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Lower early strength has been one of the bottlenecks of low-carbon supersulfated cement (SSC), consisting of slag (∼85 wt.%), gypsum (∼10 wt.%), and a small amount of an alkali activator (Portland cement (PC), ∼5 wt.%). The work aims at improving the early strength of SSC with the addition of calcium formate (CF). Different dosages (1 wt.%, 2 wt.%, 3 wt.%) of CF were added to the SSC. Then, compressive strength, heat flow, hydration product assemblage, and hydration degree of slag were obtained to reveal the role of CF. The results show that CF would significantly increase the compressive strength of the SSC, and the higher the dosage of CF, the more pronounced the effect is. 3 wt.% of CF would increase the 3-day and 28-day compressive strength of the SSC by 126% and 172%, respectively. CF would accelerate the hydration of slag, increase the hydration degree of slag and production of C-(A)-S-H gels; however, it would inhibit the rapid production of AFt during the first 3 days, and have little influence on the final production of AFt. In addition, the lower the crystal-gel ratio (the ratio between AFt and C-(A)-S-H gel by mass) is, the higher the compressive strength. The enhancement effect of CF was attributed to a more appropriate hydration environment (higher Ca2+ concentration and lower pH value), which could promote the precipitation of C-(A)-S-H gels, inhibit the rapid formation of Aft, and reduce the crystal-gel ratio of the system.

Low hydration and carbonation degree of reactive magnesium oxide cement (RMC) have always hindered the development of its properties. Motivated by the influence of amino acids on biomineralization, this work aimed to investigate the impacts of glutamic acid (Glu), serine (Ser), and arginine (Arg) on the hydration and carbonation of RMC under ambient-curing and accelerated CO2-curing. The exothermic hydration, mechanical strength, phase composition and thermal stability were studied. Results suggested that Glu significantly improved the compressive strength (0.1 M Glu increased strength by 58% at 14 days of accelerated CO2 curing) and hydration degree (71% vs. 57%). The addition of amino acids varies the pH value of the system, and different effects of hydration and carbonation were observed among different amino acid-added samples. This effect can be attributed to the state of the amino acids in the solution at that pH. The negatively charged glutamic acid in the RMC system binds to Mg2+, leads to greater MgO dissolution, enhanced hydration and carbonation, and improved mechanical strength. [ABSTRACT FROM AUTHOR]

Reactive MgO cement (RMC), which undergoes hardening through the reaction of MgO with water and CO2, is a novel cementitious material that has gained significant attention in recent years. The hydration and carbonation processes of RMC samples are significantly affected by the curing method and water content. This work compared the effects of standard curing ((20 ± 1) °C, 95% relative humidity), water curing ((20 ± 1) °C, and carbonation curing ((20 ± 3) °C, 70% relative humidity, 20% (volume fraction) CO2) on the properties of RMC with water-cement ratio (w/c) of 0.5, 0. 6, and 0.7, respectively. The types and content of the phases of the hydration and carbonation reaction products as well as their micro-morphological characteristics were investigated. The results show that the residual content of MgO in samples is 10% ~ 15% (mass fraction) at 3 d and disappears at 14 d under standard curing and water curing. When w/c is 0.6, the compressive strength of paste reaches 7.7 and 3.2 MPa at 14 d under standard curing and water curing, respectively. The strength of paste under carbonation curing is superior to that under standard curing and water curing (3 d compressive strength, 16.19, 0.42, and 0.43 MPa, 14 d compressive strength, 22.34, 7.46 and 7.23 MPa). The quantitative analysis results show that the carbonation product is mainly MgCO3 ⋅ 3H2O. When w/c is 0.6, the content of MgCO3 ⋅ 3H2O is 12.12% and 11.29% higher than that of samples with w/c = 0.5 and 0.7. The main reason for the optimal compressive strength could lies in that these needle-rod MgCO3 ⋅ 3H2O are bonded to each other form a dense microstructure. [ABSTRACT FROM AUTHOR]

