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• Ca(OH) 2 reduced the efflorescence extent of alkali-activated slag(AAS) mortar. • The mitigation mechanism of Ca(OH) 2 on the efflorescence of AAS mortar was analyzed. • Ca(OH) 2 decreased the permeability and ion leaching capacity of AAS. • The mechanical properties of AAS mortar were enhanced by addition of Ca(OH) 2. Due to the high alkali concentration, weak binding of Na+ and relatively open microstructure, the efflorescence of alkali-activated slag (AAS) cement is a critical barrier limiting its widespread application. To reduce the efflorescence, Ca(OH) 2 was added as a Ca-rich precursor into the binder with the dosage of 3, 6 and 9 wt% by slag. The efflorescence of AAS mortars was identified by visual inspection and mass weighing of efflorescence products formed on the surfaces of the mortar. Capillary porosity, water absorption rate, Na+ leaching test, XRD and TG analysis were also carried out to explain the change of microstructure and hydration products caused by the incorporation of Ca(OH) 2. The results revealed that the addition of Ca(OH) 2 significantly reduced the coverage areas and the amount of efflorescence products. However, visual inspection results were not consistent with that of mass weighing of efflorescence products, ion leaching, etc. If the impact of efflorescence on microstructure should be considered, visual inspection is not suitable. It can be concluded that the addition of Ca(OH) 2 can mitigate the efflorescence by following reasons: firstly, the capillary porosity decreased from 16.86% to 15.22% and the water absorption rate decreased from 0.0366 mm/s1/2 to 0.0179 mm/s1/2 by adding 9% Ca(OH) 2 , which means that the migration pathway of the Na+ and water within the mortar was decreased. Secondly, the leaching results indicated that fewer Na+ leached from the Ca(OH) 2 -added samples, either from powder leaching or block leaching, which means that the addition of Ca(OH) 2 may enhance the binding of Na+ by hydration products through increasing the C-A-S-H gel content. [ABSTRACT FROM AUTHOR]

This study examines the potential of using calcium‐rich precursors to improve the strength of alkali‐activated slag (AAS) cement at low temperature. To achieve this target, the effect of calcium‐rich precursors on the early strength development of AAS mortars cured at 5°C was assessed. The strength development was investigated along with its internal temperature, microstructure and reaction products. It is found that the calcium‐rich precursors can improve the early strength of AAS around 35% and the group with Portland cement clinker gave the highest strength. Calcium oxide can also increase the compressive strength of AAS cement, but its effectiveness is not as good as the sample with clinker. Adding calcium sulfoaluminate can increase the early strength, while no significant contribution to 28‐day strength could be found. The results highlight that compressive strength of modified AAS mortar can reach 15 MPa at 6 h and 20 MPa at 24 h at 5°C, suggesting that this binder could be a valuable alternative for low temperature construction. [ABSTRACT FROM AUTHOR]

In this study, the sulfate attack on mortars was accelerated using electrical pulse as an external electrical field. Meanwhile, the specimens with the same mix proportions were immersed in sulfate solution as comparison. The sulfate concentration and mechanical properties, including flexural and compressive strengths, were measured to evaluate the effects of sulfate attack. In addition, the transformation of microstructural nature and phase of the specimens were evaluated by scanning electron microscope and X-ray diffraction. The results showed that electrical pulse greatly accelerated the penetration of sulfate ions into the mortars and resulted in more ettringite and gypsum formed, leading to a greater depth of sulfate reaction. In electrical pulse test, the sulfate content in MgSO 4 solution was greater than that in Na 2 SO 4 solution, which was the opposite case in immersion test. Moreover, the electrical pulse resulted in significant mechanical strength losses in a much shorter period as compared with those in immersion test. These findings indicated that the electrical pulse accelerated sulfate attack could be a new acceleration test for evaluating the sulfate resistance of cement-based materials. [ABSTRACT FROM AUTHOR]

