Enhancing mortar performance: A comparative study on particle size of recycled concrete powder and metakaolin in binary and ternary blends

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The construction industry generates substantial volumes of construction and demolition waste (CDW), including aggregates and fine particles classified as recycled concrete powder (RCP). This study investigates the sustainable reuse of RCP by evaluating its combined use with metakaolin (MK) as supplementary cementitious materials in both binary and ternary mixtures with Portland cement, aiming to reduce environmental impact without compromising mortar performance. The experimental program assessed two particle sizes of RCP, along with MK, and various replacement levels, which were pivotal parameters in the study. A series of binary and ternary mixtures were prepared with different proportions of RCP and MK, considering replacement levels ranging from 10 % to a maximum of 30 %, using RCP alone or in conjunction with MK. The mortars were analyzed in both fresh and hardened states, focusing on critical properties such as consistency index, compressive strength, splitting tensile strength, elastic modulus, water absorption, density, void index, and capillary water absorption. Furthermore, a microstructural analysis was conducted using scanning electron microscopy (SEM), and a cement consumption efficiency index, along with CO₂ emissions data, was calculated to provide a comprehensive evaluation of performance and sustainability. The findings demonstrate that RCP with a particle size comparable to that of cement (RCP1) significantly enhances both physical and mechanical performance, particularly in ternary mixtures with MK due to its pozzolanic activity, where the silica (SiO₂) and alumina (Al₂O₃) components react to form additional hydrated products, thereby improving the properties of mortars. For instance, the ternary mixture comprising 15 % RCP1 and 15 % MK attained compressive and splitting tensile strength of 29.26 MPa and 3.36 MPa at 28 days, respectively, compared to the reference mixture, which yielded 27.96 MPa and 3.33 MPa. Conversely, larger particle sizes (RCP2) and higher replacement levels led to a decline in performance across all evaluated parameters. Ternary mixtures containing RCP1 also exhibited a higher cement consumption efficiency index and lower CO₂ emissions per MPa. Notably, the mixture with 15 % RCP1 and 15 % MK showed the most favorable indicators overall, with cement consumption and CO₂ emissions measured at 12.08 kg/m³·MPa and 10.01 kgCO₂/m³·MPa, respectively. These results suggest that the synergy between RCP1 and MK enhances particle packing and matrix density, which in turn improves the structural properties of the mortar. The incorporation of MK compensates for the limited reactivity of RCP, especially at higher replacement levels, while the utilization of RCP1 offers significant sustainability benefits by reducing Portland cement consumption. This study demonstrates that the combination of RCP and MK in ternary mixtures presents a technically viable and sustainable alternative for mortar production, optimizing cement use and reducing CO₂ emissions. The innovative application of CDW-derived RCP in construction materials represents a practical approach to promoting more sustainable practices within the industry.