The self-healing mechanisms of bacterial cementitious materials via novel isolated ureolytic bacterium species

Abstract

The self-healing approach used in this study is based on the urease hydrolysing bacterium, namely, Bacillus Sphaericus. The scope of the work is diverse as it combines and brings together both aspects of the bioengineering and civil engineering in order to provide better understanding of the self-healing mechanisms that lie behind the yielding of the calcium carbonate precipitation. The initial stage of this study, comprised of collecting different soil samples from different alkaline sources to extract and isolate urease bacterium species. In addition, a collection of more than 100 different bacterial strains belong to bacillus Sphaericus were screened for the presence of urease enzyme and ability to produce copious amount of calcium carbonate under extreme alkaline conditions. In-vitro calcium carbonate precipitation experiments were performed to stress the selected bacterial strains prior to the application for the self-healing mortar. Parameters such as temperature, pH, shaking conditions, ability to form endospores and the production of copious amount of calcium carbonate were placed under scrutiny. The biochemical properties of the selected bacterial strain were studied and investigated further by monitoring the evolution in pH, the production of ammonium, insoluble and soluble calcium and the colony forming unit. Following the characterisation, three strains were promoted forward for the use of self-healing mortar. In-vitro calcium carbonate perception in broth state showed that the yielding of CaCO3 was maximum in the case of strain 89 and strain 67 at 0.5932g/100ml and 0.8398 g/100ml, respectively. The need for an encapsulating material that provides suitable environment for the bacteria to endure the mechanical and physical forces in addition to the high alkaline environment of the cement matrix, is indeed a key component towards the self-healing applications. As a result, the autoclaved aerated recycled concrete (AAC) aggregates were selected for this study, due to the high porous structure which implies high absorption properties. Three healing systems were proposed for the application of the self-healing mortar. The first approach comprises of bacterial spores impregnated under vacuum into the AARC aggregates along with a suitable nutrient designed specifically for maximum yielding of calcium carbonate precipitation. The second approach comprised of a three-component healing system with the introduction of a mixed culture to enhance the healing capacity in addition to providing reinforcement for the cement matrix. The third approach comprised of direct incorporation of different bacterial cells into cement mortar to test for the capacity of the strain to heal cracks under high alkaline environment. Mortar specimens were cracked at 28 days of curing, ranging from 0.127 to 0.875mm, the introduction of the three-healing system provided promising results in regard to healing cracks of 0.875mm over the period of 289 days of curing. Direct incorporation of bacterial strains at different concentrations 107cells/ml and 108cells/ml showed the tendency to heal cracks of 0.127 and 0.253mm, respectively. Furthermore, direct incorporation of bacterial cells into the cement matrix, supported the analysis that urea hydrolysing bacteria is indeed an enzymatic activity. Two key enzymes were defined and strongly linked to the calcium carbonate precipitation process that is the urease enzyme and the carbonic anhydrase enzyme, where the latter is a zinc enzyme that catalyses the conversion of calcium dioxide into carbonate acid and ultimately promotes further insoluble calcium carbonate precipitation. The introduction of a three-component healing system showed enhanced healing capacity, Direct Incorporation of 107cells/ml with 30% impregnated aggregates showed the capacity to partially heal cracks of 0.791mm by 28.3% over 14 days and 100% over 28 days healing period. Increasing the number of cells showed expected higher healing efficiency, specimens in set 22.1B were able to completely seal cracks of 0.875mm by 99.085 % over the period of 28 days.