Comparative study on the performance of ultra-fine fly ash prepared by different techniques in Portland cement and alkali-activated material

•Separated UFA is spherical while ground UFA is fragmented with larger specific area.•Activity index of ground UFA in PC mortar (114%) was higher than that of UFA1 (92%).•Ground UFA optimized pore structure of PC mortar due to its high pozzolanic activity.•Separated UFA exhibited higher strength in AAM due to the dense particle packing.•Porous products facilitated by ground UFA in AAM augmented total porosity. Ultra-fine fly ash (UFA) prepared using different techniques may exhibit various characteristics, which will affect their performance as cementitious material. This experiment adopted three UFA prepared through separation (UFA1), grinding (UFA2) and grinding followed by separation (UFA3) respectively, and investigated their impact on the properties of Portland cement (PC) and alkali-activated material (AAM), which are two commonly used fly ash-based construction materials. The results indicated that UFA1 consisted mainly of spherical particles, while UFA2 was dominated by broken particles with a larger specific surface area and more active Si/Al. In this case, UFA1 was superior in enhancing the fluidity of PC and AAM due to its particle morphology effect. However, the UFA that generated the highest compressive strength in PC and AAM systems differed; UFA2 in PC and UFA1 in AAM respectively. In detail, the activity index of UFA2 in PC mortar was 114% at 28 days, significantly higher than that of UFA1 (92%), because the high pozzolanic activity of UFA2 played the most crucial role in pore structure improvement. Conversely, the particle morphology of UFA was more important in assessing AAM strength. UFA1 effectively created multiple dense initial frameworks due to its high sphericity, maintaining most of the pores within the harmless pore range post hardening. UFA2 facilitated the formation of polymerization products such as N-A-S-H gel and zeolite, while their porous structures augmented total porosity. Furthermore, polymerization products could not effectively bridge gaps between fragmented UFA2 particles, leading to high macropore volume in AAM specimens. Whether in PC or AAM, UFA3 consistently exhibited a moderate performance. The observed differing performance of UFA prepared by different techniques in PC and AAM systems offers theoretical guidance for utilizing fly ash precisely in diverse applications.