Calcium Effect in PLR–PCR Geopolymers: Peak Compressive Strength at 30% PCR and Evidence of C-A-S-H/N-A-S-H Synergy

Valorization of construction and demolition waste (CDW) through alkali activation enables the formulation of geopolymers with a lower carbon footprint. This study evaluates binary pastes prepared with powdered recycled concrete (PCR) and powdered recycled brick (PLR) in seven proportions (0–100% PCR), activated with NaOH/Na₂SiO₃/KOH and cured for 7, 14, and 28 days. Compressive strength, microstructure (SEM-EDS), and chemical structure (XRD, FTIR) were characterized. Mixtures with 30% PCR reached the highest 28-day strength, associated with a denser microstructure and lower porosity. EDS analysis evidenced an increase in Ca with increasing PCR, promoting C-A-S-H gels coexisting with N-A-S-H; XRD showed a reduction of non-reactive crystalline phases, and FTIR an intensification of T–O–Si bands, both consistent with higher polymerization. Higher PCR contents diluted reactive aluminosilicates and reintroduced microdefects, leading to a drop in strength. Altogether, an optimal window at ~30% PCR is identified to maximize densification and mechanical performance without compromising the aluminosilicate network. These results support the combined use of PCR/PLR as geopolymer precursors to manage CDW and produce sustainable construction materials.