Synergistic Effects of Multi-Component Recycled Aggregate on the Fresh Properties of Mortar: Predictive Modeling and Sensitivity Analysis

The growing demand for sustainable construction materials has spurred the use of recycled aggregates in cementitious composites to reduce the consumption of natural resources and the generation of construction waste. This study investigates the combined effects of recycled glass (RG), recycled brick (RB), and recycled concrete (RC) aggregates used as partial replacements for natural aggregate (NA) on the fresh properties of mortar. A multi-factor experimental design was employed, with RG, RB, and RC replacing NA at levels of 5–25%, 15–45%, and 10–30% of the total aggregate content, respectively. The fresh properties evaluated included the final water-to-cement ratio (w/c), fresh density, and air content. The results indicated that increasing the proportion of recycled aggregates, especially RB and RC, increased water demand and air content, which is likely attributed to their higher porosity and water absorption. Consequently, the final w/c ratio increased, while the fresh density decreased by up to 12%. In contrast, mixtures with higher NA and RG contents exhibited improved compactness and higher fresh density. Furthermore, the Response Surface Methodology (RSM) and sensitivity analysis framework established in this study provide a robust quantitative tool (R2 up to 0.95) for optimizing the proportioning of multi-source recycled aggregate mortar. The findings confirm the feasibility of using multi-source recycled aggregates to develop optimized and sustainable mortar mixtures with predictable fresh-state performance.