under Na 2 SO 4 solution attack and Na 2 SO 4 -HCl solution compound attack, including visual appearance, compressive strength, hydration products, pore structure and morphology. The results show that SSC group demonstrates superior resistance to Na 2 SO 4 solution compared to PSC group, which is reflected in no change in visual appearance over immersion time, a continuous increase in strength and stable hydration products. The pore volume (＜50 nm) of SSC increased after 270 days of erosion, while PSC experienced a sharp increase in pore volume (＞50 nm), which is also reflected in the compressive strength, with PSC (52 MPa) significantly lower than SSC (67 MPa). Na 2 SO 4 -HCl solutions significantly deteriorate both SSC and PSC groups. The strength of SSC remained 45 MPa from 28 to 180 days, decreasing rapidly thereafter, while PSC strength decreased rapidly after 90 days. Compound solution reduced alkalinity, resulting in the decalcification of C-S-H gel and ettringite decomposition. After 90 days of erosion in compound solution, the total pore volume of both SSC and PSC increased from 0.15 to 0.17 mL/g, while the pore volume (＜50 nm) of SSC was higher than that of PSC. Additionally, the strength and strength retention rate of SSC group were higher than PSC after 28 days of erosion. Therefore, SSC exhibits superior resistance to compound chemical attacks compared to PSC, especially for long term attack. • Supersulfated cement (SSC) exhibits better resistance to sulfate solutions compared to Portland slag cement. • Formed ettringite with high content and more stable in SO 4 2− ions environment elevates the properties of SSC. • The resistance to compound chemical attack of SSC is superior than Portland slag cement, especially for long term attack. [ABSTRACT FROM AUTHOR]

Ultrafine cement is widely used in micro-fracture grouting due to its very small particle size, and the expansion agent can improve the dry shrinkage and cracking phenomenon of ultrafine cement grouting materials, and at the same time produce self-stress to enhance the sealing effect, but at present, there are fewer researches on the hydration synergistic effect of ultrafine cement and expansion agent. For this reason, this paper explored the effects of two commonly used expansion agents, U-type and MgO-type, on the mechanical properties, hydration heat, hydration products and microscopic morphology of ultrafine cement materials. The results demonstrated that the expansion performance of MgO-type expansion agent (MEA) was better than that of U-type expansion agent (UEA); At the same time, the MgO-type expansion agent delays the exothermic cement hydration and inhibits the transformation of hydration products, and the stress concentration generated by its delayed expansion effect leads to brittle damage of the material. UEA participates in cement hydration and produces swelling crystals, and at this time, the specimen was mainly cemented in the form of particles, and the pre-generated internal expansion stress densifies the pore structure of ultrafine cement, which enhanced the mechanical strength of the specimen. • The effects of MgO and U-type expansion agent on the expansion and mechanical properties of superfine cement were analyzed. • The effects of U-type and MgO type expansion agent on the microstructure of ultrafine cement grouting materials were analyzed. • The relationship between hydration coordination and properties of superfine cement and expander was revealed. [ABSTRACT FROM AUTHOR]

