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In this study, polydimethylsiloxane (PDMS) was applied for overall hydrophobic modification of alkali-activated slag (AAS) mortars aiming to reduce its high water sorptivity. The effects of the amount and viscosity of PDMS on the water absorption characteristics and mechanical properties of AAS mortars were investigated. Meanwhile, MIP, the surface contact angle of AAS mortars and SEM-EDS, Inverse gas chromatography (IGC), FT-IR, XRD of hardened AAS pastes were performed for analyzing the modification mechanisms. The results indicated that the water sorptivity of PDMS-modified AAS mortar decreased by 90 %, and the contact angle of the inner surface increased from 14° to 128° when the dosage of PDMS reached 8 % (by weight of binder). The main modification mechanisms of PDMS in AAS system can be summarized as: Firstly, the formation of hydrophobic coatings on the surface due to the cross-linking reaction between C-S-H gel and PDMS reduces the surface energy and the hydrophilicity of hardened AAS pastes, achieving chemical modification; Secondly, the hierarchical structures formed by micro papillary increases the surface roughness, thus enhancing the hydrophobic effect further through physical modification. Moreover, the coarsening of capillary pores also reduces the capillary force during water absorption process. Meanwhile, the addition of PMDS maintains comparable strength development of AAS mortars, which is beneficial for the practical application. • PDMS-modified AAS mortars converted from hydrophilic to hydrophobic porous materials. • A 90 % reduction of water sorptivity of mortars was achieved by addition of PDMS. • The hardened PDMS-modified AAS pastes possessed lotus-leaf-like microstructure formed by micro papillary. • The PDMS-modified mortar exhibited stable strength development. [ABSTRACT FROM AUTHOR]

This paper reports the first structural application of site-cast, alkali-activated slag concrete in China. Special attention was given in this office building project at Chongqing Jianke to quality control, construction procedure and assessment. The experience showed that quality and costs could be successfully managed through careful control of raw materials, suitable selection of admixtures, proper mix proportions and an efficient construction team. It was also found that alkali-activated slag concrete has the potential for use in cold weather. The ambient temperature during construction was at times below 5°C, yet no specific procedures were needed to maintain the temperature or provide additional heat. The project is a useful reference for future structural use of alkali-activated slag concrete. [ABSTRACT FROM AUTHOR]

This study investigated the immobilization potential of alkali-activated slag (AAS) pastes for Cr(VI) by examining compressive strength, toxicity characteristic leaching procedure (TCLP) and generalized acid neutralization capacity tests. Alkaline digestion, total acid digestion, XRD, FTIR and SEM-EDS were carried out to clarify the immobilization mechanism. The AAS pastes gave high compressive strengths, which contributed to the physical encapsulation of Cr. The addition of Cr(VI) induced an increase in compressive strength compared to Cr(VI)-free pastes. The leaching results showed that AAS pastes exhibited effective immobilization for Cr(VI) and that the leachability was strongly dependent on the NaOH dosage, water to slag ratio, initial Cr(VI) content, leachate pH and curing duration. The initial Cr(VI) content up to 1.5 wt% by weight of slag was well immobilized with the total-Cr leachability below the TCLP regulatory limit of 5 mg/L. The digestion results demonstrated that the reduction of chromium from hexavalent to trivalent played a significant role in the immobilization of Cr(VI) in AAS without any additional reductants. The XRD and SEM-EDS results suggested the formation of a clear CrO 4 -U phase as the primary retention phase for unreduced Cr(VI). Therefore, alkali-activated slag binder is effective in the immobilization of Cr(VI)-bearing wastes. [ABSTRACT FROM AUTHOR]

The electrochemical tests commonly used for evaluating reinforcement corrosion in Portland cement-based concrete may not be appropriate for testing alkali-activated slag (AAS) concrete due to its different pore structure and pore solution composition. In this article, corrosion behaviour of the steel bars in 12 AAS concrete mixes with different alkali concentration and modulus of sodium silicate solution was monitored by using gravimetric mass loss measurement. The results obtained from other electrochemical tests were compared to the gravimetric mass loss to determine their suitability for assessing the corrosion of steel in AAS concrete. It was found that only 7.8–28% of the mass loss was accounted for by the macrocell current for AAS concretes, indicating that this type of test underestimates the corrosion of the steel in AAS concrete. The steel bars in the AAS concretes gave a much higher negative half-cell potential value, presumably due to the influence of sulfides. In summary, it has been established that the criteria for assessing the onset and progress of corrosion of steel in Portland cement concrete are not suitable for the same purpose in the range of AAS concretes studied. [ABSTRACT FROM AUTHOR]

• 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]

