•Utilization of high volume fly ash in steam-cured cement mortars was investigated.•Both the early and late compressive strength of steam-cured mortars were enhanced by UFA.•Pore structure of steam-cured high volume FA blended composites was effectively refined by UFA.•The amount of CH after steam curing was generally lower than that with standard curing.•The MCL and Al/Si of C-S-H gels under steam curing were improved by UFA. Utilization of high volume fly ash is generally limited in steam-cured concrete products due to the delayed strength development. In present work, ultrafine fly ash (UFA) with a median particle size of 2.3 μm was prepared with wet-grinding technology. The influence of different FA fineness, UFA content (30–70%) and curing regimes on the compressive strength and microstructure of steam-cured high volume fly ash cement composites were studied. Both the early and late compressive strength of steam-cured FA blended composites were enhanced by wet-milled UFA with replacement level as high as 50%, and the strength was even higher than that of pure cement group. Pore structure of steam-cured high volume FA blended composites was effectively refined by UFA. The amount of CH after steam curing was generally lower than that with standard curing. Furthermore, the main chain length and Al/Si of C-S-H gels under steam curing were improved by UFA. Compared with prolonging the curing duration, increasing the curing temperature seems more efficient for the incorporation of aluminum from fly ash into the calcium silicate hydrates.