This paper presents the effect of incorporating wollastonite microfibers and cellulose nanofibers on mechanical properties and microstructure of ultra-high-performance mortar (UHPM). Cellulose nanofibers of 0.005%, 0.015%, and 0.030% are combined with 4.8% wollastonite microfiber by weight of binder, which consists of 10% silica fume and 90% cement. Compressive and flexural strengths of the UHPM specimens are measured at 7 days. The microstructure is characterized in terms of SEM/EDX, XRD, FTIR, and nanoindentation. The results show the effectiveness of the fiber hybridization in enhancing the compressive and flexural strength of ultra-high-performance mortar with the improvement in flexural toughness and fracture energy by as much as 10% than control mortar. Moreover, hybrid fiber-reinforcement based on the microstructural analysis provided more synergic effect and uniform distribution of the fibers, thus developing an efficient micro crack bridging mechanism as well as resulting in enhanced toughening properties and an increased load carrying capacity. Additionally, the combination of wollastonite microfibers and cellulose nanofibers promoted the hydration reaction products and contributed a denser matrix between the fibers and cement matrix, resulting in an increase of the microstructure and interface area between the fibers and cement matrix. Nano indentation analysis also confirmed the increase of UHD C-S-H from 65% to 84% and lower percentage of anhydrous phase in ultra-high-performance matrix after combining wollastonite microfibers and cellulose nanofibers. • The hybridization of cellulose nano fiber with wollastonite microfiber increased the compressive strength of unreinforced mortar. • The greatest advantage in hybrid-reinforcement systems in ultra-high-performance mortar is the improvement in flexural toughness. • Hybrid reinforcement systems develop an efficient crack bridging mechanism as well as resulting in enhanced toughening properties and an increased load carry capacity. • The addition of wollastonite promoted a higher UHD C-S-H compared to the control specimen and other hybrid reinforced mortars. • Cellulose Nano Fibers enhance cement hydration due to the high specific surface area, in addition to the promotion of adhesion with cement paste and bonding properties when combined with wollastonite. [ABSTRACT FROM AUTHOR]

This paper proposes an innovative preparation measure for magnesium oxide (MgO)-based foamed concrete. Carbon dioxide (CO 2) foam has been introduced into the sample as a substitute for air foam, resulting in a novel formulation referred to as MgO carbon sequestration foamed concrete (MCFC). This innovation facilitates the setting and hardening of MgO-based foamed concrete under ambient conditions. Basic mechanical properties were tested at three different ages. At the age of 28 days, the resultant MCFC has demonstrated mechanical performance comparable to conventional Portland cement-based foamed concrete including unconfined compressive strength and stain-stress behavior. Through SEM NMR and X-ray CT, the MCFC sample exhibited an increasingly compact matrix and an evolving distribution of pore structure as time progressed. Furthermore, a comprehensive life cycle assessment has revealed that MCFC has the potential to reduce carbon emissions by up to 50% when compared to conventional cement-based foamed concrete. The results imply that MCFC presents itself as a viable alternative to conventional PC-based foamed concrete, offering equivalent mechanical performance while delivering superior environmental advantages for engineering applications. ● MCFC using MgO, soil and CO 2 foam is proposed to realize the sequestration of CO 2 and the reduction in carbon emissions. ● MgO and CO 2 react to generate carbonated phases, and CO 2 is consumed to realize its solidification. ● MCFC can consume and sequester CO 2 and avoid environmental pollution, portraying great social and economic benefits. [ABSTRACT FROM AUTHOR]

Soilcrete is produced by adding a curing agent, such as cement, to the soil and mixing it thoroughly to form a solid, stable substance, but usually with poor early strength. To improve the early strength and stability of soilcrete, in this paper, solid waste materials such as mineral powder, and natural gypsum powder were added proportionately to replace parts of cement (called composite curing agent). The unconfined compressive strength (UCS), and dry-wet cycle strength of soilcrete were tested to evaluate the optimum mix design. The scanning electron microscope (SEM) and XRD test were applied for investigating the mechanism of early strength of solid waste-based soilcrete. The results show that the optimal mix ratio of cement and mineral powder is 4:6, the 7-day UCS is 1.61 MPa, and the 28-day UCS is 3.37 MPa. The composite curing agent mixed with gypsum powder and water glass has a good curing effect on silt and clay respectively, and the 28-day strength of the clay specimen can reach 3.97. During the dry-wet cycle test, the strength of the soilcrete increases first and then decreases with the number of cycles. The SEM results show that the soilcrete particles were denser, had more hydration products, and the soil particle morphology changed from loose to aggregated, accompanied by a decrease in pores. The study provides a cheaper and high-performance composite curing agent based on solid waste materials for soilcerte than conventional cement, which can promote the application in such field. • An early-strength, low-cost solicrete was proposed using solid waste materials, namely mineral powder and natural gypsum. • The UCS (Unconfined Compressive Strength), dry-wet cycle strength, and soil types of soilcrete were tested systematically. • The mechanism of the new solid waste material on soilcrete was revealed through SEM, XRD, and hydration analysis. [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]

