Influence of silica fume on diffusivity in cement-based materials I. Experimental and computer modeling studies on cement pastes

Experimental and computer modeling studies are applied in determining the i nfluence of silica fume on the microstructure and diffusivity of cement pas te. It is suggested that silica fume modifies the inherent nanostructure of the calcium silicate hydrate (C-S-H) gel, reducing its porosity and thus i ncreasing its resistance to diffusion of both tritiated water and chloride ions. Because the pores in the C-S-H are extremely fine, the relative reduc tion in diffusion depends on the specific diffusing species. Based on the N IST cement hydration and microstructural model, for tritiated water diffusi on, the reduction in the diffusivity of the gel caused by silica fume is ab out 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 be tween experimental chloride ion diffusivity data and results generated base d 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, sug gesting that at these very high dosages, the nanostructure of the pozzolani c C-S-H may be even further modified. Based on the hydration model, a perco lation-based explanation of the influence of silica fume on diffusivity is proposed and a set of equations relating diffusivity to capillary porosity and silica fame addition rate is developed. A 10% addition of silica fume m ay result in a factor of 15 or mon reduction in chloride ion diffusion and could potentially lead to a substantial increase in the service life of ste el-reinforced concrete exposed to a severe environment. (C) 2000 Elsevier S cience Ltd. All rights reserved.