Waste clay brick powder as a sustainable precursor for structural geopolymer concrete: Mechanical, durability and microstructural characteristics

This study explores the feasibility of developing structural-grade geopolymer concrete using waste clay brick powder (WCBP), a precursor whose application has previously been limited to paste and mortar formulations due to its low reactivity. To address this limitation, WCBP was partially replaced with Class F fly ash (FFA) and the resulting concretes were compared with 100% FFA-, 100% Class C fly ash (CFA)-based geopolymers and Portland cement concretes of similar compressive strength. The experimental program included optimization of activator composition, followed by mechanical, durability and microstructural evaluation through compressive, tensile and flexural testing, chloride-ion penetration, high-temperature exposure, freeze–thaw cycling, drying shrinkage, and TGA, Micro-CT and SEM analyses. WCBP alone yielded insufficient strength (14.3 MPa), whereas a 50% WCBP–50% FFA blend achieved 32.2 MPa and exhibited mechanical performance comparable to cement concrete, with superior bond strength and thermal stability. The 100% FFA mixture demonstrated the most refined pore structure and best durability, while CFA showed higher permeability and reduced thermal stability. Findings confirm that blending WCBP with highly reactive FFA enables the production of structurally viable geopolymer concretes and provides a viable pathway for valorizing brick waste within low-carbon binder systems.