Bamboo structures are susceptible to natural weathering when exposed to outdoor conditions, making it imperative to comprehend the aging resistance of bamboo. In this paper, the study of Moso bamboo exposed to outdoor environment for one year illustrates the discoloration, expansion, mass loss and density reduction of the specimens. The changes of relative crystallinity (CrI) of bamboo were measured by using X-ray diffraction (XRD), the microstructural changes of bamboo were observed by scanning electron microscopy (SEM), and the computed tomography (CT) scanning technology was used to analyze the internal structure of the damaged specimens. Meanwhile, the damage and differences in the mechanical properties of bamboo internodes and nodes were evaluated by the tests of compression parallel to the grain, compression perpendicular to the grain and compressive elastic modulus parallel to the grain, taking into account the bamboo height. The results showed that natural weathering reduces cellulose crystallinity and has a detrimental effect on the cellular structure of bamboo, resulting in a weakening of the interfacial bonding between the vascular bundles and the parenchyma. Consequently, this leads to a decrease in the mechanical properties of bamboo. Compared to the internode specimens, the compressive strength perpendicular to the grain and compressive elastic modulus parallel to the grain are higher in the bamboo node specimens. However, the compressive strength parallel to the grain will be significantly reduced due to the complex fiber structure inside the bamboo nodes. In light of the experimental findings, a constitutive model is proposed to describe the relationship between uniaxial compressive stress and strain in Moso bamboo, both before and after weathering. The numerical results obtained from this model demonstrate a strong correlation with the experimental findings, thereby establishing a solid foundation for conducting finite element analysis on Moso bamboo structures. • The cellulose crystallinity and microstructural changes of weathered bamboo were analyzed. • Comparative studies were conducted to investigate the mechanical properties of weathered bamboo. • An axial compressive stress-strain constitutive model of weathered bamboo was proposed. [ABSTRACT FROM AUTHOR]

Incorporation of fibers can significantly enhance the mechanical properties, and crack resistance of geopolymer concrete (GPC). However, the alkaline environment in GPC can adversely affect the performance of fibers. To address this issue, this paper focuses on the surface modification of basalt fibers using a silane coupling agent (SCA) to enhance their compatibility with the geopolymer matrix. Macroscopic tests are conducted to analyze the workability, compressive strength, and flexural strength of geopolymer concrete, as well as the three-dimensional condition of its damaged surface. At the microscopic level, the fiber/matrix interface is investigated using scanning electron microscopy (SEM), Fourier infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) to reveal the enhancement mechanism of the modified basalt fibers (MBF). The experimental results demonstrate that the MBF significantly improves the flexural of GPC and has stronger impact on the degree of flexural deflection of the fracture surface compared to unmodified basalt fibers (BF). Physical aspects of SCA film: increases the roughness of the surface of basalt fiber, improves the mechanical occlusion at the interface between the fiber and the matrix and the transfer of concrete matrix stress. Chemical aspects: reduces the ground polymer matrix -OH ions on the fiber surface erosion, to ensure the stability of the SiO 4 tetrahedral network structure of the basalt fiber surface, increases the fiber and matrix interface at the densification, improves the chemical adhesion. In summary, the MBF effectively enhances the mechanical properties of GPC by optimizing the interfacial bond, enhancing cohesion, and improving the microstructure. These findings provide valuable insights into the potential application of SCA-modified basalt fibers as a promising reinforcement in GPC for sustainable construction and infrastructure projects. • The mechanism of silane coupling agent modified fiber-reinforced geopolymer concrete was investigated. • Both physical adhesion and chemical bonding aspects were studied experimentally. • The action mechanism of modified basalt fibers was elucidated through microscopic characterizations. [ABSTRACT FROM AUTHOR]

