Surface activation of acid-etched and aged fly ash and its enhancement mechanism for the mechanical performance of cement-based materials

To increase the substitution rate of fly ash (FA) as a mineral admixture in cement-based materials, it is essential to activate its latent reactivity through pre-treatment modification. However, existing methods either fail to sufficiently activate FA or make the modified FA difficult to apply effectively in cement-based materials. This study presents a modification method using acid-etching and aging to enhance the reactivity of FA. It also investigates the influences of acid activator type, acid dosage, and aging time on the mechanical properties of acid-etched and aged fly ash cement-based materials (AAFCM). Scanning electron microscopy (SEM), X-ray diffraction (XRD), particle size distribution (PSD), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and mercury intrusion porosimetry (MIP) are used to examine the effects of acid-etching and aging on the particle characteristics of FA. Additionally, the study explores how these modifications influence the phase composition, microstructure, and pore structure of fly ash cement-based materials (FCM). The findings reveal the surface activation mechanism of acid-etched and aged fly ash (AAFA) and provide an in-depth analysis of its modification and enhancement mechanisms in cement-based materials. Experimental results demonstrate that acid-etching and aging significantly increase the specific surface area and surface roughness of FA. This promotes the transformation of highly crystalline aluminosilicate minerals into highly active amorphous minerals. After 48 h of acid-etching and aging with 4 % hydrochloric acid, both early and late compressive strengths of the AAFCM are significantly higher than those of untreated FCM. Microscopic analysis further indicates that acid-etching and aging accelerate the early hydration reactions of FA and cement. During cement hydration, AAFA provides abundant active ions and nucleation sites, which speeds up the formation of hydration products, enhances matrix densification, and significantly improves the mechanical properties of AAFCM.