[Display omitted] • The cohesion interactions mainly determine plastic viscosity of AAS pastes. • The greater attractions between particles is the reason for its higher viscosity. • The friction interactions mainly determine the yield stress of AAS pastes. The workability and pumping performance of alkali-activated slag (AAS) concrete are affected by its rheological properties, which are related to the interactions between slag particles in the pastes. The rheological properties of AAS paste with different equivalent sodium oxide (Na 2 O eq) dosage activated by sodium hydroxide (NaOH) and sodium silicate (waterglass) with different SiO 2 /Na 2 O molar ratios were investigated in this study. The rheological mechanism was studied from the cohesion and friction interactions by calculations from parameters such as the Zeta potential of particles in the pastes, surface energy, hydrogen bond, and friction coefficient of the alkaline activators. The results indicated that the yield stress of AAS pastes was lower than that of the paste made with slag and water, whereas the plastic viscosity was higher. The yield stress of AAS pastes was mainly determined by the friction interactions between particles. The plastic viscosity was mainly affected by attractive interactions, including van de Waals interactions and Lewis acid-base interactions. The electrostatic repulsion interactions were felt at a very short distance because the AAS pastes were systems with the high ionic strength, which means that the electric double layer were severely compressed, therefore, the attractive interactions were predominant. Taken together, these results suggest that the improvement of plastic viscosity of AAS pastes should be achieved by changing the attractive interactions. [ABSTRACT FROM AUTHOR]

Ground granulated blast-furnace slag (GGBS) is widely used as supplementary cementitious material of interest for adjusting the performance of concrete. Unlike standard commercial products, characteristics of GGBS from different iron manufacturers vary significantly and assessment of its reactivity, especially at an early age, is not well-established despite numerous parameters specified in current standard systems. Three GGBSs with different setting features during the alkali-activated process were dissolved in either deionised (DI) water or NaOH solution in this study. To illustrate the fundamental processes of reaction kinetic, the dissolved ion concentrations, glassy structural changes, and chemically bound water were measured using ICP-OES, FTIR, and TG respectively. Meanwhile, thermodynamic calculations were carried out to estimate the precipitation of C–S–H gels, which is directly linked with setting behaviours. The results clearly show the differences in the dissolution of the three GGBSs, which could be used to indicate the activity of GGBS. The GGBS with higher activity (i.e. faster setting during alkali activation) tends to dissolve evenly in DI water, whilst those with lower activity show that the silicate groups with lower polymerisation dissolved faster. The chemically bound water determined by TG analysis and the effective saturation index (ESI) are promising parameters for reflecting the activity of GGBS during dissolution in the alkali media. Although this study demonstrated the dissolution rate of higher polymerised silicate groups in GGBS should be the root for the differences in its activity, more structural information on the glassy phase and the interactions of alkali with the surface of GGBS still require further understanding. It is speculated that the ratio of Al[IV]/Al[V] plays an important role in the activity of GGBS. This finding may shape a certain basic understanding of how GGBSs dissolve in neutral and high pH solutions and highlight a re-evaluation of assumptions in thermodynamic simulation under different conditions. [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]

The chemical compositions of the aqueous phases in alkali-activated slag (AAS) paste and Portland cement (PC) paste were determined up to 28 d with the aim of obtaining a better understanding of the stability of the hydration products in the two binder systems. The saturation levels with respect to the hydration products of PC and AAS were obtained by thermodynamic modelling. The main findings of this study were: the effective saturation index for portlandite in the AAS system was always below zero and the sulfate-bearing phases were not stable in the AAS system compared with those in the PC; strätlingite and hydrotalcite phases were stable in the AAS paste due to the high magnesium and silicon concentrations in the pore solution; both the ionic strength and alkalinity of the AAS pore solution were higher than those of the PC pore solution, which were responsible for more severe efflorescence in the AAS paste and the higher conductivity of the AAS pore solution. According to thermodynamic estimations, tobermorite-based C–S–H was dominant in the AAS system after 7 d, while jennite-based and tobermorite-based C–S–H gels were present in the PC system up to 28 d. The results suggest that in PC and AAS pastes, different solid phases are formed during the hydration, which change with time, and the reactions and equilibria in both binders are completely different. [ABSTRACT FROM AUTHOR]

Alkali-activated slag (AAS) has attracted huge attention worldwide for its low CO 2 emission. However, the high water sorptivity of AAS is an obstacle before its practical application, while conventional methods can hardly solve this issue. In this study, the water sorptivity of AAS was decreased using a novel admixture, and the admixture shows a little effect on compressive strength at long ages. In order to clarify causes for its working mechanisms, the reaction process, reaction products and pore structure were investigated. The results suggest that sodium stearate (NaSt) plays an instrumental role in reaction process of AAS. Primarily, Na+ can provide a high alkaline environment, which is conducive for the dissolving of slag and increases compressive strength at the long term. Secondly, C 17 H 35 COO− can dramatically limit the movability of ions in the liquid phase due to its long molecular chain and thus, regulates precipitation of coagulation structure, improving the of ordering structure of C(N)-A-S-H and promoting formation of hydrotalcite-like phases (LDHs), from which the pore structure is upgraded by coarsening the pore diameters at the level of harmless and less harmful pores and decreases the pore connectivity. [ABSTRACT FROM AUTHOR]

