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    Bacteria-based self-healing of cement mortars loaded at different levels and exposed to high temperature
    (Emerald Group Publishing Ltd, 2022) Yildirim, Musa; Ozhan, Hacer Bilir; Oz, Hilal Girgin
    Structures are exposed to various external effects and loads throughout their service life, which can result in failure at a load lower than the design compressive strength. There are cement-based healing systems for repairing such damage, but it is often insufficient. A more effective autonomous healing system is thus needed, and microbial-induced calcite precipitation (MICP) has been studied for this purpose. In this study, bacterial mortar (BM) samples were produced and loaded at different levels of their ultimate compressive stress. The effects of loading were determined and the effectiveness of bacterial treatments was investigated. Experiments were conducted to determine crack healing, compressive strength, water absorption, ultrasonic pulse velocity and high-temperature effects. In the BM samples, the MICP mechanism repaired cracks about 3.5 times larger than the control samples. While the treatment of cracks and damage observed at 90% and 100% loading levels were highly limited thanks to the autogenous system, some properties of the BMs improved as the loading level increased. The MICP mechanism was especially effective in damaged samples under high load levels. In addition, the BMs demonstrated superior physical, mechanical and durability properties at each loading level.
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    Enhancing the high-temperature resistance of self-healing bio-cementitious composites using tea waste as a bacterial carrier
    (Elsevier, 2025) Yildirim, Musa; Ozhan, Hacer Bilir; Oz, Hilal Girgin; Ogut, Hamdi
    Bacterial composites exhibit advanced self-healing capabilities; however, their effectiveness is often constrained by production processes and environmental conditions. To enhance bacterial viability, protective carriers are required, with natural fibers recently utilized for this purpose. Fiber reinforcement has been shown to improve self-healing efficiency by limiting crack propagation. This study investigates the potential of tea waste as a bacterial carrier in cementitious composites. Bacillus megaterium spores were absorbed into tea waste and incorporated into mortar specimens at varying concentrations. The durability of bacterial composites under hightemperature exposure, a critical yet underexplored aspect, was also evaluated. Mortar specimens containing bacterial tea waste were subjected to different high-temperature conditions in both undamaged and pre-cracked states, followed by compressive strength assessments. Post-exposure microstructural changes were analysed via scanning electron microscopy (SEM). The findings demonstrated that tea waste effectively functioned as a bacterial carrier, exhibiting behaviour comparable to natural fibers. Additionally, it contributed to enhanced residual strength by mitigating thermal stress and promoting calcite precipitation, facilitating damage repair. These results highlight the potential of tea waste as a sustainable and effective medium for improving the durability of bacterial composites against high-temperature effects.
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    Optimizing hyaluronic acid production by Streptococcus zooepidemicus using taguchi method: effects of temperature and pH
    (Springer, 2025) Oz, Hilal Girgin; Ogut, Hamdi
    The cost-effective and high-yield production of hyaluronic acid (HA) by microbial means remains challenging, necessitating the optimization of existing processes through the implementation of novel approaches. The present study investigates the production of HA under varying temperature and pH conditions utilizing independent, fully controlled fermenters. The synthesis of HA was evaluated at four temperatures (32 degrees C, 35 degrees C, 37 degrees C, 40 degrees C) and four pH levels (6.5, 7.0, 7.5, and 8.0), with optimal parameters identified through the Taguchi design methodology. The Taguchi optimization method effectively identified 37 degrees C and pH 6.5 as the optimal conditions for HA production corresponding to the highest signal-to-noise (S/N) ratios of 58.18 and 57.55, respectively. These conditions resulted in a maximum yield of 1.08 g.L-1, demonstrating the efficacy of this parameter combination in maximizing production efficiency. The carbazole method was utilized to quantify the production of HA following a five-hour fermentation period, with the culture conditions subjected to a statistical comparison. A temperature of 37 degrees C yielded significantly higher HA levels than 32 degrees C and 40 degrees C (Dunn test, respectively p = 0.019, p = 0.001). The lowest HA production was observed at 40 degrees C, while the 32 degrees C group exhibited relatively low variation in HA production. Furthermore, the hourly bacterial counts demonstrated a direct correlation between bacterial proliferation and HA synthesis, with the highest bacterial growth observed at 37 degrees C and pH 7.0. This study highlights the trends in HA concentration under different temperature and pH conditions during the pre-fermentation phase, offering critical insights for optimizing HA production.
