Effect of thermal activation on hydration heat, compressive strength and water absorption of fly ash

Abstract

In this study, effect of thermal activation on early-age properties of fly ash cement paste was systematically investigated. The fly ash cement pastes were cured under two different temperatures. The hydration heat flows, with or without thermal activation, were compared and modelled by three-parameter exponential function and boundary nucleation and growth (BNG) model. For the thermally activated cement paste with different fly ash contents, two distinct regions towards strength growth ratios are recognized in the time domain, reflecting the inconsistent reaction rates of fly ash and cement under thermal activation. This is also supported by the initial water absorption coefficient at 28d. For cement paste with 40% fly ash, its water absorption under thermal activation shows a more evident improvement than the one with 20% FA. Moreover, the apparent activation energy for fly ash cement system is calculated by several approaches. The results indicate that the activation energies calculated by simple linear approximation and average instantaneous activation energies are higher than the ones calculated by three-parameter exponential/hyperbolic function, and Ea obtained by three-parameter exponential function seems to be feasibly applied to predicting strength development under thermal activation.