• Effect of Na 2 CrO 4 on the hydration kinetics of blast furnace slag were studied. • The Na 2 CrO 4 showed a strong alkaline activating on the blast furnace slag. • The compressive strength could be significantly improved by adding Na 2 CrO 4. • The main reaction products were C-(A)-S-H and CrO 4 -U phase. • A large number of alkalis was released during the reaction between slag and Na 2 CrO 4. Blast furnace slag (BFS) is a potential candidate for the immobilization of chromium-bearing wastes. This paper investigated the effect of soluble hexavalent chromium (Na 2 CrO 4) on the early hydration behavior of BFS by means of isothermal calorimetry, X-ray diffraction (XRD), thermogravimetry (TG), scanning electron microscopy (SEM) and pH measurement of pore solution. Meanwhile, the compressive strength of BFS pastes in the presence of different dosage of chromate was tested for the structural study. The primary aim was to verify the alkaline activation of Na 2 CrO 4 on the BFS. It was found that the BFS in water had scarcely hydrated in the first 7 days, whereas the addition of Na 2 CrO 4 greatly accelerated the hydration of BFS and promoted the hardening. Increasing the Na 2 CrO 4 dosage could significantly improve the compressive strength, which was sufficient to meet the requirement for immobilization of wastes. The hydration products of BFS in Na 2 CrO 4 solutions were mainly C-(A)-S-H and CrO 4 -U phase. The CrO 4 -U phase played a key role in Cr(VI) binding due to its layered structure. During the formation of CrO 4 -U phase, a large number of alkalis was released, which largely increased the pH of pore solution, thus greatly promoting the BFS hydration. The results indicated that the Na 2 CrO 4 had a strong alkaline activation on blast furnace slag. [ABSTRACT FROM AUTHOR]

In the course of an investigation on using calcium stearate (CaSt) to improve performance of the alkali-activated slag (AAS) cement, the objective of the present work is to discovery its role in the AAS system. Special interest is devoted to understand the influence of CaSt on the reaction process, reaction products and microstructural features of the AAS cement. To achieve this, isothermal calorimetry, impedance characteristics, infrared spectroscopy, X-ray diffraction, thermogravimetry, nitrogen sorption, mercury intrusion porosimetry and scanning electron microscopy were carried out. According to results obtained, the CaSt has three important effects on the AAS cement. Firstly, it inhibited slag reaction with the activator through decreasing activity of alkalis, whereas the amount of C-(A)-S-H gels in the system depended on the usage of CaSt, because the CaSt could have chemical reactions with the alkali-solution and form similar reaction products. Secondly, there is less sodium and more calcium in reaction products of the CaSt added mix, which improve their stability and uniformity. Finally, microstructure characteristics (e.g. pore size distribution, pore connectivity) are optimised and defects are reduced significantly, when CaSt is added in the AAS mix. [ABSTRACT FROM AUTHOR]

The study presents the shrinkage behaviour of Ca(OH) 2 -added alkali-activated slag (AAS) concrete. A series of tests were performed to determine the plastic shrinkage, autogenous shrinkage and drying shrinkage. In order to explain shrinkage mechanisms, the moisture loss was monitored; the pore structure was examined by nitrogen adsorption test; hydration process was assessed by isothermal calorimetry; hydration products and microstructure were characterised by XRD, SEM/EDS and TG/DTG. It was found that the use of Ca(OH) 2 increased the plastic shrinkage of AAS concrete, but reduced autogenous and drying shrinkage. This is because the addition of Ca(OH) 2 accelerated the hydration at very early stage, while non-noticeable impact could be identified after 120 h. The results from nitrogen adsorption test showed a decrease in mesopores by adding Ca(OH) 2 , which was responsible for the less drying and autogenous shrinkage. Meanwhile, the Ca/Si of C-S-H (I) increased from 0.89 to around 1.40 at 28 d when adding 5% and 10% Ca(OH) 2 . XRD patterns indicated C 2 ASH 8 and C 4 AH 13 were found in Ca(OH) 2 -added samples. These changes in hydration products also contribute less drying and autogenous shrinkage in Ca(OH) 2 -added samples. [ABSTRACT FROM AUTHOR]

The damage process of plain and blended cement mortars subjected to sulfate attack under electrical field was investigated in this paper. The deterioration of mortars was investigated by measuring the loss of compressive strength. To identify the changes of microstructure and mineral phases after the test, scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction analysis were performed on the selected samples. The results indicated that compared with sulfate attack alone, the combination of sulfate attack and electrical field accelerated the damage process of mortars, which can be explained through two different mechanisms. On the one hand, Ca 2+ ions directionally moved and then leached out from mortars under the electrical field, leading to the dissolution of portlandite as well as the decomposition of C–S–H gel at the later stage. On the other hand, the electrical field accelerated the migration of sulfate ions and then they reacted with hydration products to form massive ettringite and gypsum, which resulted in the microcracks and strength loss of mortars. Moreover, the deterioration of the mortar blended with fly ash was still visible in spite of its better chemical resistance. The strength loss of limestone powder incorporated mortar increased in comparison with Portland cement mortar as a result of an increase in porosity. [ABSTRACT FROM AUTHOR]

