Perforated cenospheres: A reactive internal curing agent for alkali activated slag mortars

Alkali activated slag (AAS) commonly exhibits very large autogenous shrinkage, which can be reduced or eliminated by internal curing. However, using existing internal curing agents can induce significant reduction of the compressive strength. To address this issue, this study proposes to use perforated cenospheres (PCs) as an internal curing agent for AAS mortars. PCs are manufactured from cenospheres through a chemical etching process, in which micro holes perforating the shell of cenospheres are produced. Through these holes, water can be loaded into and then released from the PCs for internal curing of AAS mortars. Unlike any other existing internal curing agents, PCs can react with the alkali activator to produce geopolymer. Therefore, PCs are also a part of the binder, not just a water reservoir. As a result, internal curing with PCs can not only eliminate the autogenous shrinkage of AAS mortars, but also avoid strength reduction of the mortars. In fact, significant improvement of the compressive strength (16.2%-28.9%) over the control sample has been achieved by internal curing with PCs at 56 d thanks to the reactivity of PCs.