Utilisation of Industrial Red Gypsum for Sustainable Cementitious and Geopolymer Composites

This study investigates the potential utilisation of industrial red gypsum (RG), a by-product of titanium dioxide production, as a sustainable component in cementitious and geopolymer composites. Cement pastes, mortars and metakaolin-based geopolymers were prepared with partial replacement of the main binder by RG (5–50 wt%) to evaluate its effects on workability, porosity, hydration and mechanical performance. The experimental results showed that small additions of RG (5 %) enhanced the mechanical strength of cement mortars slightly due to its micro-filling and nucleation effects, while higher contents reduced the strength and workability because of the increased water demand and porosity. In geopolymers, RG acted mainly as an inert filler, with strength decreasing significantly beyond 10 % replacement. Response Surface Methodology (RSM) confirmed an optimal composition range at low RG dosages. The results demonstrate that limited RG incorporation can contribute to a circular economy strategy in construction, promoting waste valorisation and reducing cement consumption.