Multi-scale analysis of the influence of negative temperature invasion on the hydration degree and microstructure of hydraulic concrete

To explore the impact of negative temperature on hydraulic concrete hardening, samples underwent frost simulations at −5°C for 6, 12, 24, and 48 h in an environmental chamber. Results show that frost occurring over 24 h post-initial setting causes severe matrix cracking, pore wall rupture, and expansion of C-S-H interlayer defects. At 48 h of exposure, the relative hydration degree drops to 41.33 %, indicating irreversible damage. Prolonged cold exposure transforms gel pores into various capillary and large pores, increasing the proportion of transition, capillary, and large pores. The 28-day strength exhibits a nonlinear decline with exposure duration, while both energy storage and dissipation capacities diminish.