Role of recycled concrete powder as sand replacement in the properties of cement mortar

This paper investigates the properties of cement mortar containing recycled concrete powder (RCP) as sand replacement. Two types of model RCP prepared with designated water-to-cement (W/C) ratios in the laboratory, including recycled cement paste powder (RCPP) and recycled cement mortar powder (RCMP), were utilised in the mortar. The physical, mechanical and drying shrinkage properties of the mortars with various amounts of RCP were investigated, and their microstructure was characterised. The results indicate that increasing the RCP content increases the water demand and decreases the dry bulk density of mortars due to the inferior characteristics of RCP. Replacing sand by up to 10% RCPP prepared with various original W/C ratios or 20% RCMP has a negligible or slightly positive impact on the mechanical properties of mortars. This is mainly attributed to the decreased volume fraction of large capillary pores, air voids and total porosity, which overcomes the negative effects induced by RCP. Specifically, incorporating RCP can reduce the fraction of large voids through its filling effect and slightly promote hydration in mortars. Besides, the drying shrinkage of the mortars increases with the RCP replacement ratio due to the increased volume fraction of mesopores in the RCP mortars. However, an excessive amount of RCP increases the volume fraction of capillary pores and the total porosity of mortars due to the combined action of the decreased packing density and the high porousness of RCP particles, which subsequently decreases the mechanical properties and further increases the drying shrinkage of RCP mortars. In general, incorporating a proper amount of RCP as sand replacement can enhance the sustainability of mortars without compromising their mechanical properties.