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    Production of self-healing mortar using bacterial spores encapsulated with jute fibre
    (Josip Juraj Strossmayer Univ Osijek, Fac Civil Engn & Architecture Osijek, 2023) Yildirim, Musa; Ozhan, Hacer Bilir; Oz, Hilal Girgin
    Cement-based composites have various advantages such as low cost, easy shaping, and high compressive strength. Therefore, they are widely used in the construction industry. However, their brittle structure makes them prone to cracking, which must be repaired promptly to avoid possible loss of strength and durability and ultimate structural failures. Microbially-induced calcite precipitation (MICP) offers an effective method for healing these cracks. Unlike other treatments, MICP method is a natural and environment-friendly option for self-healing of cracks. However, bacteria can be damaged by cement and mechanical impacts; therefore, encapsulation is necessary to protect them. Encapsulation in fibres is a simpler and more cost-effective method than the other techniques. This study tested the effectiveness of encapsulating Bacillus megaterium spores in jute fibres and added them to cement-based mortars. Different nutrient supplements were used, and physical analyses and compressive strength tests were conducted on 28-day cured cube specimens. Present findings revealed that Bacillus megaterium-type bacteria encapsulated in jute fibres improved the compressive strength recovery and healed the loading-induced cracks.
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    Repair of Cracks in Concrete with the Microbial-Induced Calcite Precipitation (MICP) Method
    (Sciendo, 2023) Ozhan, Hacer Bilir; Yildirim, Musa; Ogut, Hamdi; Oz, Hilal Girgin
    In this study, the microbiologically-induced calcium carbonate precipitation (MICP) method was employed to examine its potential for repairing cracks in concrete. In addition, specific gravity and porosity values were measured to examine the effect of calcite formations on concrete surfaces and microstructures. Bacteria-supplemented concrete repaired cracks up to 0.4 mm wide by filling them with CaCO3. Furthermore, this study not only examined the healing of the width but also the length of cracks. However, as the width of the treated cracks decreased, their length increased. This indicated that the MICP treatment is more effective in a limited crack range. Specific gravity values increased, and porosity values decreased in concrete supplemented with calcifying bacteria. SEM analyses showed that calcite is a bacterial product that forms a very tight bond with a cement gel and that calcite fills visible cracks and voids and creates more of a void-free and undamaged concrete structure.
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    Utilization of tea waste as a bacterial carrier in self-healing mortars
    (Springernature, 2025) Yildirim, Musa; Ozhan, Hacer Bilir; Oz, Hilal Girgin; Ogut, Hamdi
    The protection of bacteria in self-healing composites is crucial for an effective healing process. However, materials used for protection often require advanced techniques and incur additional costs. This study investigates the use of tea waste as a bacterial carrier and natural fibre reinforcement in mortar. Tea waste, a fibrous by-product with high water absorption capacity, was used as a bacterial carrier for Bacillus megaterium spores. Mortar samples with different amounts of tea waste containing bacteria were evaluated through crack healing analysis, compressive strength, water absorption, ultrasonic pulse velocity (UPV), capillarity, and microstructural analyses. Additionally, the healing efficiency of the mortars was investigated using pre-cracked specimens. The results showed that tea waste effectively absorbed and protected the bacterial spores. Due to its fibrous nature, tea waste restricted cracks and created a more favourable condition for self-healing. Bacterial mortars with tea waste repaired cracks up to 0.68 mm wide and exhibited a 26.49% increase in compressive strength compared to control samples after 90 days. Moisture from the tea waste improved the bacterial environment and promoted internal curing, leading to denser structures with higher UPV, reduced water absorption, and lower capillarity. Scanning electron microscopy analysis confirmed that tea waste supported bacterial calcite formation. FT-IR analysis demonstrated that tea waste was highly compatible with the bacterial mortar matrix. Consequently, the incorporation of tea waste led to a notable presence of bacterial calcite products within the mortar structure. The findings indicate that tea waste can be effectively utilized to enhance the self-healing capabilities of mortars.