Freeze–Thaw Cycle–Driven Pore Structure Evolution in Cement Paste at Varying Freezing Temperatures: Quantitative CT Analysis and Strength Damage Prediction

Abstract Freezing temperature strongly affects freeze–thaw (F-T) deterioration of cement-based materials, but its role in regulating three-dimensional pore morphology and geometrical complexity remains insufficiently quantified. In this study, X-ray computed tomography (CT) was used to characterize the evolution of the pore structure of cement paste subjected to F-T cycles at −20°C and −15°C. Porosity, average pore diameter, pore size distribution, pore morphology, and fractal dimension were analyzed to clarify the temperature-dependent deterioration pathway. The results show that F-T cycling promotes pore coarsening and connectivity, shifting the pore size distribution from small to medium and large pores. Meanwhile, pore morphology progressively evolves toward blade-type pores with high aspect ratios, accompanied by an increase in fractal dimension, indicating greater irregularity and complexity of the pore network. Compared with −15°C, freezing at −20°C induces a larger increase in porosity, a more pronounced transformation toward blade-type pores, and a greater rise in fractal dimension. These results demonstrate that a lower freezing temperature accelerates the coupled deterioration of pore volume, pore morphology, and geometrical complexity. Grey relational analysis identified the fractal dimension, blade-type pore fraction, and porosity as the pore indicators most closely associated with compressive strength damage. An interpretable regression relationship based on these indicators achieved a coefficient of determination of 0.9497 and an average prediction error of 8%. These findings provide experimental evidence for understanding temperature-related F-T damage in cement paste.