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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 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]

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]

• 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]

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]

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]