Clean and low-alkalinity one-part geopolymeric cement: Effects of sodium sulfate on microstructure and properties

Abstract One-part geopolymer similar to conventional Portland cement by just adding water is a clean cementitious material. Several commonly used activators include powdered sodium metasilicate and sodium hydroxide whose productive processes consume huge energy and emit a lot of greenhouse gases. In this study, sodium sulfate was introduced into one-part geopolymer in order to replace partial Na2SiO3-anhydrous, and its effects and mechanisms were studied by property measurements and microscopic characterization. Results indicate that the strength reduces with increasing the replacement level of sodium sulfate. Sodium sulfate reduces the initial rate of heat release and prolongs the induction period to some extent. At the initial stage of hydration process, a portion of sodium sulfate exists in the form of phase with crystal water, which lead to the initial volume expansion. And then, sodium sulfate takes part in hydration which is detected by XRD analysis, even though no new hydration products can be detected in geopolymer. Sodium sulfate decreases the drying shrinkage remarkably due to the early-term volume expansion generated by the phase change of sodium sulfate. The subsequent drying shrinkage leads to a certain internal stress which can explain more micro-cracks appearing in geopolymer with sodium sulfate observed in SEM photos. Not only that, the presence of sodium sulfate increases the total porosity, especially the proportion of harmful pores in one-part geopolymer. Two indexes synthesizing compressive strength and total CO2-e emission were employed to evaluate the cost and environmental implication. Sodium sulfate was proved to be a more suitable activator to prepare cleaner one-part geopolymer because the potential CO2 emission drops at least 20% after adding sodium sulfate. Even though, the more effective activator system which can decrease the use of Na2SiO3-anhydrous and drying shrinkage further and the corresponding mechanisms should be studied in deeper insight. The method to induce producing the more homogeneous microstructure in low-alkalinity one-part geopolymeric cement is also a novel research field.