Abstract: Climatic conditions in the alpine region are complicated and changeable. A sudden temperature drop is easy to impact the hydration degree and pore structure of fresh concrete, resulting in its early frost damage, extremely unfavorable to the development of its physical properties such as strength. To explore the damage mechanism of hydraulic concrete in early freezing conditions, this study conducts experimental tests on its mechanical properties, hydration behavior, and pore structure, by varying pre-curing time (6 h, 12 h, 24 h, and 48 h), freezing time (6 h, 12 h, 24 h, and 48 h), and freezing temperature (-1 °C, -5 °C, and -10 °C). The results show that an increase in freezing time or a decrease in freezing temperature increases porosity and decreases hydration degree, while a longer pre-curing time increases hydration degree and transforms more large pores into small ones. The compressive strength of early frozen concrete is proportional to pre-curing time, and inversely proportional to freezing time or freezing temperature; its pore structure shows obvious fractal characteristics, and significant correlation exists between the hydration degree and mechanical properties.

Key words: hydraulic concrete, early freezing, degree of hydration, pore structure, fractal dimension