Integrating recycled waste materials in cementitious grouts: evaluating mechanical integrity and rheological behaviour

Abstract Conventional cementitious grouts, widely used for underground reinforcement and structural repair, rely heavily on Portland cement, a material responsible for nearly 7% of global CO₂ emissions and significant resource depletion. Addressing this environmental and material-efficiency challenge, this study develops and evaluates novel sustainable grout formulations that partially replace cement with recycled waste glass (WG), tyre rubber waste (TRW), and construction and demolition waste (CDW). By systematically linking mechanical, rheological, and microstructural performance, the research bridges a key gap between sustainability-driven material substitution and the performance requirements of practical grouting applications. Grout mixtures incorporating 0–20% WG, 0–3% TRW, and 0–10% CDW were evaluated for rheological behaviour and unconfined compressive strength (UCS). All modified grouts maintained shear-thinning properties but exhibited significantly reduced viscosity, enabling lower water demand. Notably, WG-enhanced grouts preserved or even improved UCS. For example, 2.5–5% WG substitution resulted in minimal strength loss, yielding up to 97% of the 70.5 MPa control strength. When adjusted for reduced water content, these mixes achieved a 10.6% UCS increase. TRW grouts peaked at 55.2 MPa with 0.75% inclusion, while CDW caused only minor strength drops at 2.5–5%. Enhanced flowability (lower viscosity/yield stress) compensated for some strength reductions. Microstructural analysis revealed that WG improved matrix density via pozzolanic activity and filler effect, while TRW and CDW influenced pore structure. Beyond their laboratory performance, these findings underscore the practical potential of recycled waste-based grouts to reduce cement consumption, lower embodied carbon, and divert waste streams from landfills. Such formulations retain pumpability and mechanical performance while offering viable alternatives for ground reinforcement, structural repair, and sustainable construction applications. Overall, the integration of WG, TRW, and CDW demonstrates a promising pathway toward lower-impact cementitious grouts that meet both engineering and environmental demands.