Biogenic and Chemical Sulfuric Acid Corrosion of Mortars

Abstract

The biogenic corrosion of mortars by sulfuric acid-producing bacteria was simulated in a controlled breeding chamber. Calcium aluminate cement (CAC) as well as portland and blended portland cement mortars were tested. Growth conditions for bacteria were optimized in a way that accelerated the naturally occurring corrosion (worst case) by a factor of at least 24. After 12 months of incubation in the chamber the samples being tested could be clearly classified according to their resistance properties. CAC mortars were found to resist microbiologically induced corrosion significantly better than portland and blended portland cement mortars. Measurement of the amount of biogenic sulfuric acid on CAC and portland and blended portland cement mortars demonstrated a higher amount of acid on the latter. The more effective resistance of CAC mortars was explained by the higher neutralization capacity due to their chemical composition and the smaller production of biogenic acid. Chemical immersion tests in sulfuric acid at pH 2 also revealed better resistance properties for the CAC mortars. But the mortars could not be distinguished as well as in the biological test because the samples were all exposed to the same concentration of acid. It is concluded that predictions concerning the long-term behavior of mortars used in sewerage networks are only reliable on the basis of the biogenic simulation experiment because the different metabolic activities of sulfuric acid-producing bacteria inhabiting different materials should be taken into account. Chemical corrosion tests tended to overestimate the resistance properties of portland and blast furnace cement mortars.