Crystallization-Dominated Rapid Setting of Geopolymers at Low-Preparation Temperature and Low-Modulus Alkali Activators

Current research on rapid-setting geopolymers primarily focuses on accelerating the "dissolution-gelation" reaction process (including dissolution, polycondensation, and hardening) to achieve the faster solidification of the material. In this work, the low-temperature crystallization phenomenon of sodium silicate solution is innovatively introduced into the geopolymer system, and a rapid-setting strategy is proposed by exploiting the synergistic effects of a low preparation temperature and a low activator modulus (the SiO₂/Na₂O molar ratio) to intentionally induce the rapid crystallization of the alkali activator solution (sodium silicate solution), thereby enabling rapid setting of the geopolymer. Experimental results show that, at a preparation temperature of 0-10 °C and an activator modulus of 1.0, the final setting time of the metakaolin-based geopolymer is shortened to 20 min, corresponding to a reduction of up to 91.63%. Unlike the typical "dissolution stage-gelation stage-hardening stage" route of conventional geopolymers, the rapid-setting geopolymer in this study follows a distinct reaction sequence of "hardening stage-dissolution stage-gelation stage". Meanwhile, to further enhance performance and expand pathways for solid waste valorization, vanadium-extraction shale tailings (VST) were employed to partially replace metakaolin. The results indicate that, with a tailings replacement of 30%, an alkali-to-solid ratio of 0.4, and an alkali-to-water ratio of 0.7, the rapid-setting geopolymer achieves a compressive strength of 38.32 MPa. These findings confirm the broad applicability and practical potential of the proposed approach for emergency repair in cold regions and solid waste utilization.