Effect of Targeted Application of Slag in the Interfacial Transition Zone on the Freeze–Thaw Resistance of High-Volume Fly Ash Concrete

High-volume fly ash (HVFA) concrete often encounters degraded freeze–thaw (F/T) resistance due to the detrimental impact of unburnt carbon particles present in fly ash on the air void system. Given the susceptibility of the interfacial transition zone (ITZ) in HVFA concrete to deterioration during the F/T damaging process, this work proposed an approach to enhance the F/T resistance of HVFA concrete by improving the ITZ. Through the application of slag-modified cementitious paste coating on coarse aggregate and subsequent air-drying, the treated coarse aggregate was prepared. The compressive strength and F/T resistance of HVFA concrete containing such aggregate were examined. In addition, the water sorptivity of HVFA concrete was evaluated both pre- and postexposure to F/T cycles to establish a relationship with its F/T resistance. Results showed that the compressive strength of HVFA concrete was increased by 24.4% while the number of F/T cycles it can withstand was increased from 48 to 102 times without air entrainment. At last, analysis conducted via a scanning electron microscope (SEM) and X-ray diffraction (XRD) offered a more comprehensive insight into the correlation between the microstructure compactness/hydration products of the ITZ and the F/T resistance of HVFA concrete.