Influence of fly ash on the pore structure and shrinkage characteristics of metakaolin-based geopolymer pastes and mortars

This study investigates the pore structure and shrinkage behavior of metakaolin-based geopolymer pastes and mortars containing 0-30% fly ash. Fly ash substitution decreases average reactivity of the solid precursors, resulting in a lower reaction rate and accompanying longer reaction time. Composition of the sodium aluminosilicate (N-A-S-H) gel formed in the geopolymers has been changed, and the continued reaction after hardening of the pastes generates a more compact binding gel phase with lower Al/Si ratio. Refinement of the pore structure entails a higher capillary tension developed in the binders to increase the autogenous shrinkage, but also restricts the water evaporation from the pore networks, resulting in a decreased drying shrinkage. The geopolymer mortars provide higher compressive strength and lower total shrinkage when compared with the pastes.