Ferrisilicate was considered a crystalline zeolite containing iron (III) and silicon in the zeolite lattice positions. The aim of this work is to study the influence of Fe2O3/SiO2 molar ratios contained in the Fe-silica on the properties of geopolymer materials derived from waste fired clay brick and metakaolin. Fe-silica with various Fe2O3/SiO2 molar ratios of 0.2, 0.4, 0.6, 0.8 and 1.0 were synthesised using rice husk ash and hematite. 0 and 10 wt. % of each type of aluminosilicate is added to each type of Fe-silica. A commercial sodium silicate solution with a SiO2/Na2O molar ratio of 1.6 is used as a hardener. X-ray diffractometry and Fourier Transform Infrared spectroscopy are used for the characterisation of the Fe-silica and the geopolymers. In addition to these analyses, the compressive strengths of the geopolymer materials have been measured and the fragments obtained have been examined using a scanning electron microscope. The results show that the compressive strengths of metakaolin-based geopolymers increase from 53.34 to 69.08 MPa when the Fe2O3/SiO2 molar ratios contained in the Fe-silica are incremented from 0 to 0.8. When the Fe2O3/SiO2 molar ratio is increased from 0.8 to 1.0, they are reduced from 69.08 to 63.22 MPa. In contrast, those of geopolymer materials derived from waste fired clay brick decrease from 45.42 to 21.15 MPa when their Fe2O3/SiO2 molar ratios rise from 0 to 0.6 increases from 21.15 to 24.11 MPa beyond 0.6. The micrograph of the geopolymer material from the mixture of metakaolin and Fe-silica with a molar ratio of Fe2O3/SiO2 equal to 0.8 is more compact, homogeneous and partially smooth compared to other geopolymers. The presence of hematite and cristobalite, which act as fillers, is evident from the X-ray patterns of geopolymer materials after the addition of Fe-silica. This work shows that the addition of metakaolin to multi-molar of Fe-silica significantly improves the compressive strength of geopolymer materials. On the other hand, this addition leads to a reduction in the compressive strength when waste fired clay brick is used. [ABSTRACT FROM AUTHOR]

The combined use of calcined clay and limestone (LC2) with slag cement can ease the promotion of a clay-type binder and elongate the use of traditionally used slag. Results showed that addition of 22–39% LC2 can enhance the compressive strength of slag cement concrete by 26, 86, 39 and 24% at 3, 7, 28 and 90 days, respectively. Quantitative X-ray radiation diffractometry (QXRD), thermogravimetry analysis (TGA), calorimetry and thermodynamic modelling have been used to reveal the individual role of each constituent in the quaternary blend. Results showed that the degree of hydration of slag cement can be increased by LC2, and calcined clay was understood to be the main contributing factor owing to the pozzolanic reactivity and filler effect. Limestone can increase the reaction degree of slag owing to the generation of hydrated mono-/hemi-carboaluminate (Mc/Hc). A synergistic effect of slag and limestone can contribute to the later performance in quaternary LC2–slag cement. [ABSTRACT FROM AUTHOR]

• Reuse of the mineral rich recycled concrete powder (RCP) as supplementary cementitious material (SCM) is under exploited. • Meta-analysis reveals that 15% replacement of the binder with non-beneficiated RCP can reduce target strength by up to 40% compared to granulated blast-furnace slag (GGBS). • Reduction in the eCO 2 of concrete containing the CO 2 -injected RCP becomes significant above 30% binder replacement. This can reach 10–25% of that achievable with GGBS. • Treating RCP with waste CO 2 can lead to carbon negative SCM and it presents the most promising route for managing end-of-life concrete in the circular economy. Reuse and recycling are vital practices in the circular economy. Despite progress in recycling aggregates from end-of-life concrete, the potential for reusing recycled concrete powder (RCP) as supplementary cementitious material (SCM) is now attracting research and commercial interest. Herein, a meta-analysis of concrete in which RCP was used as an SCM was conducted, and a multivariate regression model was developed for predicting strength from mix composition. The carbon footprint of alternative beneficiation strategies, including milling only, milling plus thermal treatment and milling plus CO 2 injection, were quantified and used with the regression model to investigate RCP-containing concrete's embodied carbon (eCO 2). The comparison was made with conventional SCMs and end-of-life scenarios of concrete from different cement types. The meta-analysis and regression model showed that 15% cement replacement by the non-beneficiated RCP caused a 40% reduction in 28-day compressive strength, and at 50% replacement, the strength reduction was 70%. Above 30% cement replacement, the RCP beneficiated through CO 2 injection reduced the concrete's eCO 2 per unit strength by 10–25%, while the thermally treated RCP had greater eCO 2 than conventional SCMs. Thus, circularising end-of-life concrete does not guarantee low carbon concrete production. Instead, treating RCP with waste CO 2 leads to a carbon-negative SCM, presenting the most promising route for low-carbon concrete in the circular economy. [Display omitted] [ABSTRACT FROM AUTHOR]

