Effect of Activator and Mineral Admixtures on the Autogenous Shrinkage of Alkali-Activated Slag/Fly Ash

The high autogenous shrinkage of alkali-activated fly ash/slag (AASF) poses a significant concern for the widespread application of AASF in structural engineering. The present study compares the efficacy of activator and mineral admixtures in mitigating the autogenous shrinkage of AASF, and discusses the underlying mechanism. The results show that the use of activators with a lower silicate modulus and a lower sodium content, as well as incorporating metakaolin (MK) or silica fume, can reduce the autogenous shrinkage of AAMs. These approaches delay the appearance of the second exothermic peak, which corresponds to the later formation of C-A-S-H gels and slower development of capillary pressure. The inclusion of MK not only retards the reaction but also facilitates the formation of N-A-S-H gels, resulting in a coarse pore structure and reduced water consumption. The use of the activator with a lower silicate modulus (reduced from 1.5 to 1.0) leads to a higher internal relative humidity and the reduced pore volume of silt-shaped and ink-bottle pores (2–50 nm) in AASF, thereby reducing the autogenous shrinkage without significant strength reduction.