Evaluation and optimization of various treatment methods for enhancing the properties of brick-concrete recycled coarse aggregate

Crushed brick-concrete recycled aggregate (BCRA) represents a sustainable and environmentally friendly alternative to natural aggregates (NAs) in practical engineering. However, compared with NA, the high porosity and low strength of BCRA means it struggles to meet engineering requirements, which limits its large-scale application. This paper reports upon an experimental study to enhance the quality of BCRA via soaking in different types of solutions (pozzolanic materials, sodium silicate, polyvinyl alcohol, and silicon-based additive). The physical and mechanical properties of treated and untreated BCRA were evaluated by testing the apparent density, water absorption, and crushing value. Meanwhile, the optimization schemes for different treatment techniques were recommended using the multi-objective grey target decision method. In addition, the surface morphology of BCRA was characterized by X-ray diffraction and scanning electron microscopy with energy dispersive spectroscopy, to examine the microstructure of the BCRA surface before and after different treatment processes. The results show that 80% cement, 10% fly ash, and 10% silica fume slurries had notable effects on the apparent density and mechanical strength of BCRA, in which the apparent density increased by 0.6–1.8% and the crushing value decreased by 3.5–19.7%; however, the water absorption of BCRA increased by 1.7–8.5%. The physical and mechanical properties of BCRA increased under an increase of sodium silicate concentration and soaking time, though its strengthening effects were limited. Polyvinyl alcohol and silicon-based additives were best able to improve the waterproofing of BCRA, and the water absorption of the BCRA after treatment was reduced by 5.1–17.8% and 30.5–44.9%, respectively. Clear microstructural improvements can also be observed on the treated BCRA surface. To summarize, the BCRA treated by the four treatment methods exhibited surfaces that were denser, less porous, and more homogeneous and regular. These results will provide a useful reference for the future engineering applications of treated BCRA.