Performance Evaluation of Copper Slag as Precursor and Fine Aggregate in Alkali-Activated Mortars

Alkali-activated mortars (AAMs) have emerged as sustainable alternatives to conventional hydraulic cement (HC) matrices produced with natural sand. In this context, interest in the valorization of industrial by-products to develop eco-efficient construction materials has gained crucial academic and industrial attention. This study investigates the feasibility of producing AAMs incorporating copper slag (CS) as an artificial fine aggregate (AFA) to partially or completely replace natural sand. Moreover, the binder matrix was formulated using 80% CS and 20% HC as precursors, activated with different alkaline solutions (Na2SiO3 + NaOH) at activator-to-precursor mass ratios ranging from 0.15 to 0.35. Concurrently, CS was incorporated as AFA at volumetric replacement levels of 0%, 25%, 50%, 75%, and 100%. The AAMs were evaluated in terms of workability, physical performance (i.e., bulk density, water absorption, and void content), and mechanical performance. The results demonstrate that the workability of the AAMs increased with higher AFA dosages, reaching a maximum improvement of 23.8% compared with the AAM without AFA. The bulk density of the AAMs increased monotonically with increasing AFA content (consistent with the higher density of AFA with respect to natural sand), whereas water absorption and void content decreased progressively. Although all AAMs exhibited significantly lower compressive strengths than M1 at 7 and 28 days, the differences progressively decreased with curing age. At 56 and 90 days, M5 and M6, incorporating 75% and 100% AFA, respectively, achieved mean compressive strengths that were not statistically different from those of M1, indicating that the mixtures with the highest AFA contents maintained later-age mechanical performance within the variability of the conventional reference mortar. The study demonstrates the feasibility of the synergistic utilization of CS as both precursor and AFA in AAMs. This dual-pathway valorization closes materials loops and advances circular economy principles within the construction sector.