The dispersion/distribution of nanoSiO 2 in cement-based materials is known as very influential on the property gain, but still remains a big problem that has not been well addressed. The aim of this work was trying to tailor the dispersion/distribution of nanoSiO 2 in cement-based materials with silica fume. Firstly, the surface of silica fume was amino-functionalized (AFSF) for adsorbing and dispersing nanoSiO 2 (NS), then the adsorption of NS on AFSF and its effects on the properties of fresh and hardened cement-based materials were studied. Results showed that the adsorption of NS on AFSF followed the Freundlich adsorption model, and a multi-layer adsorption characteristic were found. The adsorption of NS on AFSF helped increasing the fluidity of mortar, and mitigated the negative effects of nanoSiO 2 on the hydration and hardening properties of cement-based materials at late ages, resulting in an improved compressive strength, a higher degree of hydration of cement and a compacter microstructure at late ages than the pure AFSF-added sample, and a higher and comparable compressive strength than the control sample at the early and late age can be resulted. It can be suggested from this work that the dispersion/distribution of nanomaterial in cement-based materials with silica fume helps a better exploration of the nano-modification of cement-based materials and provide a new technique on producing nano-modified cement concrete, and work on making good absorption of nanomaterial on SCM is worth of further study. [ABSTRACT FROM AUTHOR]

• NS can reduce the sensitivity of clinker content on the strength development of the SSC low-clinker-added samples. • The NS addition continuously increases the mechanical performance and promotes hydration of high-clinker added samples, especially in the later periods. • NS changes the morphology of hydrates but not apparently effects on porosity. Alkalinity fluctuations in supersulfated cement (SSC) significantly affect its compressive strength, limiting its utilization, but there is a lack of systematic study and compensation techniques. The effects of clinker content (0.5 wt%, 1 wt%, 3 wt%, 5%, i.e., C0.5, C1, C3 and C5) and nanosilica (NS) addition on the performance of SSCs were investigated. Experimental results showed that 1 wt% NS effectively reduced the sensitivity of clinker content of SSC (the compressive strength of SSC did not change when clinker dosage changed from 0.5 wt% to 1 wt%). However, NS retards the early hydration caused by the hindering of ettringite precipitation (most apparent in the low clinker samples C0.5 and C1). The addition of NS improved the early and later age compressive strength and slag reaction degree, reduced the porosity, and refined the pore structure in the high clinker SSCs (C3 and C5) compared to the low clinker group (C0.5 and C1). [ABSTRACT FROM AUTHOR]

Ca(OH) 2 (CH) is a critical constituent that influences the durability of cement-based materials. By taking the advantages of the in situ hydrolysis of tetraethoxysilane, TEOS, and the in situ consumption of CH at the surface of hardened cement-based material, this work studied the effects of TEOS-treatment on the CH-related durability, i.e., the calcium-leaching and sulfate-attack resistivity. Results suggested that TEOS can penetrate into hardened cement-based material more easily than water. Thermogravimetric analysis showed that TEOS consumed CH on the surface of hardened cement-based material, resulting in a gradient distribution of CH, and benefited the CH-related durability. The loss of compressive strength of mortar samples suffering decalcification was reduced by TEOS-treatment, and the lower Ca-leaching of the TEOS-treated sample introduced by the reduction of CH content and the polymerization of C-S-H gel (shown by NMR results) could be two of the critical reasons. XRD results showed that the formation of the gypsum on the surface of cement-based material that cured in Na 2 SO 4 solution could be reduced by the in situ consumption of CH of TEOS, which benefited the reduction of the loss of the compressive strength in the corrosive environment. It can be deduced from this study that the in situ consumption of CH of TEOS on the surface of hardened cement-based material would favor the improvement of the durability, showing its potential for surface-treatment of hardened cement-based material. [ABSTRACT FROM AUTHOR]

