Biomineralization in cement based materials : inoculation of vegetative cells
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Recently, self-healing applications of cement-based materials have received a lot of interest. One major area of interest with respect to self-healing applications in cement-based systems focuses on using biomineralization processes. Biomineralization is biochemical process in which microorganisms stimulate the formation of minerals. The existing research on biomineralization in cement-based systems has showed promising results and the studies suggest that biomineralization could be a useful approach for remediation of cracks on the surface of concrete. This dissertation presents the results of an intensive study undertaken to understand the influence of vegetative bacteria, specifically Sporosarcina pasteurii (S. pasteurii), when it is incorporated within cement paste. Vegetative S. pasteurii cells were suspended in a urea-yeast extract medium and this medium was mixed with cement. The influence of the vegetative S. pasteurii cells on Portland cement paste properties, such as compressive strength, hydration kinetics, and setting time was evaluated. It was determined that the hydration kinetics was highly influenced when the bacterial medium was used to prepare cement paste, and severe retardation was observed. It was also observed that an increase in calcium carbonate precipitation, particularly calcite, occurred within cement paste when the bacterial medium was used. Furthermore, use of the bacterial medium resulted in reducing the porosity and increasing the compressive strength of the hardened paste. Ex-situ culture experiments were conducted to determine the impact of pH and calcium concentration on the morphology of calcium carbonate precipitate; the results indicated that the morphology of the precipitate was more influenced by calcium concentration. A key focus of this dissertation was to examine the viability of the vegetative cells that were inoculated in cement paste. Viable S. pasteurii cells were found to be present in hardened cement paste samples that were as old as 330-days, and 50% of the viable cells detected were defined as vegetative cells. At last, the use of including internal nutrient reservoirs as a means to extend the viability of the bacterial cells within hardened cement paste was explored. The results showed that the percentage of vegetative cells remaining was affected when internal nutrient reservoirs was incorporated into the system.