Formation, growth and corrosion effect of sulfur oxidizing bacteria biofilm on mortar

In this paper, a protocol has been developed to evaluate the corrosion of mortar by sulfur oxidizing bacteria through formation of biofilm on mortar surface. The bacteria formed a biofilm at the air–liquid interface over time and caused mortar corrosion in liquid. The bacteria biofilm was characterized in terms of changes in composition and thickness during its formation and growth. From biofilm formation to its stabilization, the impact of the biofilm on mortar was studied in terms of the changes of mortar mass, compressive strength, mineral composition and microstructure. The results showed that: (1) The sulfur-oxidizing bacteria adhered to the mortar surface within 7 days. The biofilm formed at air–liquid interface after 15 days and entered a rapid phase starting after 45 days. The thickness of biofilm reached a maximum about 1570 μm on day 75, and then stabilized between 75 and 90 days. Similar trends were observed in microbial quantity, and the contents of polysaccharide and protein in the biofilm. (2) Corresponding to the growth of biofilm, the mass of mortar started to increase at 45 days and stabilized after 75 days. The compressive strength of mortar increased from 67.2 MPa on Day 7 to 82.9 MPa on Day 75, then declined to 68.8 MPa on Day 95. The property change correlated to the increasing amount of gypsum in the part of mortar exposed to liquid, where sulfate was accumulated by microbial oxidation. In contrast, the gypsum content in the mortar exposed to biofilm was much less, suggesting biofilm reduced gypsum formation by limiting the transportation of sulfate into mortar.