The effects of silica fume and superplasticizer on cement hydration under low water cement ratio

The water-to-cement ratio of ultra high strength concrete is extremely low; therefore the addition of high-range water reducing agent (suerplasticizer: SP) and silica fume (SF) is indispensable at mixing. This research was conducted to elucidate how a high-range water-reducing agent of plycarboxylic acid type for high strength and a high-range water-reducing agent of plycarboxylic acid type for ultra high strength, both different in molecular structure, would affect the compressive strength and hydrate of low-heat portland cement if added to the cement along with silica fume. The influences were discussed from the perspectives of cement hydration required by the Rietveld method using powder X-ray diffraction (XRD).
When the high-range water-reducing agent of plycarboxylic acid type for ultra-high strength concrete and silica fume were added, compressive strength slightly decreased in the first seven days of aging but showed no change after 28 days of aging.
Meanwhile, the C3S hydration rate reached 70 to 85% after 7-day aging, regardless of SP type and the amount of SF addition, but then became stagnant. However, SF addition increased the C2S hydration rate at the initial age. The SF hydration rate became 40 to 50% after 7-day aging and 60 to 70% after 91-day aging. However, adding more SF was found to reduce the reaction rate. Therefore, neither SP type nor molecular structure was found to affect C-S-H generation significantly.

Author: Y. Umemura, M. Satou, K. Koizumi, N. Tsuyuki

Section: Science and production

Keywords: silica fume, superplasticizer, C—S—H, Rietveld method