This paper covers the result of physico-mechanical, microstructural, optimization, and environmental impacts of alkali-activated paste (AAP) produced from recycled waste concrete (RWC) powder and metakaolin (MK). The experimental findings revealed that RWC powder up to 40 % effectively reduced setting time. The specimens with M s ratios of 1.25 and 2.00 had similar setting periods, while the mixes activated with 0.50 had greater setting time variance. The compressive strength significantly increased with an increase in RWC powder, and the mix formulated with 40 % RWC powder and M s ratio of 2.00 showed maximum strength of 79.1 MPa at 28 days. However, further inclusion of RWC powder beyond 40 % constantly decreased the strength. A statistically significant model to predict compressive strength was obtained by response surface methodology study, and the obtained optimum mix design (RWC=36.33 % and M s :1.89) was experimentally validated with an absolute error of 0.17 %. The microstructural analysis indicated that N-A-S-H type gels were the major reaction products, and the inclusion of RWC powder led to the generation of C-(A)-S-H type gels along with N-A-S-H type gels. Furthermore, an increase in the RWC powder ratio increased the crystallinity index values of AAP samples, as shown by X-ray diffraction results. Life cycle assessment (LCA) analysis was performed for all AAP mixes, including the optimum mix. LCA results indicated that the addition of RWC powder consistently reduced the CO 2 emission, cumulative energy demand, and cost of the resulting AAP mixes; however, these parameters slightly increased with an increase in M s ratio of alkaline solution. [Display omitted] • Investigation on AAP containing recycled waste concrete (RWC) and metakaolin (MK). • Compressive strength of paste mixes enhanced up to 40 % RWC powder substitution. • A statistically significant RSM model was obtained to predict compressive strength. • CO 2 emission and energy demand decreased with an increase in RWC powder ratio. • RWC powder can be used to produce economical and green building materials. [ABSTRACT FROM AUTHOR]

In the process of service, the coating is vulnerable to ultraviolet radiation, which leads to aging failure. The metakaolin base geopolymer coating studied in this paper has excellent mechanical and anti-corrosion properties through the previous mix ratio study. However, the UV aging resistance of the coating is unknown. In response to this issue, the present study devised UV aging experiments for metakaolin based geopolymer coatings. Employing UV irradiation in conjunction with EIS tests, the aging resistance of the geopolymer coatings was assessed through electrochemical fitting and analyses such as SEM. The study delves into the structural evolution of metakaolin based geopolymer coatings under UV aging conditions. After 500 h of ultraviolet irradiation, the resistance of the geopolymer coating is enhanced, its water diffusion coefficient is reduced, two orders of magnitude increase the resistance of the coating, and the protection performance of the geopolymer coating is enhanced. In the control group, the waterproof performance of epoxy resin decreased significantly, the impedance decreased, and the resistance decreased by three orders of magnitude. SEM results show that the post-curing of the coating under UV irradiation continuously generates NASH gels, and the coating microstructure is dense. The results show that geopolymer has strong aging resistance. • This experiment investigates the mechanism of UV ageing resistance of metakaolin base geopolymer coatings. • Comparison with epoxy resin coatings concludes that the geopolymer coatings have very good UV aging resistance. • The system of evaluating the water resistance and porosity of the coatings by capacitance method based on Fick's law of diffusion is applicable to the geopolymer coatings. • UV irradiation induces a post-curing reaction of the geopolymer coatings. [ABSTRACT FROM AUTHOR]

