Recycling waste claystone into value-added and eco-friendly cementitious materials for concrete industry decarbonisation

The increasing demand for sustainable cement production has prompted the exploration of alternative supplementary cementitious materials (SCMs). This study explores the influence of incorporating waste-derived Australian claystone (WAC) as SCM on the hydration kinetics, internal microstructure evolution, and macro-scale properties of blended cement systems. WAC was used in both raw and calcined forms, with calcination conducted at 400 °C, 600 °C, and 800 °C to enhance pozzolanic reactivity. The results demonstrate that calcined WAC, particularly at 800 °C, significantly promotes the generation of C–S–H and C–A–S–H gels while reducing calcium hydroxide (CH) content, thereby accelerating hydration and refining pore structure. The Ca/Si ratio of hydration gels decreases with curing age and is further reduced by WAC addition, especially in the samples containing thermally activated WAC. Among all cementitious mixtures incorporating WAC, the formulation containing 10 % WAC thermally activated at 800 °C achieves superior mechanical and transport performance. Specifically, this mix demonstrates the highest compressive strength and the lowest rates of water uptake and capillary absorption, indicating optimized matrix densification and hydration efficiency. The results highlight the potential of the thermally activated WAC as a high-performance, eco-friendly SCM for concrete decarbonisation.