Alkali activation of industrial waste fly ash to produce high-value-added C-S-H seeds

Solid waste fly ash modified by alkali activation was used as a raw material to prepare calcium silicate hydrate (C-S-H) seeds as early-strength agents using a mechanochemical method. The C-S-H seeds, both before and after modification, were characterised using X-ray diffraction, thermogravimetric analysis, silicon nuclear magnetic resonance, Fourier transform infrared spectroscopy and transmission electron microscopy to systematically assess their properties. The early and later mechanical properties of cement pastes mixed with different types of C-S-H seeds were investigated. Additionally, theoretical calculations were performed to ascertain whether the modified C-S-H seeds complied with the pertinent restrictions on alkali content when incorporated into cement. The research findings revealed that the modified C-S-H seeds provided a greater variety and a higher quantity of effective seeds compared with the non-modified seeds. The early strength of cement paste containing modified C-S-H seeds was enhanced by 131.4%, indicating a more pronounced seeding effect than that of non-modified seeds, without negatively impacting later strength. Theoretical calculations validated that the incorporation of 2% modified C-S-H seeds did not trigger an alkali–aggregate reaction. The method is a novel approach for utilising less active solid waste in cement to produce C-S-H seeds, offering a reference for the convenient and cost-effective use of aluminium-rich waste in the preparation of C-S-H seeds.