This paper presents a feasibility study on the production of ternary blended cements with low-kaolinite calcined coal gangue (CCG) and limestone, considering kaolinite contents varying from 11.7 % to 40.6 % in coal gangue (CG). The ternary systems were designed with 15 %, 30 %, 45 % and 60 % by mass of cement replaced by CCG and limestone with a fixed mass ratio of 2:1. The fluidity and compressive strength of the mortars were measured, followed by the strength efficiency analysis combined with CO 2 emission. The potential hydration mechanism was revealed by analysing the hydration kinetics, hydrates and microstructural evolution of these ternary pastes. The results showed that it is possible to use low-kaolinite CCG and limestone to prepare ternary blended cements with adequate mechanical strength that can fulfill the 28-day strength requirement of 32.5 or 42.5 grade cement. In addition, Increasing the kaolinite content in CG can increase the substitution of cement up to 60 %. Microstructural studies indicated that the compressive strength of these ternary mortars at 3 and 7 days was mainly dominated by cement hydration and the pozzolanic reaction between CCG and portlandite (CH), respectively. The precipitation of carboaluminates from the synergy of CCG and limestone significantly reduced critical pore size and increased the volume fraction of fine capillary pores from 7 days, well supporting the strength development of the ternary mortar. The findings of this work underscores the potential of using low-kaolinite CCG as a supplementary cementitious material (SCM) to reduce carbon footprint in the field of construction, and also in contributing to a high-value green utilization of solid waste. • Good linear correlations are obtained between the strength of ternary mortars and the kaolinite content in CCG. • The pozzolanic reaction between CCG and portlandite supports the strength development of ternary mortars before 7 days. • The synergistic addition of CCG together with limestone enhances the precipitation of carboaluminates. • CCG20LS10 has a higher promising for engineering application with considering CO 2 emission and strength development. [ABSTRACT FROM AUTHOR]

• LC2 can enhance both compressive and flexural strength of slag cement mortars. • LC2 can optimize the phase assemblages of slag cement. • The synergistic effect could be beneficial for the property gain at all ages. The replacement of slag cement with limestone and calcined clay (LC2) is beneficial to expand the sources of SCMs and reduce CO 2 emissions in the global cement industry. In this paper, the synergistic interactions between slag cement and LC2 are studied using multi-technique characterization including TGA, XRD and SEM. The results indicate that the addition of LC2 enhanced both compressive and flexural strength of slag cement mortars. Characterization of the microstructures show LC2 can optimize the phase assemblages of slag cement by stabilizing ettringite whilst promoting the formation of hemicarboaluminate and monocarboaluminate. It is concluded that faster reaction of calcined clay and nucleation effect of limestone compensate for the slow early hydration of slag cement whilst slag contributes to the later performance. The synergistic hydration effects of slag and LC2 offers unique opportunities to reduce CO 2 footprint without compromising early and long-term performances. [ABSTRACT FROM AUTHOR]

Calcium sulphoaluminate cement (CSA) has the characteristics of quick hardening, high early strength and high impermeability, however its strength growth persistence in the middle and late stages (after the age of 3 days) is poor. In order to improve this disadvantage, the pilot production of alite (C3S) modified CSA (AMCSA) clinker was carried out by liquid phase manipulation and barium ion doping technology. The effects of different dosages of gypsum on the hydration and hardening properties of AMCSA, such as setting time, hydration rate, compressive strength and hydration products, were studied. The results show that the mineral content of ye'elimite, C2S, C3S and iron phase in the calcined AMCSA clinker are 48.5 wt.%, 32.6 wt.%, 11.7 wt.% and 7.2 wt.% respectively, which are close to the designed mineral composition. The stable coexistence of ye'elimite and C3S in the same clinker system is realized. The initial and final setting time of AMCSA are retarded with the increasing gypsum dosage. When the gypsum dosage is 15 wt.% under the experimental conditions in this study, the AMCSA mortar reaches the highest compressive strength at every age. The strength of AMCSA mortar at 28 days is still significantly improved compared with that at 3 days, which indicates that the shortcoming of the low strength growth persistence of CSA in the middle and late stages is improved. [ABSTRACT FROM AUTHOR]

This research investigates the influence of colloidal nanosilica (CNS) on the hydration and hardening properties of Limestone Calcined Clay Cement (LC3). The sulfation degree for LC3 was first optimized based on the hydration heat, and the results suggested that gypsum increases the cumulative heat release during the hydration process of LC3, with a dosage of 2% by weight gypsum leading to the highest heat release. The effects of CNS on hydration reaction, fluidity, mechanical properties and microstructure of LC3 were then investigated. According to the results obtained from isothermal calorimetry and thermogravimetric analysis, CNS can considerably accelerate the reaction rate of the LC3 system. 3% and 5% by weight CNS can significantly improve the compressive strength of LC3 blends, especially at early ages of 3 and 7 days. The findings from this study lead to a better understanding of the modification effects of CNS on LC3 which subsequently provides an insight into regulation mechanism of CNS on LC3. • The optimal dosage of gypsum is 2wt% which balances the silicate and alumina reactions in the LC3. • 1% and 3% by weight addition of CNS intensifies silicate peaks, while 5% dosage depresses the silicate peak. • An addition of 5% by weight CNS significantly increases the compressive strength of LC3 system, particularly at early ages. • CNS accelerates the hydration process of LC3. [ABSTRACT FROM AUTHOR]

