An overview of the effect of SAP and LWS as internal curing agents on microstructure and durability of cement-based materials

Internal curing (IC) is an effective way to improve key engineering properties and enhance the durability of cement-based materials. Lightweight sand (LWS) and superabsorbent polymers (SAPs) are widely employed as efficient IC agents. The underlying mechanism of such IC agents is to maintain the internal relative humidity (IRH) in the matrix. This can sustain further hydration of the binder over time and reduce self-desiccation and the associated chemical and autogenous shrinkage. This paper presents a comprehensive review of the effects of LWS and SAP as IC agents on the microstructure, transport properties, and durability of cement-based materials. This includes the effects of IC agents on water permeability and chloride ingression as well as frost durability, carbonation, corrosion resistance, deicing salt scaling, and acid attacks. The incorporation of LWS and SAP can significantly improve the durability of cement-based materials by densifying the microstructure, shifting the pore structure towards finer pores, and ultimately decreasing the permeability of cement-based materials. The size, content, type, and pre-treatment of LWS and SAP, as well as the binder content and water-binder ratio, are key factors that affect their efficiency on the durability of cement-based materials.