Effect of sulfoaluminate expansive additive on mechanical properties, internal relative humidity, and shrinkage of early-age mortar

Incorporating a suitable amount of sulfoaluminate expansive additive (SEA) into cementitious materials can compensate the shrinkage deformation. However, the influence of SEA on the relative humidity inside cementitious materials is still unclear. This study conducted a series of tests to investigate the mechanical properties, internal relative humidity, and shrinkage of early-age cement mortars containing different contents of SEA. Then finite element simulations of the drying of cement mortars with different contents of SEA were conducted to derive the moisture diffusion coefficient. The results reveal that the amount of expansive ettringite generated in cement mortar tends to increase with increasing SEA content, which can decrease the porosity of cement mortar and refine its pore structures. Both the compressive strength and flexural strength of cement mortar decrease with increasing SEA content. The decrease in mortar strengths is more significant during the later age. The mortar shrinkage under both sealed and drying conditions decreases with increasing SEA content. However, the compensation effect of SEA on the shrinkage deformation is less effective under drying condition. The relative humidity inside the cement mortar under sealed condition would decrease by 9.3% at 36 days when 8% SEA was used. While the decrease would be 3.4∼7.2% when the cement mortar was exposed to drying condition with RH of 60%. Moreover, since SEA densifies the microstructures and refines the pores in mortar, the moisture diffusion coefficient of mortar decreases with increasing SEA content.