Concrete deterioration, which is commonly observed in cooling towers and drainage pipeline structures, is considered to be more severe and faster than that in off-shore structures subjected to salt attack and carbonation [1], because the former structures are typically exposed to microbiological and sulphate attacks. Such drainage structures as sewer pipes and culverts provide a favorable habitat to sulfur-oxidizing bacteria, such as Thiobacillus strains. Thiobacillus significantly accelerates the deterioration of concrete through the results of its growth action mechanism [2]. Thiobacillus forms strongly acid biofilms, which leads to a decrease in the alkalinity of the cement matrix, thereby degrading the concrete substrate. This microbiologically influenced corrosion is perpetual and accumulative. In addition, anaerobic bacteria populating the drainage structures metabolize with sulfate ions and organic waste, which generates hydrogen sulfide gas. The deoxidation action by Thiobacillus converts the hydrogen sulfide gas into sulfuric acid. Consequently, the sulfuric acid accelerates the deterioration of concrete through the reaction with calcium hydroxide and calcium silicate hydroxide which are the main hydration products of cement [3]. This mechanism of the sulfuric acid deterioration of concrete is permanent in drainage environments under conditions that are difficult to observe and manage.

Maintenance against to prevent the deterioration of concrete in drainage structures has become a major concern, because the strength degradation of such structural elements can result in traffic accidents and catastrophic mass mortality events because of road settlement and sinking. In particular, most drainage structures that were constructed only 10–20 years ago require comprehensive repair and strengthening because of severe concrete deterioration caused by microbiological and sulphate attacks. However, it is not easy to observe the defects and damages in the drainage structures and to provide reliable maintenance because of the unusual accessibility. Thus, new technologies have begun to attract attention from a maintenance-efficiency perspective to combat the microbiological and sulphate attack environments in drainage structures [4].

Some photosynthetic bacteria such as Rhodobacter capsulatus, Rhodobacter blasticus, and Rhodopseudomonas palustris create glycocalyx as a saccharide complex on cell colonies through the interaction and cohesion activities of bacteria [5]. The glycocalyx acts as a protecting sheath for the cell and activates the interaction with other bacteria under hazardous atmospheres. The photosynthetic bacteria restrain the deoxidation action to sulfide acid and sulfide hydrogen gas through the immobilization of sulphate and sulfide hydrogen ions in the drainage structures [6]. As a result, the biological activity of the photosynthetic bacteria can effectively counteract the microbiological and sulphate attacks caused by exogenous strains in the drainage structures. Yang et al. [4] and Yoon et al. [7] investigated the significance and feasibility of a conceptually new biological coating mortar that was developed on the basis of the immobilization of the photosynthetic bacteria creating glycocalyx as a biomimetic material to protect the substrate of concrete. Their pioneering experiments showed that the glycocalyx thoroughly blocked the penetration of sulfuric acid into concrete. Thereby, the formation of the ettringite and gypsum that are generated as a result of the reaction between sulfuric ion and hydration products of cement was restricted.

The objective of this study was to estimate the sulfate resistance of concrete coated with the biological mortars developed in a previous investigation. Considering the glycocalyx production capacity and viability in the hardened cement mortars [4], Rhodobacter capsulatus was immobilized in expanded vermiculite together with a bacterial culture medium. Concrete cylinders were coated with the developed biological mortars at a thickness of 2.5 mm and then immersed in 5% sulfuric acid solutions to examine the sulfate resistance of the mortars. The variations of the compressive strength and mass of concrete and the transition of hydration products of the cement matrix were examined at different ages. The consumption of calcium hydroxide by reaction with H2SO4 and thereby the production of gypsum in the cement matrix were quantitively traced by thermogravimetric analysis. The glycocalyx formation and the survival of the bacteria in the coating mortars exposed to sulfuric acid solution were assessed by using scanning electron microscopy (SEM) imaging and the viable cell count method, respectively.