Sustainable design of rubberised geopolymer-based ultra-high performance concrete using waste tyres: Mechanical, microstructural, and environmental evaluation

This study experimentally investigates a cement-free rubberised geopolymer ultra-high performance concrete (GUHPC), incorporating waste tyre rubber as a partial replacement for sand across controlled size fractions (medium, coarse) and hybrid gradations at 10–30 % replacement levels. The fresh and mechanical properties were evaluated, alongside sustainability metrics such as embodied CO 2 and material cost. The findings revealed that rubber substitution above 20 % led to significantly reduced workability and prolonged setting time. At 30 % replacement, the 28-day compressive and flexural strengths declined by 30 % and 17 % (from 164.3 to 113.9 MPa, and from 15.3 to 12.8 MPa, respectively). However, compressive energy absorption and early-age flexural toughness were notably enhanced. Hybrid rubber mixes minimised strength loss, raised toughness indices, and restrained matrix-penetrating cracks relative to single-size counterparts, attributed to enhanced packing and aggregate skeleton continuity. The alkaline geopolymer matrix improved rubber hydrophilicity and interfacial bonding, mitigating strength reductions compared to cement-based rubberised concretes. Microstructural analysis revealed strong matrix-fibre bonding in the control, evident mechanical interlocking at serrated medium rubber interfaces, and weak bonding with fibre debonding at coarse rubber surfaces. Moreover, replacing 10–30 % of sand with rubber reduced embodied CO 2 by 26–79 % and decreased material cost by 10 %. Within the scope of fresh, mechanical, and sustainability metrics, the 20 % hybrid rubber mix provided the best performance balance, with a 53 % carbon footprint reduction while retaining high strength and ductility. These results underscore the potential of rubberised GUHPC for sustainable infrastructure and protective applications such as impact-resistant façade panels, energy-dissipating safety barriers, and durable pavement overlays, where high toughness and resilience are critical performance requirements. • Geopolymer-based UHPC (GUHPC) was developed using waste tyre crumb rubber (CR). • GUHPC with 20 % CR addition exhibited compressive and flexural strengths of 134.4 and 14.0 MPa, respectively. • Rubberised GUHPC showed smaller strength losses than cement-based concrete due to better CR bonding in the alkaline matrix. • Medium CR maintained partial interlock via serrated surfaces, while coarse CR showed smooth, weakly bonded inclusions. • Replacing 30 % silica sand with CR reduced GUHPC cost by 9.7 % and embodied CO 2 by 79 %.