Shrinkage Characteristics of Alkali-Activated High-Volume Fly-Ash Pastes Incorporating Silica Fume

Abstract

The alkali-activation of high-volume fly ash (HVFA) produces a low-carbon sustainable cementitious binder with remarkable strength and chemical durability at room temperature. In this work, the influence of activator type (i.e., deionized water, NaOH, Na2SO4, Na2CO3 solutions), silica fume incorporation, and curing duration on the phase assemblage and strength and shrinkage developments of HVFA is studied. The results show that Na2SO4-activated HVFA shows the highest strength achievement, followed by Na2CO3, NaOH, and water. Alkali incorporation in HVFA considerably increases the shrinkage magnitude and is mainly attributed to the detrimental effect of alkalis on the viscoelasticity and stiffness of cementitious solids. The HVFA activated by the NaOH solution shows the largest shrinkage, followed by Na2CO3 and Na2SO4 solutions, despite comparable moisture loss. The curing duration does not significantly affect the shrinkage development of alkali-activated HVFA without silica fume. However, with silica fume incorporation, extending curing duration considerably reduces the shrinkage of alkali-activated HVFA. The phase and molecular analysis suggest that silica fume noticeably retards the reaction of HVFA systems but tends to increase the level of silica polymerization in gel products, regardless of activator type.