The high water absorptivity of recycled concrete fine aggregate (RCA) and recycled brick fine aggregate (RBA) restricts their utilization in concrete. This study investigates the effects of incorporating RCA and RBA on the macroscopic properties and microstructure of geopolymer concrete under three pre-wetted conditions: unpre-wetted, semi-pre-wetted, and fully-pre-wetted. Results indicated that both RCA and RBA exhibited inferior physical properties, with water absorption rates of 8.5% and 16.0%, and crushing indices as high as 15.2% and 20.4%, respectively. Direct replacement of natural sand with these aggregates was found to degrade the macroscopic performance of concrete. However, the semi-pre-wetted treatment significantly improved both the macro-properties and microstructure of the resulting concrete. For instance, mixtures incorporating semi-pre-wetted RCA and RBA, which were designated as RCAS100 and RBAS100, showed an increase in slump flow by 11.9% and 19.5%, respectively. These mixtures achieved compressive strengths of 80.0 MPa and 68.6 MPa, representing an enhancement of 8.5% and 7.8%, respectively, compared to the unpre-wetted control mixtures, RCAD100 and RBAD100. Concrete incorporating semi-pre-wetted recycled fine aggregate exhibited the narrowest interfacial transition zone, or ITZ. Specifically, the ITZ width of the RBAS100 mixture was approximately 15 μm smaller than those of the RBAM100 and RBAD100 mixtures. This improvement is attributed to the fact that the semi-pre-wetted treatment optimally utilizes the internal curing effect provided by the aggregates. This process not only enhances the mechanical properties of concrete but also refines the microstructure of the ITZ. Consequently, this study establishes a theoretical basis for optimizing geopolymer concrete performance by controlling the moisture content of recycled aggregate. It further contributes key theoretical support for the efficient, large-scale utilization of construction waste.
Han et al. (Tue,) studied this question.