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Influence of Silica Fume on Diffusivity in Cement-Based Materials. I. Experimental and Computer Modeling Studies on Cement Pastes.


pdf icon Influence of Silica Fume on Diffusivity in Cement-Based Materials. I. Experimental and Computer Modeling Studies on Cement Pastes. (1032 K)
Bentz, D. P.; Jensen, O. M.; Coats, A. M.; Glasser, F. P.

Cement and Concrete Research, Vol. 30, No. 6, 953-962, June 2000.

Keywords:

cements; computer models; cement paste; diffusion; hydration; silica fume

Abstract:

Experimental and computer modeling studies are applied in determining the influence of silica fume on the microstructure and diffusvity of cement paste. It is suggested that silica fume modifies the inherent nanostructure of the calcium silicate hydrate (C-S-H) gel, reducing its porosity and thus increasing its resistance to diffusion of both tritiated water and chloride ions. Because the pores in the C-S-H are extremely fine, the relative reduction in diffision depends on the specific diffusing species. Based on the NIST cement hydration and microstructural model, for tritiated water diffusion, the reduction in the diffusivity of the gel caused by silica fumee is about a factor of five. For chloride ions, when a diffusivity value 25 times lower than that used for conventional high Ca/Si ratio C-S-H is assigned to the pozzolanic lower Ca/Si ratio C-S-H, excellent agreement is obtained between experimental chloride ion diffusivvity data and results generated based on the NIST model, for silica fume additions ranging from 0% to 10%. For higher addition rates, the experimentally observed reduction in diffusivity is significantly greater than that predicted from the computer models, suggesting that at these very high dosages, the nanostructure of the pozzohmic C-S-H may be even further modified. Based on the hydration model, a percolation-based explanation of the influence of silica fume on diffusivity is proposed and a set of equations relating diffusivity to capillary porosity and silica fume addition rate is developed. A 10% addition of silica fume may result in a factor of 15 or more reduction in chloride ion diffusion and could potentially lead to a substantial increase in the service life of steel-reinforced concrete exposed to a severe environment.