Realization of high-percentage addition of fly ash in the materials for the preparation of geopolymer derived from acid-activated metakaolin

This study focuses on using fly ash to prepare phosphoric acid-activated metakaolin geopolymer, in which the replacement of metakaolin by a high percentage (up to 30%) of fly ash was achieved. The effects of the addition of fly ash on the compressive strength of the resulting geopolymer were tested and statistically analyzed, and the related mechanism was explored through using X-ray diffraction, field emission scanning electron microscope, Fourier-transform infrared, and magic angle spinning nuclear magnetic resonance analysis. The results indicated that the compressive strength of geopolymer maintained at a high level even as a high percentage of fly ash was used to replace metakaolin. The key reason is that fly ash cenospheres effectively optimized the acid-activated geopolymerization of metakaolin. On one hand, cenospheres prevented the plate-like metakaolin particles from being bonded by the aluminum phosphate rapidly generating on the particle surface, which promoted the geopolymerization degree of metakaolin. On the other hand, the three-dimensional network of Si–O–Al–O–P became stronger with the increase of the crosslinking degree of Si, Al, and P under the influence of fly ash cenospheres. These findings demonstrate the feasibility of using fly ash as raw materials for the preparation of high strength phosphoric acid-activated geopolymer, which shows potential for the proper disposal and effective recycling of fly ash.