Concrete structures exposed to aggressive aqueous media (waste water, soft water, fresh water, ground water, sea water, agricultural or agro-industrial environments), due to their porous nature, are susceptible to a variety of degradation processes resulting from the ingress and/or presence of water. In addition to chemical and physical degradation processes, the presence of water contributes to undesirable changes in the material properties resulting from the activities of living organisms, i.e. biodeterioration. Since microorganisms are ubiquitous in almost every habitat and possess an amazingly diversified metabolic versatility, their presence on building materials is quite normal often, they can infer deterioration that can be detrimental (loss of alkalinity, erosion, spalling of the concrete skin, corrosion of rebar’s, loss of water- or air tightness…). The deleterious effect of microorganisms, mainly bacteria and fungi, on the cementitious matrix has been found to be linked, on the one hand, with the production of aggressive metabolites (acids, CO2, sulfur compounds, etc.) but also, on the other hand, with some specific, physical and chemical effects of the microorganisms themselves through the formation of biofilm on the surface. Moreover, the intrinsic properties of the cementitious matrix (porosity, roughness, mineralogical and/or chemical composition) can also influence the biofilm characteristics, but these phenomena have not been understood thoroughly as of yet. Nonetheless, a serious review about understanding interactions between cementitious materials and microorganisms has been reported .
These deteriorations lead to a significant increase in the cost of repairing structures and to loss of production income, but may also lead to pollution issues resulting, for example, from waste water leakage to the environment. Also, building facades, and notably concrete external walls, can be affected by biological stains, which alter aesthetical quality of the construction, sometimes very quickly, and lead to significant cleaning costs. Microorganisms, mainly algae, responsible for these alterations have been quite well identified. Research is now rather focused on determining colonization mechanisms, and notably influencing material-related factors, and on development of preventive or curative, and preferentially environmentally friendly, solutions to protect external walls. However, up to now, no clear results about the efficiency of these various protection solutions are available.