Microbial Effects on Physico-Mechanical and Microstructural Properties of Commercial Portland Cements
Munyao, Onesmus Mulwa
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Structures made using cement are prone to deterioration over time. Majority of structural failures have been attributed to inappropriate concrete mix designs, poor engineering designs, substandard aggregates, mixing water and sand. While many studies have focused on use of alternative cementitious materials to improve on the durability of existing Ordinary Portland cement (OPC), little attention has been given to microbial influence on concrete durability. This study investigated the influence of Thiobacillus intermedius, Starkeya novella and Acidithiobacillus thiooxidans bacteria on physico-mechanical and microstructural properties of selected Kenyan made cements. Commercial Ordinary Portland Cement (OPC) and Portland Pozzolana Cement (PPC) were used to cast the mortar prisms. The control mortar prisms were prepared using distilled water whereas the bacteria solutions were utilized as mix water, curing medium or a combination of two. The test cements were further subjected to 1.5 % sulphuric acid solution as a curing media. Physical characterization of both control and bacterial test cement pastes was carried out to determine normal consistency, setting time and soundness. Compressive strength was determined at 2nd, 7th, 28th, 56th and 90th day of curing. XRD and FTIR were used to characterize the hydration products of the test mortars after curing for 28 days. Scanning Electron Microscopy (SEM) was carried out to study the surface morphology of the hydrated cement mortars after curing for 28 days. Both control and bacterial mortar prisms were subjected to accelerated chloride and sulphate ingress test after curing for 28 days. Apparent Diffusion Coefficients (Dapp) were estimated from the solutions to Fick’s Second Law of Diffusion. Results showed conformity of physical characteristics of control and microbial cement pastes to Kenya cement standards. The compressive strength however decreased significantly for all the bacterial prepared mortar prisms. OPC exhibited the highest strength decrease of 33.5% at 90th day of curing when mixed and cured in Starkeya novella bacteria solution. SEM analysis showed extreme erosion of hydration products with formation of deleterious expansive ettringite crystals for the bacterial prepared mortars in the pore matrices. Control mortar prisms exhibited systematic formation of hydration products within the pore system. Mortars prepared with bacteria solutions as mixing water exhibited pronounced ingress of chloride and sulphate ions. OPC SK-SK mortars exhibited chloride and sulphate apparent diffusion coefficients (Dapp) of 8.2995 x 10-10 m2/s and 1.3562 x 10-9 m2/s respectively. There was increased water absorptivity in the microbial mortars. OPC SK-SK registered the highest water absorptivity of 72.75%. The exposed mortar prisms in 1.5% sulphuric acid solution exhibited low compressive strength at all curing ages with OPC demonstrating the highest decrease in compressive strength of 73.56% at 90th day of curing. Results from this study show that the selected bacteria commonly present in sewer systems play a vital role in deteriorating cement- based materials. This reduces the service life and the durability of the exposed cement structures with time. The work recommends development of remedial methods to prevent the bacteria attack on concrete and mortar structures placed in sewer and other aggressive environments.
- PHD-Chemistry