Fly-ash-based geopolymer modified by metakaolin for greener grouting material

Fly ash usually requires high-temperature maintenance owing to its low volcanic ash activity, which greatly limits its application. In this study, fly-ash-based geopolymer grouting materials were prepared by modifying them with suitable alkali activators and metakaolin at room temperature. The results of the orthogonal experiments showed that the content of metakaolin had the most significant effect on the 1-, 7- and 28-day mechanical properties of the material. The optimum mix proportion was determined as metakaolin content (m (MK)/m (P)) = 15%, sodium silicate modulus (M (s)) = 1.0 and sodium silicate content (m (SS)/m (P)) = 50%. The compressive strengths of the materials at 1, 7 and 28 days were 12.6, 24.9 and 36.3 MPa, respectively. The results of X-ray diffraction, Fourier transform infrared spectroscopy, mercury intrusion porosimetry and scanning electron microscopy showed that the reaction mechanism of fly ash was subject to a bidirectional alkaline attack. Meanwhile, the unreacted fly ash particles proved the inertness of fly ash, while metakaolin mostly dissolved and formed hydrated sodium aluminosilicate gel to increase the mechanical strength of the geopolymer. The material developed in this study can meet the specification requirements of inorganic cementitious material for road grouting in terms of compressive strength and fluidity, increasing the usage of fly ash and contributing to environmental protection.