Roles of recycled concrete powder on the properties of alkali-activated slag: Reaction products and microstructure development

This study investigated the potential utilization of recycled concrete powders (RCP), obtained from demolished old buildings as precursors to produce alkali-activated slag (AAS). These RCPs contained significantly higher amounts of SiO₂ and CaCO₃ while low level of hydration products, thus limiting their potential application for high substitution in Portland cement-based materials. The present study examined the influence of RCP on the heat evolution, reaction product, pore structure, and mechanical strength of the AAS-RCP system. The results revealed that the incorporation of RCP delayed the alkali-activated reaction at the early age. At later ages (28 days), however, the reactive silica and calcium components in RCP gradually dissolved, facilitating the formation of gel phases with high silicate polymerization and contributing to a higher gel pore volume. Additionally, a zeolite-like product, i.e., gismondine, was identified in AAS-RCP. As a result, AAS-RCP exhibited satisfactory mechanical properties at 28 days even with a high RCP addition. The present study demonstrated the promising potential of large-scale incorporation of RCP with alkali-activation technology in producing low-carbon concrete.