Effect of calcined concrete slurry waste on the hardened properties and microstructure of fly ash-based alkali-activated materials

Large volumes of concrete slurry waste (CSW), accounting for approximately 0.8% of concrete production in ready-mix plants, are generated and pose notable environmental challenges. To address these issues, this study assesses the potential of utilising CSW as a precursor material in the production of alkali-activated materials (AAMs), as AAMs are recognized for their lower carbon footprint compared to OPC-based materials. In this study, CSW were partially replaced fly ash (FA) at replacement levels ranging from 10 to 50%, followed by activation with sodium hydroxide and sodium silicate. Both uncalcined and calcined CSW (400, 600 and 800 °C) were employed to evaluate the effect of calcination of CSW on the properties of AAMs, specifically compressive strength, water absorption and microstructure. The results demonstrated that calcination of CSW significantly enhanced the compressive strength of AAMs compared to uncalcined CSW. Among the samples, AAMs prepared with CSW calcined at 800 °C achieved the highest compressive strength of 46.9 MPa. A similar trend was observed for water absorption, where AAMs prepared with CSW calcined at 800 °C show a reduction from 24.0 to 20.5% compared to those using uncalcined CSW. The strength enhancement was mainly attributed to the filler effect of calcined CSW, which densified the microstructure by filling voids. This study demonstrates the potential of recycling CSW into AAMs, thereby reducing concrete waste and promoting sustainable construction.