Strength and sustainability of recycled alkali-activated concrete across temperature and curing regimes

The construction industry increasingly adopts sustainable materials and strategies to mitigate its environmental impact. This study evaluates geopolymer concrete incorporating recycled aggregates (GOCRA) as an eco-friendly alternative to conventional cement-based materials. A cement-free geopolymer binder composed of aluminosilicate-rich waste materials and alkaline activators (Na₂SiO₃/NaOH) was used to prepare mixtures containing natural coarse aggregate (basalt) and recycled coarse aggregates (RCA), including crushed concrete (CC) and crushed ceramic (CE). Specimens were cured at 40 °C, 60 °C, and 80 °C for 60 h to assess the influence of curing temperature on compressive strength development. The results showed that geopolymer concrete incorporating recycled aggregates exhibited comparable mechanical performance to control mixes with natural aggregates. The highest compressive strength was recorded when curing at 60 °C for 60 h, reaching 41.5 MPa in the reference mix and approximately 37–38 MPa in mixes containing crushed concrete and crushed ceramic. These results confirm that recycled aggregates can produce sustainable geopolymer concrete with good strength. Scanning electron microscopy revealed a dense microstructure with strong interfacial bonding. These findings highlight the potential of geopolymer concrete with recycled aggregates to achieve both environmental benefits and satisfactory mechanical properties. A novel approach is presented for estimating the porosity of crushed aggregate mixtures using ImageJ software, providing a simple and cost-effective alternative to traditional laboratory methods.