Biogenic sulfuric acid corrosion within wastewater environments
Luca-Alexander Kempf, Institute for Building Materials, Ruhr-University Bochum, Bochum, GermanyRolf Breitenbücher, Institute for Building Materials, Ruhr-University Bochum, Bochum, Germany
Concrete is an established and commonly used building material in the field of sewage construction. In sewage systems, concrete is exposed to harsh environment concerning chemical attacks, which can lead to a rapid degradation. This degradation is caused by a microbiologically induced type of corrosion, known as biogenic sulfuric acid corrosion (BSA). After a few years under service, damages can occur in sewage pipes as a result of exceeding the contaminant concentration limits of the wastewater (user), the use of insufficiently designed concretes (planner) or the development of unfavorable operating conditions (operator). Anaerobic areas in sewers, formed by certain conditions such as slow flow rates, partial filling and low circulation of air, release hydrogen sulfide gas (H2S) into the space above the sewage. Sulfur oxidizing bacteria subsequently converts the hydrogen sulfide gas into sulfuric acid (H2SO4) with a pH value as low as 1, which in turn leads to a strong acidic attack on the concrete structure. The sulfuric acid causes a combined dissolving and due to the formation of ettringite expanding attack within the microstructure of the cement paste. In order to counteract this exposure, Eurocode 2 proposes a coating of the entire concrete surface. Since such coatings are expensive and not sufficient durable to withstand the chemical attack over the service life of sewer pipes, further solutions are necessary. Taken into account that ordinary Portland cement is highly affected by acidic attack, prior investigations have pointed out a positive effect to the acid-resistance of concrete by the use of ground-granulated blast-furnace slag (GGBS), a by-product of the steel fabrication. To prove this positive effect of GGBS and find the best possible proportion considering also different fineness of this compound, an accelerated test method, which reproduces the attacks as accurately as possible, has been developed incl. the relevant characteristic limiting values.