Understanding ureolytic calcium carbonate biomineralization for crack repair in cementitious materials
Cracks in concrete can weaken structures, destroy integrity, impair function and open pathways for corrosive chemicals. Biomineralizaion is a common phenomenon in which most bacteria can produce precipitates that can be utilized to block the pathways and heal the cracks eventually. Researchers have developed different ways to apply bacteria on the outside of cracked buildings. Commercial self-healing concrete products are also available on the market. However, studies on the formation process and distribution of CaCO3 in cracks, and how it strengthens the cracked concrete on the micron scale are insufficient. On the performance level, quantitative evaluation of the healing efficiency of biomineralization in concrete also requires further verification. By isolating the bacteria, chemicals and nutrients required in the biomineralization process, this dissertation investigates their influence on CaCO3 precipitation. Although cement paste can go through carbonation when exposed in air, no CaCO3 was found to form in a wet environment in the presence of nutrient medium with CaCl2 or calcium lactate but in absence of bacteria. Furthermore, calcium lactate was found to be a more effective calcium source than calcium chloride and cement paste powder for biomineralization. Urea was found to not only increase the extent of CaCO3 formation but also promote vaterite formation over other CaCO3 polymorphs. Crack healing through precipitation of biominerals was examined using mechanical tests and a non-destructive technique. Interestingly, from the mechanical test results, treating cracks with a combination of nutrient medium and CaCl2 at room temperature was found to provide the best healing effect compared to the other treatment solutions. The addition of urea did not appear to promote increased precipitation of CaCO3. In order to link the performance of the treated beams with CaCO3 precipitates, various small-scale characterization techniques were employed, such as X-ray diffraction, scanning electron microscopy, thermogravimetric analysis method, and Fourier Transform Infrared spectroscopy. Those characterization techniques were utilized to analyze the polymorphs, morphology, distribution and transformation of CaCO3 precipitates. Furthermore, additional experiments were performed to understand the effect of extended treatment time. Extensive image analysis of CaCO3 precipitates in cracks revealed that the precipitation tends to happen on specific locations in cracks. Through careful observation of the precipitate bridging the two sides of the crack walls, a hypothesis on the nucleation process of crystal precipitates was provided to correlate the strength recovery of cracked concrete and bonding strength of precipitates. The research output from this study provides an improved understanding of how bacteria-induced mineral precipitation occurs in cementitious material and how it can potentially remediate cracks in cementitious material. Furthermore, for the first time, the duration of treatment was discovered to be one of the most influential factors based on research results.
Biomineralization, Bacteria, Ureolysis, Concrete, Carbonate
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign