MICROSTRUCTURAL CHANGES OF FINE-GRAINED CONCRETE EXPOSED TO A SULFATE ATTACK

Sulfate attack is one of the major threats to durability of concrete constructions and it becomes a major destructor of sewage-collection systems where concrete sewer pipes are exposed to sulfates. The most frequent biodeterioration in sewage pipes is caused by biogenic sulfuric-acid corrosion. During this attack, the pH of the surfaces of concrete sewer pipes is reduced and chemical reactions lead to the cracking and scaling of the concrete material, accelerated by the sewage flow. This paper is focused on the sulfate attack on fine-grained concrete where the effect of a 0.5 % sulfuric-acid solution on the microstructural changes of various types of concrete after a treatment for a period of 6 months was investigated with mercury intrusion porosimetry and scanning electron microscopy. It was found that the total porosity of most samples was decreased after the sulfate attack, indicated by the products of the sulfate corrosion filling in the pores of the concrete. The smallest changes in the microstructure were observed in the samples made from sulfate-resisting cements. The formation of the locations rich in sulfur, iron and aluminum during the sulfate attack of the concrete was determined by mapping the chemical-element distribution.