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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]

• 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]

• 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 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]

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]