Abstract: Microwave-curing and mechanical grinding of fly ash have both beenadopted as effective methods for improving the early-age strength of alkali-activated fly ash (AAFA) binders. This study combined these two approaches by synthesizing AAFA using original, medium-fine, and ultrafine fly ash as precursors, and then specimens were cured with a five-stage temperature-controlled microwave. The compressive strength results indicate that the original AAFA develops the highest strength initially during microwave-curing, reaching 28 MPa at stage 2. Medium-fine AAFA exhibits the highest strength of 60 MPa when cured to stage 4-I, which is 26% higher than the peak strength of original AAFA. It is attributed to the significant rise in their specific surface area, which accelerates the dissolution of Si and Al from the precursor and facilitates the subsequent formation of N-A-S-H gels. Additionally, nanoscale zeolite crystals formed as secondary products fill the tiny gaps between amorphous products, thereby significantly improving their microstructure. In contrast, ultrafine fly ash, primarily composed of fragmented particles, necessitated a substantial amount of water, which adversely affects the absorption efficiency for microwave of AAFA specimens. Thus, ultrafine AAFA specimens consistently exhibit the lowest compressive strength. Specifically, at the end of curing, the compressive strength of these three specimens with microwave-curing is approximately 32%, 59%, and 172% higher than that of the steam-cured sample, respectively. These findings demonstrate the compatibility of microwave-curing and fly ash refinement in enhancing the early compressive strength development of AAFA.
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2025, Vol. 39 
