Hydration products, internal relative humidity and drying shrinkage of alkali activated slag mortar with expansion agents

Abstract In this paper, two types of composite expansion agents, CSAE (calcium sulphoaluminate type expansion agent) with more calcium sulphoaluminate than CaO, and CE (CaO type expansion agent) with more CaO than calcium sulphoaluminate, were used to improve the drying shrinkage of AAS (alkali activated slag) system, separately. For comparison, SRA (shrinkage reducing agent) was used for the same purpose as well. Results showed that the addition of CSAE, CE and SRA in AAS mortar reduced the drying shrinkage by about 41–45%, 54–56% and 35–44% at 56 d, respectively. At late ages, the IRHs in the three groups with admixtures were all higher than the reference group, though the addition of CSAE caused a significant decline of IRH (internal relative humidity) in AAS mortar at the first three days. Compared to CSAE, CE showed a better effect in drying shrinkage control due to the comprehensive effect of both low water consumption of CaO hydration and the high elastic modulus of CH in hydration products. It is found that drying shrinkage was linearly related with IRH in AAS, and the incorporation of the admixtures reduced the sensitivity of drying shrinkage to IRH in AAS mortar.