Preparation and mechanical properties of red mud-based geopolymer recycled concrete

To advance sustainable construction, in this study, red mud-based geopolymer recycled aggregate concrete (GRAC) was synthesised using industrial waste (red mud) and construction waste (recycled coarse aggregate (RCA)). Orthogonal experiments optimised the geopolymer mortar mix design, identifying sodium silicate concentration (30%), ground granulated blast furnace slag (GGBS) content (40%), lime (5%) and gypsum (0%) as the optimal proportion. Sodium silicate concentration was the most critical factor influencing compressive strength, followed by GGBS content, while lime and gypsum exhibited minimal effects. The optimised mortar achieved a 28 days compressive strength of 61.48 MPa – which is 36.90% higher than ordinary Portland cement (OPC) mortar. GRAC specimens with 0–100% RCA replacement were subsequently prepared. Compressive strength decreased with increasing RCA content, peaking at 50% replacement (45.96 MPa at 28 days, 2.64% above OPC concrete). Microstructural analysis by way of scanning electron microscopy/X-ray diffraction analysis revealed: depolymerisation of red mud and GGBS minerals released active Ca, Si and Al components; geopolymerisation formed a hybrid zeolite-like framework and calcium aluminosilicate hydrate (C–A–S–H) gel; the geopolymer matrix filled RCA surface microcracks, enhancing interfacial bonding. GRAC’s superior strength stems from synergistic RCA–geopolymer interactions, reducing cement consumption and natural resource extraction. This work demonstrates high-value recycling of dual waste streams for sustainable infrastructure development.