Influence of biomass fly ash additive on blast-furnace cement hydration and hardening

The aim of this study is to investigate the possibility of using biomass fly ash (AFA) as an additive for blast-furnace cement, where 5, 10, 15, 20 and 25% (by weight) of the Portland cement is replaced by AFA. The specimens are then hardened for 7, 28 and 90 d under normal conditions. It is found that the AFA additive accelerates the initial hydration of blast-furnace cement and promotes hydration of the slag component, but has a negative effect on the early compressive strength of samples. Nevertheless, at a later period of hardening (after 28 and 90 d of hydration), AFA additive (up to 10 wt%) increases the compressive strength of the samples. This effect is related to the formation of gismondine (CaAl₂Si₂O₈·4H₂O) and a significant amount of calcium silicate hydrates. It is determined that AFA activates the latent hydraulic properties of blast-furnace slag, but the slag pozzolanic reaction in blast-furnace cement samples with the AFA additive proceeds slowly and occurs more intensively at a later period of hardening.