In order to enhance the application of epoxy mortar in pavement repair engineering, an epoxy-based repair material with short molding time, easy fabrication, high early strength and good durability was developed. In the past, the fillers of epoxy mortar were mainly fly ash and silica fume. In this paper used loess (LS) and river sand (RS) which were lower cost and more readily available, mixed with epoxy resin to make a new epoxy mortar system. The influence of epoxy resin mass fraction (20%,25%,30%), filler type (LS, RS), and LS-to-RS ratio (2:1,1:1,1:2) on the mechanical properties and durability of the epoxy mortar were investigated. It was observed that the early compressive strength of the specimens increased significantly with the increase of the proportion of epoxy resin in the mixture, reaching 49.2 MPa after 2 h of curing. Moreover, when the mass fraction of epoxy resin increased from 20% to 30%, the flexural strength increased by more than 20% after curing for 2 h. Additionally, compared to single filler, mortar containing both LS and RS exhibited superior durability. Unlike the specimens of the all-river sand group L0, which showed a 40% to 60% decrease in mechanical properties in sulfate and chloride saline solutions, almost no loss of strength occurred in the L1R1 group. This phenomenon can be explained by the findings from SEM and MIP tests that the L1R1 group has a more compact pore structure compared to the L0 group. [Display omitted] • Epoxy resin with loess and river sand to form a new pavement emergency repair material system. • Under the curing effect of epoxy resin, the compressive strength of epoxy mortar can reach 49.2 MPa for 2 h. • Epoxy mortar had excellent durability in water and saline solutions. [ABSTRACT FROM AUTHOR]

This study delves into the influence of various nanomaterials on the mechanical performance and durability of sludge-based geopolymer. In terms of overall strength, it was evident that nanosilica (NS), when introduced at the same doping level, exhibited the most significant enhancement in mechanical properties, surpassing nano-calcium carbonate (NCC) and graphene nanoparticles (GNPs). For instance, the addition of 2% NS resulted in a remarkable increase of up to 108.2% in compressive strength and 69.9% in flexural strength. Furthermore, the early reaction rate of sludge-based geopolymers was positively affected by the incorporation of NS, NCC, and GNPs. After 28 days, thermogravimetric analysis demonstrated that the mass loss for the reference sample and geopolymers enhanced with NS, NCC, and GNPs was approximately 15.06%, 14.78%, 14.85%, and 14.95%, respectively. Regarding durability, the sludge-based geopolymers displayed enhanced resistance to sulfate erosion and freeze-thaw cycles. This improvement was attributed to the ability of nanomaterials to fill pores and promote hydration reactions, resulting in a denser matrix. This paper delves into the influence of nanomaterials on the gel microstructure and gel micromechanical properties, shedding light on the mechanisms responsible for the enhanced mechanical and durability properties. It also presents a comparative analysis of results at different doping levels, offering a promising solution for waste treatment and carbon emission reduction, while making a compelling case for practical engineering applications. • First study of mechanical and durability properties of nanomodified sludge-based geopolymers. • Nanomodified sludge-based geopolymers can be used for practical engineering applications. • The mechanical properties of nanomodified sludge-based geopolymers depend on factors such as gel strength, pore filling and chemical bonding. [ABSTRACT FROM AUTHOR]

Fly ash is known as a common geopolymer precursor. However, it is important to search for suitable alternatives as the availability of fly ash will reduce with the move to more renewable energies. This paper evaluates the feasibility of using waste clay brick powder (WCBP) as a partial alternative for fly ash in geopolymer mortars with ground granulated blast furnace slag (GGBFS), under ambient and heat curing conditions. The physical, mechanical, and mineralogical properties of geopolymer mortar with 10 to 40% WCBP were investigated. Microstructural properties of WCBP and geopolymer mortars were investigated using X-ray fluorescence (XRF), X-ray diffraction (XRD), tescan integrated mineral analyzer (TIMA), scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS). Reductions of the flow of fresh mortar were observed with the increase of WCBP. The highest 28 days compressive strengths were obtained in the mortars containing 40% WCBP, both at ambient and heat curing conditions. Microstructural analysis highlighted compact structure and reduced porosity, which are attributed to the role of WCBP in enhancing fly ash-GGBFS mortar properties by promoting geopolymer gel formation. • Characterization of Waste Clay Brick Powder (WCBP) showed its potential use as a geopolymer precursor. • The effect of WCBP on the mechanical and mineralogical properties of fly ash-GGBFS-WCBP geopolymer mortar was investigated. • High strength and a compact microstructure development could be achieved by using up to 40% WCBP. [ABSTRACT FROM AUTHOR]

