Microbially mediated carbonation of marine alkaline minerals

Abstract

Concrete constructions in the marine environment suffer from chemical attack of sea salts which can induce damage to both the concrete matrix and embedded steel reinforcement. For example, ingress of sulfate and chloride ions can respectively result in detrimental ettringite formation and enhanced corrosion of the steel rebars. The first degradation mechanism is due to development of expansive minerals within the concrete matrix, a process where increased internal pressure can result in crack formation. Cracking dramatically increases matrix porosity further enhancing the ingress rate of detrimental chemicals. However, with controlled mineral expansion comes the possibility of an employable mechanism for autonomous sealing of cracks. In this research project our aim is to study the potential for carbon dioxide-producing bacteria to act as an agent to control expansive carbonation reactions following serpentinization (hydration reactions) of alkaline precursor minerals in marine concrete. Early experimental results showed that seawater derived magnesium ions and carbon dioxide molecules show high potential for self-healing due to the formation of crack-filling expansive minerals. Environmental scanning electron microscopy combined with X-ray element analysis revealed that dolomite (CaMg(CO3)2) formation via intermediate conversion of brucite (Mg(OH)2) is the mechanism underlying this process. Several previously documented studies have reported that dolomite formation, analogous to delayed ettringite formation, can be detrimental to concrete as it can in fact induce crack formation due to increased internal stresses. In this project our aim is to control expansive mineral formation by bacterial CO2 production channelling the process for healing- but not for causing cracks in concrete constructions in the marine environment.