Controversial effect of Silica fume in concrete

Use of silica in concrete is one of the example that something that has been labeled as harmful can become a useful thing if we know its property in details. A branch of scientist are reporting that the Aggregate-Silica Reaction is responsible for deterioration of structure, while another group of researcher are using silica to improve the quality of concrete. What is going on?

A study was done at the University of Texas to dissect this controversy.

Silica fume is added in concrete due to its small grain size that helps to efficiently pack the concrete mass resulting in higher density. In addition silica also helps to hydrate the cement and form C-S-H thus helping to increase the concrete’s strength. However, there are impurities in silica fume i.e. there is mixture of larger size silica particles which are formed during grinding. It has been reported that such larger size silica are reactive and aggravate ASR. Nonetheless, there is no concluding results because results from various researchers conflicts with each other.

It has been widely accepted that reactive silica exhibits pessimum effect i.e. the expansion peaks at a certain percentage of silica content rather than increasing linearly. When ratio of silica to  alkali is high expansion is lowered because the reaction occurs vigorously and the reactants are consumed before the concrete is hardened. On the other hand, when ratio of silica to alkali is low, there is not sufficient ASR to contribute to total expansion. A sample of experimental result shown below indicates that at silica fume content of about 4% (by weight of cement), the expansion is maximum for both reactive and non-reactive aggregates. This result indicates that silica fume should be used above 10% if we want to make the structure more durable. Addition of lower silica fume can infact backfire and cause degradation of concrete in a shorter time period instead of adding to its strength.

asr

 

Reference

  • Maas, Andrew J., Jason H. Ideker, and Maria C. G. Juenger. “Alkali Silica Reactivity of Agglomerated Silica Fume.” Cement and Concrete Research 37, no. 2 (February 1, 2007): 166–74. https://doi.org/10.1016/j.cemconres.2006.10.011.
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