High-performance self-compacting geopolymer concrete synthesised from black tea ash, waste-glass-derived activator, and recycled construction and demolition waste

This study develops a high-performance self-compacting geopolymer concrete through systematic optimisation of three waste streams: black tea ash as precursor; waste glass converted into alkali activator (89% silica extraction efficiency); and recycled construction and demolition waste as partial coarse aggregate replacement. Optimisation identified 40% BTA, 100% waste-glass-derived activator, and 30% RCA as optimal. The mixture achieved a 90-day compressive strength of 58.6 ± 1.2 MPa (6% above control: 55.3 ± 0.9 MPa), a split tensile strength of 3.55 ± 0.11 MPa, and a flexural strength of 5.86 ± 0.15 MPa. Durability was exceptional: water absorption of 1.75 ± 0.08% (17.8% reduction vs. control), 59.2% strength retention after 900 °C (control: 49.6%), very low chloride penetrability (780 ± 35 coulombs), and superior acid/sulphate resistance (strength losses of 27% and 14%, respectively). Life cycle assessment revealed a 38.7% reduction in embodied CO2, 27.1% lower material cost, and 559 kg waste utilised per cubic metre. Microstructural analysis revealed a dense, crack-free matrix with coexisting N-A-S-H and C-A-S-H gels and a thin 15–25 μm interfacial transition zone. This study demonstrates that three waste streams can be systematically optimised into high-performance, cost-effective concrete suitable for aggressive environments, offering a tangible pathway towards circular construction.