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