Durability of Innovative Cementitious Coatings for Concrete Wastewater Treatment Plants: Coupling of Biodeterioration and Cracking
Résumé
Calcium aluminate-based materials have shown high chemical and biological resistance to the attack in sewer systems1 and thus are likely to protect Portland cement-based structures and increase their durability. In this direction, a protective thin calcium aluminate-based coating, to be sprayed onto newly built wastewater infrastructures, was developed by IMERYS. It is intended to provide highest durability against severe H2S biogenic corrosion. The performance of these materials in terms of resistance to biodeterioration under the conditions of sewer networks has been demonstrated in real situations and under representative laboratory conditions2,3,4. The question arises of the ability of the coating to maintain its protective function when a crack is initiated in the concrete support (e.g. shrinkage, movement, etc.) and possibly propagate into the coating. Several phenomena may develop and condition the behavior of the cracked aluminous coating, including the self-healing of the cementitious material5,6 and the penetration of micro-organisms to reach the substrate7. The durability of the cracked coating and the chemo-mechanical behavior of the coating-substrate composite, subjects on which no data is available in the literature, were evaluated considering the specific nature of the application under evaluation. To study this coupling of cracking and biodeterioration in the lab, a method for the generation of representative cracks was developed. The coating was sprayed on the support mortar whose formulation meets the requirements of severe chemical attacks in acidic media. The three-point bending test was performed on these coated mortars to initiate cracks with a specific opening width, with a range of widths between 100 μm and 1000 μm, to be maintained after unloading. Then, to understand the mechanisms of biodeterioration, cracked and uncracked specimens were exposed to the biological BAC test8,9 which reproduces biodeterioration conditions observed in sewer environments. A non-biogenic acid test was also performed to discriminate the biological activity of the microorganisms. The biodeterioration process was monitored during a 3-months campaign in terms of analyses of the composition of the leached solutions to quantify the calcium and aluminum leaching. Additionally, chemical, microstructural and mineralogical analyses on the coated mortar specimens, with scanning electron microscopy coupled to energy dispersive spectroscopy, and X-ray diffraction were conducted to identify mechanisms of biodeterioration and eventual healing phenomena during the exposure to biological sulfur-oxidizing activity.