Development of a biodeterioration accelerated test to understand the behavior of cementitious materials made with Calcium Aluminate Cement in sewer networks
Résumé
Sewer networks contain many aggressive and corrosive agents for pipe materials. One type of damage can be attributed to concrete corrosion by biogenous sulfuric acid. Under particular conditions biogenic hydrogen sulfide may be released in the headspace. This latter sulfur species is biologically converted into sulfuric acid by the action of a succession of sulfur-oxidizing bacteria in the pipe headspace. According to field data; cementitious materials made with Calcium Aluminate Cement (CAC) offer far better performance on site than those made of Ordinary Portland Cement (OPC). The development of an accelerated and pertinent laboratory test is necessary to better understand the mechanisms involved in both cementitious materials. Biodegradation mechanisms can be illustrated by four steps: Abiotic oxidation of H2S into elemental sulfur on the surface of cementitious material, Oxidation of sulfur to sulfuric acid by bacteria, Migration of sulfuric acid into gypsum layer, Acid attack of cementitious materials producing gypsum expansive layer. The study of each step allows identifying elements necessary to establish an accelerated test reproducing the conditions found in the aerial part of sewer networks. Such a test is nowadays developed in Ifsttar. The first samples are analyzed and are expected to reveal differences in behavior between the different samples: OPC/sand (use as a reference), CAC/sand and CAC/Alag® (CAC aggregate). Meanwhile, an in situ exposure campaign is performed in France with the same type of samples to compare laboratory results with those obtained on site. This comparison will be helpful to validate the laboratory test.