Curing Regimes and Fly Ash Effects on UHPCM Properties Post High‐Temperature Exposure

To promote the application of fly ash (FA) in UHPC at elevated temperatures and achieve green development in the construction industry. In this study, the effects of curing regimes and FA on the evolution of compressive properties, hydrates, and microstructure of ultra‐high performance cementitious materials (UHPCM) at elevated temperatures were systematically investigated by macroscopic mechanical performance testing and microscopic analysis. The results indicated that the combination of steam curing and dry‐hot air curing could accelerate the secondary hydration to significantly enhance the compressive strength of UHPCM. As the FA content increased from 0% to 15%, the residual compressive strength at elevated temperatures was improved, the specimen cracks reduced in size, and the mass loss rate decreased after elevated temperatures. The mechanism of FA improving the fire resistance of UHPCM is that the secondary hydration reaction of FA was intensified by thermal curing, especially the suitable dry‐hot air curing, to generate more stable calcium silicate hydrates, which filled the internal defects. This process had been well verified by analyzing the samples through X‐ray diffractometer (XRD) and scanning electron microscope (SEM). Furthermore, as the exposure temperature increased, the residual strength of UHPCM initially increased and then decreased, reaching a maximum of 217.4 MPa at 400°C. Finally, based on the analysis of the test results, a compressive strength model of UHPCM at elevated temperatures was established considering the influence of FA content.