Alkali-Activated Grouting Materials for Underground Coal Mines: A Critical Review of Rheology, Mechanical Performance, and Engineering Applicability

The development of sustainable grouting materials is essential for enhancing underground strata stability while reducing the environmental impact associated with ordinary Portland cement (OPC). This review summarizes previously published studies on alkali-activated grouting materials (AAGMs) prepared using fly ash (FA) and ground granulated blast furnace slag (GGBFS), activated by sodium hydroxide and sodium silicate solutions. A comprehensive literature-based analysis was conducted to evaluate both fresh and hardened properties, including fluidity, setting time, yield stress, compressive strength, and durability-related performance. Particular attention was given to the influence of FA-GGBFS proportions and activator composition on rheological behaviour, mechanical performance, and engineering applicability. The reviewed studies indicate that increasing GGBFS content significantly accelerates geo-polymerization and setting behaviour and enhances early-age strength development due to its higher calcium reactivity. In contrast, FA contributes to improved workability and flowability, attributed owing to its spherical particle morphology and slower reaction kinetics. The reviewed literature further suggests that balanced FA–GGBFS alkali-activated systems can provide a favourable combination of fluidity, injectability, setting behaviour, and mechanical performance, making them particularly suitable for underground grouting and rock mass reinforcement applications. Compared with conventional OPC-based grouts, AAGMs demonstrate superior mechanical performance together reduced environmental impact through the utilization of industrial by-products and reduced clinker consumption. However, several critical challenges still hinder the large-scale implementation of alkali-activated grouting materials in underground mining, particularly with respect to field-scale validation, shrinkage mitigation, safe handling of alkaline activators, and the current lack of standardized specifications and design guidelines for underground grouting applications. These findings provide a robust scientific basis for the design and application of eco-efficient grouting materials in deep underground mining environments and support the advancement of sustainable practices in underground engineering.