Effects of Portland cement on activation mechanism of class F fly ash geopolymer cured under ambient conditions

This study addresses the influence of a small Portland cement addition on the reaction kinetics and phase evolution of ambient cured class F fly ash-based geopolymer. The results show that setting time and reaction kinetics can be controlled by tailoring the precursor and activator characteristics, primarily the Ca/Si molar ratio. Calcium ions, dissolved from cement phases, contribute to a faster precipitation of products and in consequence, to a denser microstructure of composite pastes and a higher early age mechanical performance. However, excessive cement content impedes the gel formation due to hindered hydration of cement in high alkalinity at later ages and low compatibility between calcium deficient and calcium rich phases which lower the silica availability. It is found that the optimal dosage of cement in fly ash F based geopolymeric system is depending on the total silica content, 5 wt% and 7.5 wt% for silicate-based activator with a silica modulus of 1.2 and 1.5, respectively.