Modification Effects of Nano-SiO2 on Early Compressive Strength and Hydration Characteristics of High-Volume Fly Ash Concrete

Utilization of nanomaterials in high-volume fly ash (HVFA) concrete as a means of improving concrete’s properties is feasible. This paper presents experimental data to evaluate the modification effects of nano-SiO2 (NS) on early compressive strength and hydration properties of HVFA concrete. To this end, a mixture design method was used to experimentally investigate the early compressive strength development in the NS-incorporated HVFA concrete. For a better understanding of the modification process, several important hydration characteristics of the selected NS-incorporated HVFA cement pastes were analyzed for hydration kinetics, phase composition, and pore structure through isothermal calorimetry, X-ray diffraction (XRD), thermogravimetric/derivative thermogravimetric (TG/DTG), and mercury intrusion porosimetry (MIP) measurements. Experimental results confirmed that NS can partially compensate for the early compressive strength loss of HVFA concrete resulting from high levels of fly ash (FA) replacement. The regression model from mixture design based experiments indicated that the mixture proportion of 58% ordinary Portland cement (OPC), 40% (FA), and 2% (NS) produced the highest early compressive strengths of concrete at 22.9 and 29.0 MPa, for 3 and 7 days, respectively. Hydration characteristics revealed that the addition of NS enhanced early age cement hydration and the formation of calcium silicate hydrate (C-S-H) gels. It also showed a reduction in calcium hydroxide (CH) and porosity in HVFA cement pastes, further contributing to the early compressive strength development of HVFA concrete combined with NS.