Accurate determination of the time-zero of autogenous shrinkage in alkali-activated fly ash/slag system

Autogenous shrinkage based on different time-zero may result in underestimation/overestimation on the cracking potential of alkali-activated fly ash/slag (AAFS). This study reports an experimental investigation on the time-zero of AAFS pastes characterized by different techniques. It is found that the internal capillary pressure in AAFS pastes significantly increased after 5-7.5 h, which is far beyond the final setting time that is normally regarded as time-zero for Portland cement. The transition time derived from capillary pressure method directly captures the alteration of capillary stress in AAFS, which matches well with the change of the shrinkage slope. Due to the moisture equilibrium between the specimens and humidity sensor, it is hard to use the measured internal relative humidity to determine the real time-zero of AAFS. This research finds that the onset of capillary pressure in AAFS paste provides a more scientific and accurate determination of time-zero.