Agriculture waste generation is increasing daily, and agricultural waste management has become one of the most challenging issues for many countries. To contribute towards a sustainable waste management system, this experimental study evaluates the impact of using agricultural waste materials in concrete. Concrete that does not harm the environment and has improved engineering properties can be produced using agricultural waste ash such as green gram pod ash. Green concrete has been examined for its mechanical, durability and microstructure properties when partially replaced with Portland cement by green gram pod ash (GGPA). Conducting various tests using concrete containing different weight proportions of green gram pod ash in a mix at 2%, 4%, 6%, 8%, and 10%, respectively. This paper examines the mechanical, durability and morphological characteristics of green gram pod ash blended with cement concrete. It has been shown that concrete containing 8% green gram pod ash enhances the properties of GGPA concrete. It was observed that the addition of 8% green gram pod ash enhances the compressive strength. Split tensile strength, flexure strength, modulus of elasticity and impact strength of concrete by 4.26%, 4.07%, 35.19%, 16% and 10.3% at 28 days of curing. It was concluded that green gram pod ash was shown to satisfy engineering application design requirements and significantly reduce concrete production costs and environmental pollution. The cement replacement with 8% of GGPA is optimum for achieving better concrete properties. • Evaluations of the impacts of using Green Gram Pod Ash (GGPA) in cement and cementitious material production were conducted. • Incorporation of Green Gram Pod Ash (GGPA) in concrete improves the strength and ductility performance of concrete. • Substitution level of GGPA in concrete should be limited to 8% (by weight) • GGPA concrete samples containing 8% green gram pod ash displayed maximum static modulus of elasticity and higher impact energy. [ABSTRACT FROM AUTHOR]

Ultra-high-performance concrete (UHPC) surpasses normal strength concrete in engineering properties such as durability under extreme conditions, strength, and ductility characteristics. However, the dense structure, impermeable nature, and lack of capillary pores in ultra-high-performance concrete may diminish its resistance to spalling when subjected to intense heating conditions, unlike normal and high-strength concrete. The behavior of UHPC under such conditions has not been extensively researched, especially in the context of engineering properties and spalling behavior. Existing knowledge is primarily inferred from normal and high-strength concrete behavior under high temperatures, but this approach has sparked contention due to differing phenomena at play. Given ultra-high-performance concrete's distinct composition and excellent mechanical features, the principles applied to normal and high-strength concrete may not sufficiently explain its complex behavior during thermal exposure, particularly regarding spalling and engineering properties. Furthermore, enhancing its fire resistance becomes increasingly crucial as UHPC gains popularity in new and existing structures. To bridge this knowledge gap, this review paper presents various facets of UHPC during intense heat exposure, emphasizing engineering properties, fire-induced spalling, its factors, underlying mechanisms, microstructural analysis, performance, and preventative and remedial strategies. Future research recommendations are also offered to improve UHPC's performance under elevated temperatures, potentially leveraging current knowledge in the field. • Adding fibers can mitigate spalling in UHPC. • Dense microstructure of UHPC helps lower spalling. • Intense heat can cause explosive spalling, if not treated well. [ABSTRACT FROM AUTHOR]

