Improving the properties of metakaolin/fly ash composite geopolymers with ultrafine fly ash ground by steam-jet mill

[Display omitted] •The steam-jet mill reduces the fly ash (FA) particle size and converts the particles into an irregular shape.•Small particle size FA have higher reactivity than original FA.•Small particle size FA helps to optimise the pore structure of the geopolymer.•The mechanical strength of the composite geopolymer was improved by the crushed FA. The particle size distribution and shape of fly ash (FA) are important factors affecting the properties of geopolymers. In this study, a steam-jet mill was used to optimize the fineness and properties of FA as a replacement for metakaolin (MK) in the production of geopolymers. The structural changes of FA by different grinding processes were observed by particle size analysis and SEM. As the treatment frequency increased, the particle size of FA decreased and the particles changed from spherical to irregular shape. XRD, FT-IR, SEM, EDS and LF-NMR were used to reveal the hydration mechanism of geopolymers with different treatment processes and FA substitution ratios and to investigate their mechanical properties. The results showed that the small particle size and irregular shape of the FA had high activity and excellent filling effect. After 28 days, the compressive strength of B-F30 (20% substitution MK and 30 Hz treatment) was 67.80 MPa, which was similar to that of A (0% substitution MK) at 66.36 MPa. FA crushed by the steam-jet mill allows optimization of the pore structure and further increases the compressive strength of geopolymers. This provides an effective way to prepare high-performance geopolymers using solid waste.