High Strength and Concrete Binders Part A: Reactivity and Composition of Cement Pastes With and Without Condensed Silica Fume

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Title: High Strength and Concrete Binders Part A: Reactivity and Composition of Cement Pastes With and Without Condensed Silica Fume

Author(s): H. Justness, E. J. Sellevold and G. Lundevall.

Publication: Special Publication

Volume: 132

Issue:

Appears on pages(s): 873-890

Keywords: binders (materials); cement pastes; pozzolanic reactions; hydration; silica fume; Materials Research

Date: 5/1/1992

Abstract:
Cement pastes of interest for high-strength concrete technology were investigated by high-resolution solid state magic angle spinning (MAS) Si-nuclear magnetic resonance (NMR) in combination with thermal analysis (DTA/TG). NMR reveals the degree of hydration for C3S/C2S in cement, pozzolanic activity of condensed silica fume, and average chain length of the silicate anions in the CSH-gel. A combination of NMR and DTA/TC data gives the empirical formula of the CSH-gel. The binders investigated were made from blended portland cement containing 0, 8, and 16 percent cement replacement with condensed silica fume and water-binder ratios of 0.20, 0.30, and 0.40. The specimens were allowed to cure in sealed conditions for 1, 3, 7, 28, 126, and 442 days. The results confirmed that condensed silica fume is a very reactive pozzolan. The conversion rate of condensed silica fume to hydration products after 3 days of curing was, in fact, higher than for the neat cement at the same age. After 3 days of curing, condensed silica fume reduced the degree of hydration of the cement in the blended cement pastes when compared with pastes without it. The effect was enhanced at later ages when the cement hydration process stopped while the pozzolanic reaction continued to near completion. In regard to the composition of the CSH-gel, it was found that the average chain length for the linear polysilicate anions increased with decreasing w(c + s) and, in particular, with increasing dosages of condensed silica fume. Furthermore, the c/s of the gel decreased considerably with increasing dosages of condensed silica fume. The mechanism of the pozzolanic reaction of condensed silica fume is discussed.