Toward high sustainability using fully recycled geopolymer concrete: mechanical, rheological, and microstructural properties†

This study explores the sustainable production of fully recycled geopolymer concrete using waste ordinary Portland cement (OPC) concrete, aiming to reduce environmental impact and promote circular construction practices. Crushed OPC concrete was utilized as both recycled aggregate (RA) and recycled concrete powder (RCP), while recycled clay brick powder (RBP) was incorporated as a supplementary aluminosilicate additive. Alkaline activation was performed using sodium hydroxide solutions at three molarities (8, 12, and 16 M) combined with water glass (WG) at a WG/NaOH ratio of 2. The mechanical, rheological, and microstructural properties of geopolymer recycled concrete (GRC) and paste (GRP) were assessed alongside a Life Cycle Assessment (LCA). Results showed that incorporating RA and RCP significantly enhanced compressive strength—up to 90% at 90 days compared to geopolymer paste—while increasing NaOH molarity improved overall mechanical performance. Although RBP decreased compressive strength, density, and elastic modulus, it notably enhanced workability, surface quality, and crack resistance. Microstructural analysis revealed strong adhesion between RCP and RA due to their chemical compatibility, contributing to matrix homogeneity. The LCA confirmed that GRC exhibits a lower environmental footprint than conventional OPC concrete. These findings support the classification of GRC as a high-performance structural material and demonstrate its potential as a viable, eco-friendly alternative for sustainable construction applications.