The effect of variation in pore structure on the frost resistance of porous materials

Processes of freezing of water and melting of ice were studied using samples of hardened suspensions of plaster-of-paris with different mean pore sizes and constant porosities. Changes in the structure of the material were analyzed by measurement of the thermal and volume changes during cooling and heating cycles, and by the measurement of pore-size distribution and the changes of strength after the freezing cycles. It was shown that changes of volume within a temperature limit T=±20°C depend on the mean pore size and on the degree of water saturation. The disruption of the hardened plaster-of-paris structure was evaluated by determining the decrease in strength, if the nucleation of the ice crystals occurs during supercooling and the rate of their growth is greater than the rate of the amount of water forced out of the pores. The experimental observations support Power's theory concerning the frost resistance of porous material.