Strength and Heat Development in Concrete: Influence of Fly Ash and Condensed Silica Fume


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Title: Strength and Heat Development in Concrete: Influence of Fly Ash and Condensed Silica Fume

Author(s): Magne Maage

Publication: Special Publication

Volume: 91


Appears on pages(s): 923-940

Keywords: age-strength relation; blended cements; compressive strength; concretes; curing; fly ash; heat of hydration; silica; temperature.

Date: 2/1/1986

In order to reduce energy, save raw materials and improve mechanical properties, different pozzolans are now commonly used in cement and concrete production. A comprehensive research program was undertaken where cement and concrete properties, influenced by fly ash and condensed silica fume in different combinations, were investigated. This paper presents their influence on strength and heat development. The program included an ordinary portland cement and two blended cements with 10 % and 25 % fly ash respectively. The three cements were combined with 0 %, 5 % and 10 % condensed silica fume. Curing temperatures used were 5°C, 20°C and 35°C. Condensed silica fume is very finely graded and the content of amorphous Si02 is very high. The pozzolana reaction starts therefore early, at 20°C from around 7 days, at 35°C from around 2 days. At 5°C, no pozzolana reaction was observed for the first 28 days. The pozzolana reaction from fly ash was found to be slower than the reaction from condensed silica fume, probably due to the coarser grinding and the lower Si02-content. The compressive strength results indicated that the pozzo-lana reaction was more sensitive to the temperature than the reaction involving cement hydration alone. The slow strength development of concrete when using fly ash in blended cements can be avoided by grinding the cements to a higher fineness. The effect on strength development when using condensed silica fume was approximately the same in all three types of cement investigated. The heat development was higher in pure portland cement than in blended cements. However, when adding condensed silica fume, the heat development increased. Maturity functions were found to be valid up to maturities corresponding to curing in 20°C for approximately 2 days.