Highlights • The effects of nanoSiO 2 and ordinary SCMs on sulfate attack resistivity of cement pastes/mortars were studied and compared. • NanoSiO 2 effectively improved the mechanical property of samples subjected to sulfate attack. • The volume expansion and cracking of nanoSiO 2 -added sample was greatly reduced when exposed to sulfate attack. Abstract Durability of cement-based materials served in sulfate environments is a big concern because of the devastating effects of sulfate attack. By taking the advantages of nanoSiO 2 (NS) on the physiochemical properties of cement-based materials, especially at early age, small dosages (1 and 3 wt%) of NS were added into cementitious materials, and the effects of the sulfate attack on the strength development and volume stability were studied. Comparison studies were made with conventional supplementary cementitious materials (SCMs), i.e., silica fume (SF), fly ash (FA) and ground granulated blast-furnace slag (GGBS) at their conventional dosages. Samples were initially standard-cured for 7- and 28-day before curing in 5 wt% Na 2 SO 4 solution for 6 months. It was found that NS effectively improved the early age properties of cement paste/mortar, thus leading to the enhanced mechanical property of samples undergoing sulfate attack and all of which outstood samples containing SF, FA and GGBS. Moreover, after 28-day initially curing, a comparable compressive strength was found in the NS-added sample when compared to FA/GGBS-added sample at 180 days' sulfate attack. At the same time, the volume expansion of cement paste/mortar, as well as the cracking, in the NS-added sample was greatly reduced and the effect was more pronounced in the 28-day initially cured mortar samples. Work in this study shows NS's potential of improving the sulfate attack resistivity of cement-based materials. [ABSTRACT FROM AUTHOR]

To better understand the influences of nano-TiO 2 on the properties of cement-based materials, the hydration process, the compressive strength development, and the drying shrinkage of cement-based materials with the addition of nano-TiO 2 of 25 nm were investigated. Results showed that nano-TiO 2 increased the compressive strength of cement mortar through its acceleration on cement hydration and the pore-refining effect: 5 wt.% nano-TiO 2 accelerated cement hydration process for 2 h and reduced the most probable pore size by 19.4%, 48.5%, and 54.4%, respectively. In addition, the influences of the hydrophilicity of nano-TiO 2 on the water loss and volume stability of hardened cement pastes were primarily investigated and discussed in this work. For a comparison study, nano-SiO 2 of 30 nm size was also used. Results showed that the decline of the contact angle and the refining of pore structure with the addition of nano-TiO 2 slowed down the water loss of hardened cement paste, which resulted in a comparable drying shrinkage to that of the control sample at 1 month age. [ABSTRACT FROM AUTHOR]

The poor frost resistance of supersulfated cement (SSC) is of great concerns for its application. This research assessed the frost resistance performance of SSC with 1 wt. % and 3 wt. % cement clinker activator (C1 and C3) and explored the effects of nano-SiO 2 (NS) (1 wt. % and 3 wt. %, N1 and N3) on the frost resistance of SSC through macro- and micro-evaluation on loss of the compressive strength/the mass/the relative dynamic elastic modulus and XRD/MIP/SEM. Results showed that the frost resistance of C1 sample was superior than those of C3 sample. The addition of NS enhanced the frost resistance of SSC: the maximum mass loss was reduced by about 92.8% and the maximum relative dynamic elastic modulus loss was decreased by about 34.6%. This could be ascribed to the change of the porous microstructure: the portion of micro-pores smaller than 20 nm increased and the microstructure was densified. Moreover, Moreover, NS changes the long needle-like crystalline phase (AFt) shape to a short rod-like shape, the latter of which was more tightly wrapped by the gel phase, leading to a more compact microstructure, and the detachment of AFt from gel matrix (would be introduced by the difference of their contraction/expansion rate under F-T cycles) could result in the poor performance of SSC, while this could be relieved by the reduction of the AFt-to-gel ratio as seen from the NS-added SSC sample. • Nano-SiO 2 (1%) can improve the frost resistance of supersulfated cement (SSC). • The compaction of the microstructure of SSC by Nano-SiO 2 is the reason for the improvement of frost resistance. • The crystalline AFt peels from the C-S-H gel and forms a fragile structure after freeze-thaw cycles. • The poor frost resistance of SSC is related to the higher crystalline-to-gel ratio. [ABSTRACT FROM AUTHOR]