A review on the use of LWA as an internal curing agent of high performance cement-based materials

Lightweight aggregate (LWA) has been used in concrete for a long time, but its internal curing (IC) effect has been realized recently. This paper offers a review of LWA as internal curing materials for high performance cement-based materials. The effect of LWA on IC is related to its amount, pretreatment method, particle size, and distribution in concrete. Pre-wetted LWA has a better IC effect than dry LWA. The particle size of LWA and its distribution in concrete are two important factors to achieve satisfactory IC effectiveness such as concrete strength and autogenous shrinkage, which need to be further studied. The effects of LWA on the strength depends on two opposite facts: the side effect of LWA's porous structure and beneficial effect of internal curing on the hydration. The desorption of IC water is related to the size ranges of pores in LWA and paste, and large pore sizes of current LWA cause a quick desorption of IC water in concrete with low water-to-binder ratio. Water release of LWA promotes the hydration and refines the pore structure. Interfacial transition zone (ITZ) between cement paste matrix and LWA is also improved and the permeability is reduced. The characterization of ITZ surrounding LWA is a key point to understand the IC mechanism and a feasible method needs to be put forward. Those studies will provide a better theoretical foundation for the preparation of high performance LWA cement-based materials. (C) 2019 Elsevier Ltd. All rights reserved.