Performance optimization of red mud-based geopolymer modified grouting materials via response surface methodology

To reduce the carbon footprint and resource intensity of conventional cementitious grouts, we develop a geopolymer-based system in which red mud and fly ash synergistically replace a portion of ordinary Portland cement (OPC). Mixture performance is tuned using sodium hydroxide and sodium silicate activators. Single-factor tests and response surface methodology (RSM) were conducted to quantify factor effects. The investigated variables included red mud, fly ash, cement content, water-to-binder ratio, and activator dosages. The responses comprised pH, bleeding rate, flowability, viscosity, setting time, and compressive strength. Based on these data, multi-response regression models were established. Red mud–fly ash synergy is evident at appropriate replacement levels; moderate activator dosages promote C–S–H and N–A–S–H formation and densify the matrix, whereas excessive dosages disturb ionic balance and increase porosity. All models achieve R² > 0.98. The optimal mix (water-to-binder ratio = 0.573; red mud = 24%; sodium hydroxide = 6.25%; sodium silicate = 2.63%) shows prediction errors < 5% and superior overall performance. Compared with an OPC grout, cement consumption, total cost, and embodied carbon are reduced by 42.1%, 11.9%, and 40.7%, respectively.