Self-desiccation is one common phenomenon of high-performance cementitious materials characterized by low water to cementitious material ratio (w/c). Autogenous shrinkage is closely related to the internal relative humidity (RH) drop and capillary pressure induced by self-desiccation in the cement pastes. However, there is debate about the determination of time-zero, the time at which autogenous shrinkage begins to develop.
The objective of this study is to provide an accurate determination of time-zero based on the relationship between the internal RH and autogenous shrinkage of low w/c ratio cement pastes. And according to the time-zero, cement pastes blended with zeolite were prepared to investigate the potential of zeolite as internal curing agent.
The autogenous shrinkage was conducted according to the standard method ASTM C1698. Internal RH was performed on the sealed cement pastes at very early age by conventional method of hygrometer. Setting time was determined by the Vicat needle apparatus according to the standard method ASTM C191.
Experimental results revealed that no internal RH drop was observed around the final setting time determined by the Vicat method. Besides, a knee point was observed in the shrinkage curve at the time when the internal RH began to decrease. This is the so-called time-zero. And zeolite was found to be a potential internal curing agent according to the autogenous shrinkage tests measured from the new time-zero.
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