Influence of alkaline on sodium sulfate activated slag: strength, phase assemblage and microstructure

Abstract

Abstract Alkali-activated slag (AAS) is gaining popularity as a low-carbon and energy-saving cementitious material. The use of Na2SO4 as an activator in AAS is a promising strategy that has yielded relatively favorable results. However, Na2SO4-activated slag have low compressive strength due to the low initial pH, which limits the consumption of AAS. In this study, the effect of Na2SO4 dosage and NaOH concentration on the compressive of Na2SO4-activated slag pastes were investigated. The phase assemblage and microstructure evolution of hardened pastes were analyzed by X-ray diffraction (XRD), thermogravimetric analysis (TGA) and scanning electronic microstructure (SEM). The results indicated that the 28 days compressive strength of AAS pastes developed progressively with the increase of Na2SO4 dosage, while the effect of NaOH on the compressive strength was related to the concentration of NaOH due to the changes of phase assemblage and microstructure. In the AAS pastes activated by sole Na2SO4, the main hydration products included C-(A)-S-H, ettringite and hydrotalcite-like phase. The addition of NaOH could promote the hydration of AAS, but inhibited the formation of ettringite, and the two mechanisms worked together to affect the compressive strength. When the NaOH concentration was less than 0.75 mol/L, the amount of ettringite decreased with the increase of NaOH concentration, and the amount of SO42− rich hydrotalcite-like phase gradually increased, which was much smaller in size, leading to a decrease of the compressive strengths of the pastes. After the NaOH concentration increased to more than 0.75 mol/L, the high concentration of OH− played a dominant role in promoting the hydration of AAS, leading to the improvement of compressive strength.