Early-Age Strength Development of Concrete Incorporating Fly Ash and Condensed Silica Fume

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Title: Early-Age Strength Development of Concrete Incorporating Fly Ash and Condensed Silica Fume

Author(s): G. Carette and V.M. Malhotra

Publication: Special Publication

Volume: 79

Issue:

Appears on pages(s): 765-784

Keywords: age-strength relation; compressive strength; concretes; fly ash; plasticizers; pozzolans; silica.

Date: 5/1/1983

Abstract:
Early-age strength development of concrete in which part of the portland cement has been replaced by low-calcium fly ash tends to be slow, because fly ash acts as a relatively inert component during this period of hydration, though at later ages it contributes significantly to strength development. It was considered that the problem of low early-age strength of portland cement-fly ash concrete could be overcome by the incorporation of small amounts of condensed silica fume, a very fine and more rapidly reactive pozzolan. This report presents the results of an investigation on the early-age strength development of concrete incorporating 30% low-calcium fly ash, and to which small amounts of condensed silica fume have been added. The amounts of the fume ranged from 0 to 20% by combined weight of the portland cement plus fly ash. A total of thirty 0.06-m3 concrete mixtures with water-(cement + fly ash) ratios ranging from 0.40 to 0.80 were made; 240 cylinders were tested in compression and 180 prisms were tested in flexure. A supplementary series of six concrete mixtures was made to deter-mine the effect of silica fume and fly ash on the long-term strength development of concrete. Test data showed that the incorporation of condensed silica fume increased the compressive strength of concrete at all ages as compared with the compressive strength of the control concrete (70% portland cement + 30% fly ash). At 7 days, the loss of compressive strength due to the partial replacement of cement by fly ash was completely overcome by the addition of 10% condensed silica fume for concretes with water-(cement + fly ash) ratios ranging from 0.40 to 0.60; 15 to 20% was required for concretes with higher water-(cement + fly ash) ratios, At 28 days, regardless of the water-(cement + fly ash) ratio, the effect was generally achieved with less than 5% silica fume addition. The laterage strength development of portland cement-fly ash concrete did not appear to be impaired by the use of condensed silica fume indicating availability of sufficient lime for the fly ash pozzolanic activity.