PHD-Department of Chemistry

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Browsing PHD-Department of Chemistry by Subject "Biodeterioration"

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    Effect of Selected Bacterial Biocementation, Biodeterioration, Bioremediation on Selected Properties and Microstructure of Hydrated Cement Mortar
    (Kenyatta University, 2023) Ngari, Reginah Wangui; Joseph Karanja wa- Thiong'o; Jackson Wachira Muthengia
    Cement-made structures encounter durability and sustainability challenges. This is due to degradation by aggressive agents such as Cl-, SO42- and CO2. The presence of micro-organisms, such as Sulphur reducing bacteria (SRB) and Sulphur oxidizing bacteria (SOB), have been found to lead to production of cement degrading agents. On the contrary, there are micro-organisms that may benefit the mortar. Bacteria, for example that precipitate CaCO3, have found use in improving the performance or remediation of degraded concrete / mortar. In this study, the effect of Bacillus flexus, Bacillus cohnii and Thiobacillus thioparus on selected physico-chemical, mechanical properties and bioremediation of mortars was investigated. Growth of bacteria was done in a laboratory set-up. Bacteria concentration of 1.0 x 107 cells per mL of mix media was used. Mortar prisms were prepared using distilled water or bacteria solution as mix water and cured diistilled water or in bacteria solution. Control experiments had mortar prisms prepared using distilled water as mix and curing water. Pastes for setting time and soundness tests were similarly prepared using the above mix waters. Compressive strength, soundness, water absorption, setting time tests analyses were carried out for each category of mortars. Compressive strength test analyses were carried out on the 2nd, 7th, 14th, 28th, 56th and 90th day of curing of the mortars. Microstructural analyses of the hydrated mortars were conducted using Scanning Electron Microscopy (SEM). 28-day cured control and microbial mortars were subjected to water absorption analyses, chloride and sulphate ion ingress tests. From the results, the incorporated bacteria did not affect the soundness of cement significantly though setting time was significantly affected. Compressive strength was observed to increase with curing period for the Bacillus flexus and Bacillus cohnii with the highest gain observed at 90 days as 31.60 % for w/c of 0.4. On the contrary, there was a significant decline in compressive strength for all mortar where Thiobacillus thioparus was incorporated across all the curing regimes. The lowest percentage loss was observed as 44.38 % at 90 days for w/c of 0.6. Diffusivity of Cl- ions was highest for mortars subjected to Thiobacillus thioparus and lowest for Bacillus flexus. The same mortars exhibited Dapp of 2.5479 x 10-10 m2/s and 3.2977 x 10-10 m2 /s respectively. Dapp for SO42- showed a similar trend as Cl- ion. From the SEM images, microstructural analysis indicated presences of calcite (CaCO3), calcium silicate hydrate (CSH), ettringite, portlandite (Calcium hydroxide, CH) with the microbial mortars. A denser and a more refined microstructure with Bacillus flexus and Bacillus cohnii was observed as opposed to massive presence of voids, micro cracks and pores with Thiobacillus thioparus mortars. Generally, Bacillus flexus was a better CaCO3 precipitating bacteria than Bacillus cohnii. The results show that MICP was an effective method of remediating mortar that has been degraded by bacteria.