This paper developed a cost-effective and highly carbonatable industrial-grade γ-C 2 S binder sintered from limestone and sandstone, aiming to advance the industrialization of γ-C 2 S binder in construction fields. The carbonation behavior of the industrial-grade γ-C 2 S binder, including the evolution of carbonation kinetics, compressive strength, phase composition, and microstructure with carbonation duration was studied. Results showed that a temperature of 1400 °C and limestone-to-sandstone mass ratio of 2.1 can be regarded as the optimal sintering regime, which can secure a relatively complete self-pulverization of industrial-grade γ-C 2 S binder. The industrial-grade γ-C 2 S binder had a milder carbonation reaction, which can reduce the evaporation of water and lead to 48% greater compressive strength, compared to that sintered from analytically pure reagents. During carbonation, the proportion of γ-C 2 S phase significantly decreased, while the proportions of CaCO 3 and silica gel notably increased in the first 1 h, followed with a slight change in proportions of those phases with the carbonation further extending to 24 h. Similarly, the degree of carbonation rapidly increased to 36.3% in the first 1 h, followed with a slight increase to 43.7% at 24 h. The carbonation of industrial-grade γ-C 2 S, accompanied with the precipitation of CaCO 3 on γ-C 2 S phase surface and the bond of silica gel among CaCO 3 grains, contributed to the formation of nacre-bionic interlocking brick-mud microstructure. This led to the rapid enhancement of compressive strength with carbonation of industrial-grade γ-C 2 S. • Industrial-grade γ-C2S binder from limestone and sandstone was sintered. • Industrial-grade γ-C2S showed a greater compressive strength after carbonation. • Limestone-to-sandstone mass ratio of 2.1 sintered at 1400 °C was the optimal regime. • Nacre-bionic interlocking brick-mud microstructure was formed during carbonation. [ABSTRACT FROM AUTHOR]

Geopolymer concrete has been increasingly used in industry. However, if the geopolymer concrete is blended using a single percussor its performance is often very limited by the properties of the percussor. Thus, it is preferable to use multiple percussors to produce geopolymer concrete. In this paper an experimental study is reported on the residual micro- and macro-mechanical properties of the steel-fibre reinforced geopolymer concrete blended by using combined percussors of ground granulated blast-furnace slag, fly ash, and salic fume. The experimental work included the examination of the effects of exposure temperature, the volume fractions of coarse aggregate and steel fibre on the compressive strength, elastic modulus, peak strain, toughness, and stress-strain constitutive relationship of the mixed geopolymer concrete, and the characterisation of the micro-structure of the materials after they were exposed to various elevated temperatures by using X-ray diffraction, scanning electron microscopy, thermal gravimetric analysis, and mercury intrusion porosimetry techniques. The experimental results showed that the residual compressive strength and residual elastic modulus decrease with the increase of exposure temperature regardless of the mix designs. The residual compressive strength of the geopolymer concrete is higher than that of the geopolymer mortar when they both are not reinforced with steel fibre. However, when they are reinforced with steel fibre the residual compressive strength of the geopolymer concrete is lower than that of the geopolymer mortar. Based on the experimentally obtained results, a temperature-dependent uniaxial stress–strain empirical equation is also proposed for the purpose of practical use. • Residual mechanical properties of GPM/GPC after exposed to elevated temperatures are reported. • Micro-structure and phase change of GPM and GPC after exposed to elevated temperatures are examined. • The effect of coarse aggregate on the residual mechanical properties of GPM and GPC is presented. • The effect of steel fibre on the residual mechanical properties of GPM and GPC is presented. • Residual stress-strain constitutive equations of GPM and GPC with and without steel fibre are proposed. [ABSTRACT FROM AUTHOR]

