Microstructural investigation of biomineralization in cement-based materials

Many factors affecting the durability and integrity of concrete structures can be related to micro-cracks. Due to its brittle characteristics, the cracks can be induced in concrete elements under different stresses. Recent studies confirm the possibility of microbial induced calcium carbonate precipitation with self-healing ability. This ability of micro-crack healing can be achieved via different methods and one of the recent novel methods for this purpose is the biomineralization in cement-based materials. In this method, the calcium carbonate precipitates as a result of the biochemical process of the microorganisms. There are different studies to evaluate the self-healing in cement-based materials for the purpose of compressive strength recovery and durability of the structures. However, there is not enough information regarding the effects of biomineralization on the hydration process and its products. To fill the knowledge gap in the literature, this study aims to investigate the effect of biomineralization and calcium carbonate precipitation on possible changes in cement hydration products due to biomineralization as well as the effect of superplasticizer on this process.
This work aims to understand the biomineralization process in cement-based materials by using different testing methods such as 29Si NMR, 27Al NMR, XRD and SEM-EDX analysis. In the current work, the Sporosarcina pasteruii strain has been used for bacterial induced calcium carbonate precipitation and it has been cultivated in corn steep liquor medium with the addition of calcium nitrate as a calcium ion source. The cement paste samples and bacterial cement paste samples used in the study have been treated in water and nutrient media respectively until the testing age.
Based on the collected results, it can be concluded that the addition of S. pasteurii bacterial strain has a minor effect on the C-S-H and its polymerisation. However, the 27Al NMR findings show a major change in the AFt and AFm phases and their transitions. Furthermore, the XRD results did not indicate any major changes to calcium carbonate content in bacterial cement paste samples but the STA analysis clearly indicated a higher amount of calcium carbonate in the same samples. As such, the presence of poorly crystalline calcium carbonate in the bacterial samples is concluded. The presence of calcium carbonate in cement based materials such as cement paste, mortar and concrete is considered to increase the density resulting decrease in porosity and increase in mechanical properties under compression. This confirms the increase in strength correlated with biomineralization in cement based materials observed through literature review.