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by Clyde W Fourie, Mark G Alexander
11th International Congress on the Chemistry of Cement. ()


Concrete for use in sewers that are prone to the biological production of sulphuric acid needs to be manufactured using materials capable of resisting acid attack. The acid readily attacks certain constituents in the concrete, such as cements and calcareous materials, exposing the steel reinforcing, which can lead to collapse of the pipe. Altering the nature of the binder can significantly improve the acid resistance of the concrete. The physical and chemical nature of the binder can be modified by incorporating mineral extenders into the mixture or by replacing the normal Portland Cement with Calcium Aluminate Cement. Generally, acid-soluble calcareous aggregates are used in concrete mixtures to prolong the life of pipes exposed to acidic environments. The dissolution of the calcareous aggregate neutralises a mild sulphuric acid environment (pH = 3,00) and ensures that the acid attack is spread over the entire surface of the concrete, and is not only concentrated on the binder portion. It is speculated that the neutralisation of the acid environment not only slows down the rate of the sulphuric acid attack but also reduces the activity of the acid-loving thiobacillus bacteria, thus decreasing the rate of sulphuric acid production. However, in aggressive sulphuric acid environments (pH = 1,00) the calcareous layer is dissolved rapidly. Under these conditions, Calcium Aluminate Cement is more effective in improving the acid resistance of the concrete. The chemistry of the Calcium Aluminate Cement is such that it is claimed to have a higher neutralisation capacity than Portland Cement, and the Calcium Aluminate Cement appears to be toxic to the thiobacillus bacteria. The acid resistance of the mixture is further improved by using a synthetic aggregate such as Alag™ in place of the calcareous aggregate.

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