Better durability of calcium aluminate cement regarding biocorrosion in sewer networks
Résumé
A material, from its manufacture and throughout its service life, may suffer attacks that impact its initial properties. Depending on the environment in contact with this material it could be a physical, a chemical, a biological, or a combined aggression. Microbially induced corrosions which affect all materials are frequently encountered in various fields such as oil industry, power electricity production, aerospace industry… and may lead to the entire material destruction. Sewer networks are not an exception. Indeed, materials used in these structures are most generally made of cementitious materials that are subjected to corrosion by biogenous sulfuric acid. Under particular conditions, biogenic hydrogen sulfide may be released in the pipe headspace. This latter sulfur species is then biologically converted into sulfuric acid by the action of a succession of sulfur-oxidizing bacteria developing on the aerial part of the material surface. Sulfuric acid will stream on the wall of the structure and dissolve the cementitious matrix resulting in the collapse of the structure. However, all cementitious materials are not equivalently facing biodeterioration in sewer networks. According to field data, those made of Calcium Aluminate Cement (CAC) are well preserved with a surface pH reaching 3-4 while Ordinary Portland Cement (OPC) (largely used) are highly deteriorated with a surface pH less or equal to 1. This work aims at exploring the interest of CAC in sewer networks through the comparison of various formulations studied both on site and in laboratory. The presence of alumina in CAC materials confers a triple action against iocorrosion. Firstly, the oxidation of sulfur species on CAC materials is less favourable than on OPC ones. Sulfur-oxidizing microorganisms have fewer nutrients to grow and therefore produce much less sulfuric acid. Secondly, it buffers concrete surface pH at 3-4 limiting therefore the colonization of the most deleterious sulfur-oxidizing microorganisms. Finally soluble aluminium content of CAC materials has a bacteriostatic effect on some microorganisms, limiting the development of the biofilm. Due to the better CAC material durability in such conditions, it could be envisaged to use this material in other environments where acidic compounds are biologically generated.