• Proper use of LS can increase fluidity and prolong setting time without losing strength. • LS can affect reaction products of AAS and its function depends on mix proportions. • Variations of pore structure of modified AAS cement relies on the dosage of LS. • Around 30% hydration degree of LS was achieved at 28 d for AAS with 40% LS. This study aims at discussing physical and chemical influence of limestone powder (LS) on the AAS system. General properties, including fluidity, setting time, drying shrinkage, and mechanical strength of AAS pastes/mortars, were examined. Pore structure change and phase assemblages of LS-AAS composites were studied. The results showed that the addition of 50 wt% LS increased the fluidity and setting time of AAS pastes, but reduced the total drying shrinkage of AAS mortars. Meanwhile, around 30% reduction in mechanical properties were found at the same replacement ratio. The pore structure was refined in the pastes with LS replacement<25 wt%, but was coarsened in the sample with 40 wt% LS. Limited new phase was formed in the sodium silicate activated slag samples, which was believed to depend on the modulus of the activator. Around 30% hydration degree of LS was achieved in 40 wt% LS-replaced AAS pastes hydrated for 7 ~ 28 d as determined by TG-DSC analysis. The main drawbacks of AAS, fluidity, setting time, and drying shrinkage, are improved by using high-volume LS, so use of high-volume LS in AAS system can be an option for the purpose of environmental protection. [ABSTRACT FROM AUTHOR]

• Chloride transport coefficient of alkali-activated slag (AAS) cement could be predicted using electrical responses. • Temporal variation of strength, pore solution and transport properties of AAS is assessed. • AAS cement is more sensitive to mix proportions and additional care is needed to achieve high quality. Relatively few studies currently exist concerning temporal variation of the overall performance of alkali-activated slag (AAS) cement; a topic essential for quality control in practice. This study uses microstructural characteristics and electrical responses to reflect performance variation of AAS cement. To achieve this, four key performance parameters were assessed, pore formation factor, chloride transport coefficient, permeability coefficient and compressive strength. Classical theories used for PC-based materials were applied to estimate the performance of AAS paste. It is found that the formation factor of AAS can be assessed by combining bulk conductivity and pore solution conductivity and the permeation properties of AAS cement cannot be reliably assessed by the Katz-Thomoson (KT) model, while the Millington-Quirk (MQ) model works better. To assess the relationship between compressive strength and permeable porosity, the Balshin and Ryshkevitch equations performed best in terms of describing the relationship between these two parameters. Meanwhile, AAS cement is more sensitive to the mix proportions and additional care is needed to ensure its quality. It is the first time to use electrical response to predict the chloride transport coefficients of AAS cement, which would provide a powerful tool for its quality control and service life prediction. [ABSTRACT FROM AUTHOR]

Fourteen raw slag samples were collected from different steel companies from China, using which setting, mechanical and early-age microstructural behaviours of alkali-activated slag (AAS) cements were investigated. To assess the reaction process between ground granulated blast furnace slag (GGBS) and activator, isothermal calorimetry tests were conducted on three selected AAS pastes for 50 h, while TG, FTIR, and Raman tests were conducted to characterise the early microstructural changes. Main findings include that the setting behaviour of AAS pastes depends primarily on the fineness and chemical compositions of GGBS, and statistical analysis results indicate that three of the constituents namely, SiO 2 , MgO and K 2 O, have a strong impact compared to the rest constituents. It is also found that commonly-used parameters to describe characteristics of GGBS are not sufficient to predict setting behaviours of AAS paste made by different GGBSs. Furthermore, the 28-d compressive strength of AAS mortars cannot be considered as a reliable indicator to assess its reaction potential at early age, at least, has no relationship to its setting behaviour. The early age microstructural analysis indicates that one possible reason behind the huge different setting and early age compressive strengths of AAS pastes is the variation in the releasing speed of Ca ions. [ABSTRACT FROM AUTHOR]