New approach for evaluating chloride ion penetration of mortar in cold environments

The aim of this paper was to use the surface fractal dimension (D_s) to evaluate the chloride ion penetration of cement mortar in extremely cold regions. Pore structure is a critical factor that influences the permeability of cement mortar. This study investigated surface fractal dimensions of the pore structure of cement mortars, which were subject to one-time freezing for 7 days after casting. A thermodynamic model was applied to calculate D_s on the basis of mercury intrusion porosimetry data. The results show that the pore structures of all specimens exhibit the scale-dependent fractal property, and three distinct fractal regions (I, II, III) are obtained. According to the regression analysis results of fractal dimension and pore structure parameters, it was found that the fractal dimension can characterise the complexity and chaos of pore structure and predict pore size distribution. Based on this, a linear regression between chloride diffusion coefficient (D_nssm) and D_s in fractal regions was carried out. The results show that D_s of micro pores (region I) have good correlation with D_nssm, up to 0.92908, which is more accurate than the porosity. An index of surface fractal dimension is proposed to evaluate the durability of concrete buildings in extremely cold regions.