Evaluating the chloride diffusion coefficient of cement mortars based on the tortuosity of pore structurally-designed cement pastes

The chloride diffusion coefficient of mortar was estimated commonly based on the porosity or critical pore size of cement paste, presenting large deviation between theoretical calculation values and measurement results, and then low reliability. The diffusion pathway of chloride can be theoretically characterized by pore tortuosity, which combines the effects of both porosity and pore size distribution. However, correlation between chloride diffusion coefficient and pore tortuosity has not been established yet. In present study, the pore structurally designed of cement pastes were prepared by changing the particle size of Portland cement and water-to-cement ratio, then porosity, pore size distribution, and pore tortuosity were calculated based on mercury intrusionextrusion profile. A conception of ?tortuosity contribution factor? was proposed to quantify the contribution of each pore size fraction to the tortuosity of cement paste. The small capillary pore with radius of 5?20 nm had highest contribution to the pore tortuosity. For a given porosity, 10% increase of small capillary pore led to a 22.6% increase of pore tortuosity, resulting in a 33.5% drop of chloride diffusion coefficient. While 10% increase of large-sized capillary pore with radius of 20?50 nm resulted in 5.7% increase of pore tortuosity and consequently 10.5% drop of chloride diffusion coefficient. The results will provide a better understanding in evaluation of chloride diffusivity of cement-based materials and optimization of pore structure to improve chloride resistance efficiently.

Automated summary

The study examined the relationship between chloride diffusion coefficient and pore tortuosity in cement pastes, finding that small capillary pores with radius of 5-20 nm have the highest contribution to pore tortuosity and chloride resistance. The results suggest that optimizing the pore structure of cement-based materials can effectively improve chloride resistance.