Microbial interactions with mineral building materials

A research on the interactions between micro-organisms and mineral building materials (concrete and natural stone), performed at the Magnel laboratory for concrete research of Ghent University, in collaboration with other research groups was presented. The first research topic is the deterioration process taking place in concrete sewer systems and manure storage facilities, called biogenic sulphuric acid (BSA) corrosion. This process can be mimicked by pure chemical or microbial degradation tests. The problem of chemical tests with sulfuric acid is that they do not take into account a possible biocidal activity of the concrete. Nevertheless, microbial tests confirmed that it is preferable to use a material with a high neutralisation capacity to limit bioteterioration. Therefore, a model for biogenic sulfuric acid attack should include the concrete alkalinity as a parameter, next to other durability related parameters such as porosity. The second topic is the application of bacteria for bioconsolidation and self-healing. Bacterially induced carbonate precipitation was investigated as a surface treatment for cementitious materials and limestone. The biodeposition treatment resulted in an increased resistance of concrete towards water absorption, carbonation, chloride penetration and freezing and thawing. In porous limestone, consolidation by biodeposition could be achieved at depths up to 30 mm and more. The overall strength increase in this zone amounted to more than 300%. With regard to microbial self-healing, bacteria were immobilized on diatomaceous earth or inside microcapsules before introduction into the concrete mixture. Incorporation of diatomaceous earth with Bacillus sphaericus in mortar specimens allowed self-healing of 0.15 mm-0.17 mm cracks by calcium carbonate precipitation for 40 days after crack formation. Introduction of bacterial microcapsules enabled self-healing of cracks having a width of almost 1 mm in three weeks. Another application of micro-organisms on concrete is for the creation of green concrete walls. For these walls, the concrete was purposefully designed to obtain a specific microstructure and bioreceptivity as substrate for biological growth. Concrete with magnesium phosphate cement and carbonated Portland cement concrete were applied to obtain sufficiently low pH values. Other important factors that determine bioreceptivity such as concrete porosity and surface roughness were also optimized for biological growth.