Localized Utilization of Silica Fume in High-Volume Fly Ash Concrete Through Densification of the Interfacial Transition Zone

Interfacial transition zone (ITZ) is usually the weakest area in concrete, especially in high-volume fly ash (HVFA) concrete, where most fly ash particles remain unreacted due to their relatively low reactivity. In this case, some supplementary cementitious materials featuring high reactivity, such as silica fume, are usually added directly into HVFA concrete to improve its ITZ. This study proposed a targeted aggregate pretreatment method, which only employed silica fume in the ITZ to enhance its material-use advantages. The approach involved pre-coating coarse aggregate with a silica fume-modified cement paste layer and casting HVFA concrete using such pretreated coarse aggregate. The influence of five key parameters during the preparation of silica fume-modified paste, namely the water-to-cementitious material ratio, the content of silica fume, the mass ratio of coarse aggregate to cement paste, the mixing time for blending coarse aggregate and paste, and the drying time after mixing, on the void fraction of the ITZ of HVFA concrete cast with such coated coarse aggregate was investigated. Response surface optimization identified W/CM = 0.3, SF/CM = 9%, Ca/CP = 4, and a mixing time of 3 min as the factor settings for minimizing the ITZ void fraction within the investigated domain. A drying time of 5 h was selected for experimental confirmation, yielding an experimentally measured ITZ void fraction of only 21.4%. Furthermore, compared to the control groups, the optimized HVFA concrete exhibited improved compressive strength and water absorption performance and densified ITZ microstructure. Notably, the silica fume consumption in this approach was 2.8–3.5 times lower than the commonly reported dosage range of 8–10%, demonstrating material-use advantages alongside considerable performance enhancement.