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

Fly ash usually requires high-temperature maintenance owning to 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 under room temperature. The results of the orthogonal experiments showed that the content of metakaolin had the most significant effect on the material’s 1, 7, and 28 d mechanical properties. The optimum mix proportion was determined as metakaolin content (mMK/mP)=15%, sodium silicate modulus (Ms)=1.0, sodium silicate content (mSS/mP)=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 subjected to a bi-directional alkaline attack. Meanwhile, the unreacted fly ash particles proved the inertness of fly ash. While metakaolin mostly dissolved and formed N-A-S-H gel to increase the mechanical strength of 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, extending the usage amount of fly ash and contributing to environmental protection.