• The rheological properties of AAS pastes have been improved by low surface free energy mineral admixtures. • The surface free energy properties of low surface free energy mineral admixtures have been determined by IGC method. • The interparticle forces were calculated to reveal the mechanism for rheological improvement. This study investigates the effect of low surface free energy mineral admixtures (CaF 2 -dominated fluorite (FL) and fluorine-containing lithium slag (LS)) on the rheological behavior of alkali-activated slag (AAS) pastes. The fluidity, rheological behavior, surface charge properties and surface free energy properties were tested. The interparticle forces based on extended Derjaguin-Landau-Verwey-Overbeek (EDLVO) theory were calculated to understand the mechanism for the rheological behavior improvement. The results revealed that both FL and LS could improve the fluidity and decrease the yield stress and plastic viscosity of AAS pastes. Furthermore, the attraction is manifested by the sum of electrostatic, van der Waals, and Lewis acid-base forces, and this value decreases with increasing LF or LS content, implying that the particle dispersion in AAS pastes is improved. Furthermore, observing flocculent structures captured by the metallographic microscope proves the better dispersion of AAS pastes after adding FL and LS. Moreover, incorporating LS or FL decreases the flexural and compressive strengths of AAS mortars. [ABSTRACT FROM AUTHOR]

• Adding 0.5 wt% calcium stearate can reduce 25% of the autogenous shrinkage of AAS; • The use of stearates can significantly reduce the internal capillary pressure of AAS; • The dynamic elastic modulus of AAS will reduce by 20% after adding stearates; • Elastic deformation only accounts for 10% of the total autogenous shrinkage of AAS. The autogenous shrinkage of the stearates-modified alkali-activated slag (AAS) mortar was investigated in this study. The experimental results suggested that the maximum reduction (25 %) of autogenous shrinkage of AAS can be achieved by adding 0.5 wt% calcium stearate, while potassium stearate and sodium stearate seem less effective in decreasing shrinkage. According to the results obtained, the mechanisms of shrinkage mitigation are mainly associated with the retardation of hydration, the coarsening of pore structure, and the increased contact angle. It is estimated that the capillary pressure of pores was reduced by 30 % ∼ 60 % when stearates were used. However, the elastic modulus of AAS was reduced by 20 % due to the bubble-stabilizing effect of stearates, especially at higher dosages, which would partially offset its decreasing effects on autogenous shrinkage. It is also found that the traditional physical model could be used to predict the autogenous shrinkage of plain and modified AAS samples, provided the effect of both elastic and creep parts is considered. [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]

Increasing attention is concentrated on the alkali-activated slag (AAS) as an alternative binder owing to its low environmental impact and high mechanical strength, whereas the carbonation resistance of AAS can be quite problematic. In this work, calcined Mg/Al layered double hydroxide (CLDH) was synthesized and used to improve the carbonation resistance of AAS mortars activated by NaOH. The effect of CLDH on the carbonation depth, mechanical strength, and microstructure before and after carbonation was investigated. The results show that CLDH reconstructed the lamellar structure and formed a hydrotalcite-like phase because of its memory effect. The hydrotalcite-like phase acted as a buffer system to absorb CO 2 intruding into the AAS mortars from the atmosphere, which retarded the decalcification of C-(A)-S-H. Meanwhile, the porosity decreased and the mortars become denser by the addition of CLDH, which prevented the diffusion of CO 2 into the mortars. The compressive strength of AAS mortars was enhanced by the carbonation regardless of CLDH addition. Calcite and vaterite were the main carbonation products. The addition of CLDH did not significantly change the polymorphs of carbonates but mitigated their production. • The effect of CLDHs on the carbonation depth of AAS mortars is studied. • The effects of CLDHs on the phase composition and microstructure of AAS are discussed. • The CO 3 2− is absorbed by the process of CLDHs recrystallized to LDHs. • The porosity of AAS mortars is significantly reduced by adding CLDHs. • Adding CLDHs effectively reduces the susceptibility of AAS mortars to carbonation. [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]

This study investigates the long-term (570 days) performance of alkali-activated slag (AAS) mortar exposed to combined wet-dry cycles and sodium sulphate solutions (i.e. 5 wt% and 10 wt%). Physical and mechanical characteristics of AAS mortars (i.e. visual appearance, compressive/flexural strength, mass change, capillary porosity, water sorptivity) as well as mineralogical and chemical parameters were determined using XRD, FTIR, DSC and BSE. Findings were compared to Portland cement (PC) and high sulphate resistant (HSR) samples. Results indicate that AAS mortars perform better than PC and HSR samples with minimal changes to compressive strength at 570 days (1.7% increasement). The main failure mode for AAS mortar was external spalling, which could be due to the crystallisation/dissolving pressure of sodium sulphate. Moreover, the results indicate key differences in the deterioration mechanism of AAS. Unreacted slag, exposed during sodium sulphate attack under wet-dry cycles, can continue to react in sodium sulphate to form silicon-rich gels. The formation of highly siliceous gel regions has beneficial impacts, such as the increase in flexural strength. While no calcium sulphate phases were detected via XRD and FTIR after 570 days of exposure, it is evident that the molecular changes in the microstructure reveal depolymerisation and enhanced formation of Si–O phases after long-term sodium sulphate exposure. These results are important to understand the long-term degradation mechanisms of AAS materials exposed to sodium sulphate under wet-dry cycles. [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]