Evolution and damage threshold of pores for natural pumice concrete under freeze–thaw cycles

Natural pumice concrete (NPC) is a building material with the advantages of light weight and high thermal resistance. In cold regions, NPC undergoes damage from freeze–thaw cycles. Freeze–thaw damage is closely related to changes in the pore structure of concrete. Therefore, it is meaningful to investigate the evolution characteristics and damage threshold of pore structure for NPC under freeze–thaw cycles. In this study, freeze–thaw cycle tests and nuclear magnetic resonance (NMR) tests were designed. The characteristics of the evolution of the pore structure during freeze–thaw cycles were discussed. The results showed that the porosity in NPC specimens increases with the number of freeze–thaw cycles and the main evolution of the pores showed the degradation of fine capillary pores (10 nm < r < 100 nm) into coarse capillary pores (100 nm < r < 1000 nm) and non-capillary pores (r > 1000 nm). After freeze–thaw cycles, the proportion of coarse capillary pores and non-capillary pores increased by 4.83–10.59%. This evolutionary feature directly leads to the degradation of the mechanical properties of NPC. Additionally, a pore damage model was established, and the pore damage threshold was also calculated based on the experimental results. The obtained damage threshold of pore structure can provide the theoretical foundation for the application of NPC in cold regions.