Study On Pore Structure,Impermeability And Strength Of Early Frozen Cement-Based Materials

The early frost may cause serious injury to the internal pore structure of concrete,and hence has a strong influence on the impermeability and mechanical properties of concrete.It is of great theoretical and engineering application to study the change of pore microstructure and to study the development regulation of strength and impermeability after freezing and to master the process of early frost damage.In this paper,fractal theory is applied in the early frozen cement-based materials,the pore range is divided according to the scale-dependent nature of fractal theory,and the physical meaning of fractal dimension is verified,that is,to quantify the complexity of the pore structure.The development and variation of chloride ion diffusion and strength of early frozen cement-based materials with different experimental factors are studied,and the internal mechanism of the change regularity is given based on the pore structure.The effect of negative temperature on the development of pore structure is studied,and the process of early frost is described.The results show that the early frozen cement-based materials have obvious fractal characteristics,and the pore range is divided according to the scale-dependent nature.It is found that the pore range is not affected by the test factors,but the pore size distribution of the paste and mortar is not the same.Under the condition of early freezing,for the cement paste with precuring for 24 hours,the pore range of 10 ~ 350 nm increases the short-term strength,and the pore range of 350 ~ 6000 nm affects the long-term strength.For the mortar with precuring for 24 hours,the pore range of 10 ~ 2000 nm is the main factor affecting the strength.Due to inadequate pre-curing,the loss of strength is very large,and the pore range of more than 6000 nm is stable after precuring for 24 hours.The pore range of more than 2000 nm affects the strength and it is also affected by precuring time.It tends to be stable in this range after precuring for 24 hours.The pore in each range has a certain effect on the impermeability of cement-based materials.For the mortar with precuring for 24 hours,the effect is greater than others in the range of 10 ~ 100 nm,and for the cement paste with precuring for 24 hours,The pore in the range of 10 ~ 6000 nm is the main factor of affecting the diffusion coefficient of chloride ion.Early frost will change the pore size distribution of cement-based materials,in which the negative temperature will affect the pore range.And it is found that the early frost is the process of pore structure deterioration and repair.During this process,liquid water plays an important role,moisture migration and ice propagation cause irreversible damage to the pore structure.