Experimental evaluation of hybrid concrete produced from recycled concrete aggregates and alkali-activated fly ash for sustainable construction

The growing demand for sustainable construction materials has intensified interest in hybrid concrete systems incorporating recycled aggregates and industrial by-products. This study investigates durability-related properties, specifically water absorption and density characteristics of hybrid concrete produced using recycled concrete aggregates (RCA) and alkali-activated fly ash as partial binder replacements. Concrete mixtures were prepared with RCA replacement levels of 0%, 25%, 50%, 75%, and 100%, while maintaining consistent mix proportions. A total of six specimens (150 mm) were tested for each mixture to evaluate compressive strength development at 7, 14, and 28 d, alongside density and water absorption characteristics. Results revealed that compressive strength increased with curing age for all mixtures but decreased progressively with higher RCA incorporation. The 28-day compressive strength ranged from 42.8 MPa for the control mix to 28.9 MPa at full RCA replacement. Density values decreased, and water absorption increased with RCA replacement, reflecting increased pore connectivity and moisture transport pathways. The findings showed that hybrid concrete with up to 50% RCA can achieve balanced durability-related behaviour based on water absorption characteristics while significantly reducing dependence on natural aggregates and conventional cement, indicating the beneficial role of alkali-activated fly ash in enhancing matrix densification and interfacial bonding.