Assessment of coatings for protection of cement paste against microbial induced deterioration through image analysis

In this study, a laboratory method and equipment was developed to accelerate and study the microbial induced deterioration of cementitious materials, and three types of coatings, namely, cement-based capillary crystalline waterproofing coating (CCCWC), cement-based bactericidal coating (CBC) and epoxy coal tar pitch coating (ECTPC) were applied onto the cement paste surface, and their protecting effects were investigated. After immersion in sewage, the microstructure and element distribution of cement paste were characterized by scanning electron microscopy-electron dispersive spectrometry (SEM-EDS). Besides, the distribution of dead/live cells within the biofilm attached to various specimens was examined by confocal scanning laser microscopy (CLSM). To make a quantitative study, the image analysis was adopted. Since both the images obtained belonged to RGB color system, it was found that the image processing program of MATLAB is simple but very helpful for such task. The results indicate that for uncoated specimen, there were the most content of S and the least contents of Ca and Si. However, for coated specimens, the contents of S decreased and that of Ca and Si increased accordingly. ECTPC had the best protective effect on the microbial induced deterioration of concrete, followed by CBC and CCCWC. Besides the physical barrier effect, the bactericidal function of coating has significant contribution for protecting concrete against sewage attack. Furthermore, the image analysis was used to determine the penetration depth of element S in cement paste quantitatively, and based on the change of the S concentration with penetration depth, the surface specimens can be divided into three zones: interior deterioration layer, transition layer and non-corroded layer. The combined use of micro-scale characterization and image analysis can provide a quick, accurate and labor saving method for the deterioration study of coated concrete in sewage environment.