Optimisation of magnesium oxysulfate cement incorporating MgO-rich industrial waste using Box–Behnken design: Mechanical and microstructural insights

The use of MgO-rich industrial waste in magnesium oxysulfate cement (MOSC) offers a sustainable approach to producing low-carbon cementitious binders and facilitates the valorisation of industrial waste. This study determined the optimal MgO:MgSO₄·7H₂O ratio and waste incorporation level through mechanical performance assessment, indicating an 8.5:1 MgO:MgSO₄·7H₂O ratio and 35% MgO-rich industrial waste as the ideal composition. The pozzolanic efficacy of the additional cementitious materials was validated using Frattini and Strength Activity Index (SAI) assessments before optimisation. The effects of diatomite, silica fume, ground granulated blast-furnace slag (GGBFS), and citric acid were optimised using Response Surface Methodology (RSM) with a Box–Behnken design. The optimised mixes attained 28-day compressive strengths ranging from 58 to 64 MPa, along with notable enhancements in flexural and tensile cracking strengths. Microstructural analysis employing XRD, FTIR, TGA, SEM, and EDS identified the presence of brucite, hydrated magnesium sulphate phases, and magnesium silicate hydrate (M–S–H) gel, resulting in a dense, compact microstructure that enhances mechanical performance. The results indicate that MgO-rich industrial waste can be efficiently used to produce high-performance, environmentally sustainable MOSC binders.