This paper aims to investigate the influence of introducing calcium sulfate whiskers (CSW) and basalt fiber (BF) on the gas permeability coefficients of concrete. Employing the Cembureau method, the research quantifies gas permeability while utilizing Nuclear Magnetic Resonance (NMR) to precisely measure pore structure parameters. The results highlight that the incorporation of CSW leads to a substantial reduction in gas permeability, exhibiting a remarkable 57.6% decrease when utilized at a dosage of 3.0 kg/m³ compared to the standard concrete. In contrast, the introduction of BF at the same dosage does not yield a commensurate reduction in permeability. Notably, the combined application of CSW and BF at 3.0 kg/m³ each yields the most effective mitigation of gas permeability. Furthermore, the judicious addition of either CSW or BF demonstrates the capacity to diminish the overall porosity of concrete, alongside a reduction in the proportion of harmful pores (R 100 , featuring diameters larger than 100 nm). Optimal outcomes manifest when both CSW and BF are introduced at the 3.0 kg/m³ dosage, effectively minimizing R 100 and the gas permeability coefficient. It is interesting to find that in comparison to the correlation observed between the total porosity and gas permeability of concrete, the relationship between the proportion of R 100 and gas permeability emerges as more statistically significant (R 2 approximately 0.800), especially for CSW concrete. This can reveal the interconnection between the concrete's pore structure and its gas permeability characteristics more appropriately. • Optimal gas impermeability achieved with 3.0 kg/m3 of CSW and BF in concrete. • Optimal CSW and BF dosage mitigates total porosity and reduces harmful pores (R100). • Gas permeability coefficient has a strong correlation with R 100 proportion. [ABSTRACT FROM AUTHOR]

In the mudslide prone areas of western China, concrete piers were severely damaged by washing.In our previous research, the anti-abrasive UHPC prepared with saturated prewet high titanium heavy slag sand (ST-UHPC) is proposed to prevent such damage. And we studied the effect of grinding and erosion on the performance of ST-UHPC before, but the mechanism has not been studied in depth In order to study the microstructure evolution of ST-UHPC cement by the grinding and erosion, we use XRD, SEM, 27Al NMR, 29Si NMR test methods in this paper. By this way, we studied the effects of shock and salt erosion on hydration products composition, migration and transformation of Al-containing hydration products, hydration degree of cement and C-A-S-H gel microstructure. The results showed that the impact of bedload damaged the microstructure of UHPC. Then the damage of UHPC microstructure accelerates the transport of aggressive ions. Next, SO 4 2- erodes from the site of damage and produces expansive erosion products such as gypsum and calcareous vanadite. The erosion of SO 4 2- and Mg2+ resulted in the dealuminization and decalcification of C-A-S-H gel in UHPC cement and mechanical properties degradation. Thus, the expansion damage of UHPC and the impact damage of UHPC structure are aggravated. So the microstructure damage of UHPC was further aggravated. Under the action of impact, the water impacting material forms various chemical bonds with the C-A-S-H substrate. The chemical bonds results in the evolution of C-A-S-H microstructure. In conclusion, the structure of C-A-S-H substrate deteriorates obviously under the coupling effect of scouring and erosion. And the impact resistance of C-A-S-H substrate decreases significantly. • The anti-abrasive UHPC prepared with saturated prewet high titanium heavy slag sand was proposed. • XRD, SEM, 27Al NMR, 29Si NMR were used to study the microstructure evolution of ST-UHPC. • The microstructure evolution of UHPC was studied by molecular dynamics simulation. [ABSTRACT FROM AUTHOR]

Ultra-high-performance concrete (UHPC) made of limestone calcined clay cement (LC3) exhibits excellent mechanical properties as well as good environmental friendliness. This paper studied the improving effect of nano-SiO 2 on the mechanical properties and microstructure of LC3-UHPC. The experimental results show that the addition of nano-SiO 2 demonstrates obvious advantages. The mechanical properties of LC3-UHPC are enhanced, and the frictional bond performance between steel fiber and cementitious matrix is also improved. The fiber/matrix bond strength is increased. A constitutive model describing the single fiber pullout behavior is established, and the values of related parameters are suggested. Besides, nano-SiO 2 also refined the cementitious microstructure of UHPC and made the matrix denser. Based on the comprehensive analysis of mechanical properties and microstructural characteristics, the optimal content of LC3 and nano-SiO 2 were proposed. [Display omitted] • The performance of LC3-UHPC was improved by using nano-SiO 2. • The microstructure of UHPC using nano-SiO 2 was optimized. • The fiber/matrix bonding behavior of LC3-UHPC was improved. • Comprehensive evaluation of the influence of nano-SiO 2 was conducted. [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]