Effect of Fly Ash Mechanical Activation on the Rheology, Hydration, Strength, and Microstructure of Cementitious Systems

Mechanical activation is a promising approach to improving the performance of low-calcium fly ash (FA) in cementitious materials; however, its effect on time-dependent rheology remains insufficiently quantified. This study investigates the influence of mechanical activation of fly ash from the Almaty combined heat and power plant No. 3 (CHP-3) on the rheological, hydration, mechanical, and microstructural properties at cement replacement levels of 5–20 wt.%. Grinding increased the specific surface area from 3710 to 6134 cm2/g. Cement mortars containing as-received FA or mechanically activated fly ash (AFA) were evaluated by rotational rheometry after 5 and 90 min of resting using the Herschel–Bulkley model, while cement pastes were examined by semi-adiabatic exothermic measurements and microstructural analysis. Mechanical activation generally increased flow resistance and affected the time-dependent rheological response, with pronounced changes in viscosity and hysteresis after 90 min that remained detectable after standardized remixing. At identical replacement levels, AFA pastes exhibited maximum temperatures 1.3–7.0 °C higher than FA pastes (temperature rise higher by 2.1–6.8 °C), indicating enhanced early hydration. At 28 days, AFA mortars reached compressive strengths of 43.7, 41.4, 36.8 and 35.0 MPa at 5, 10, 15, and 20% replacement, respectively, remaining above the reference mortar (33.3 MPa), whereas FA mortars exceeded the reference only at 5–10%. The semi-quantitative X-ray diffraction (XRD) portlandite-to-quartz intensity ratio was approximately 18–31% lower for AFA15-P than for FA15-P, consistent with enhanced portlandite consumption, while scanning electron microscopy (SEM) of the same 15% pastes revealed a denser matrix and improved ash–hydrate intergrowth after activation. Overall, mechanical activation extended the fly ash replacement level at which the 28-day strength remained above the reference from 10% to 20%, while increasing rheological resistance.