Harun Tanyildizi

This study examined the bacterial healing of metakaolin‐based geopolymer concrete modified with colemanite (K) and red mud (RM) exposed to high temperatures. The metakaolin‐based geopolymer concrete samples modified with 4% K and 6% RM were prepared. Then, the geopolymer concretes were subjected to 200, 400, 600, and 800°C to form high‐temperature cracks. Also, half of these samples were cracked when loaded to 70% of their tensile strength. Then, these cracks developed in the specimens subjected to varying temperatures healed using Sporosarcina pasteurii . The healing was conducted by injecting the bacterial medium into the cracks. Weight change, ultrasonic pulse velocity, split tensile strength, and water absorption experiments were performed on healed samples and control specimens subjected to high temperatures. Also, various microstructural analyses and micro‐computed tomography analyses were conducted to examine the healing performance of S. pasteurii and the development of calcium carbonate. This study found that the recovery percentages of split tensile strength of specimens exposed to high temperatures ranged between 84.60% and 21.17%. Also, this study found that S. pasteurii can successfully heal 98.86% of high‐temperature cracks.

In this study, the performances of bacteria‐based autogenous and autonomous healing of metakaolin, red mud, and colemanite‐based geopolymer mortar exposed to sulfate attack were compared. The red mud and colemanite in geopolymer mortar mixtures were used at 0%, 6%, and 4% of metakaolin by weight. Bacteria‐free samples for autonomous healing and bacteria‐containing samples for autogenous healing were produced. S. pasteurii was used as the bacteria in this study. Then, these specimens were exposed to sulfate attack for periods of 1, 2, 3, 6, and 12 months. After the samples were exposed to sulfate attack, the autogenous and autonomous healing process was carried out. In the healing process, a solution containing 9 × 10 ⁸ cells/ml bacteria (Bac‐media) was injected into the cracks with a syringe to heal the geopolymer mortar samples. The mass change, length change, compressive strength, ultrasonic pulse velocity, flexural strength, and capillary water absorption tests of autonomous and autogenous healing geopolymer mortar samples were performed. Also, SEM/EDS, FTIR, Micro‐computed tomography (Micro CT), and XRD were carried out for microstructure analysis. This study found that the mechanical properties of autonomous healing samples were higher than those of autogenous healing samples. Also, Micro‐CT analysis found that 98.51% of the total porosity of the geopolymer mortar exposed to the sulfate attack was filled with CaCO ₃ precipitated by using S. pasteurii . These results provide an effective method for the recovery of geopolymer composites subjected to durability effects such as sulfate attack.

This study examined the mechanical properties of 3D‐printed geopolymer mortars (GPMs) containing multi‐walled carbon nanotubes (MWCNTs) subjected to freeze–thaw (F‐T). The fly ash (FA) and metakaolin (MK) were used as binders in the 3D‐printed geopolymer composites (GPCs). MWCNT was used at 0%, 0.25%, 0.5%, and 0.75% rates in the mixtures. The rheological characteristics of the mixtures were ascertained through experiments conducted on viscosity, buildability, and flow. After this stage, samples with dimensions of 40 × 40 × 160 mm were manufactured using a 3D printer and cured for 28 days at a laboratory temperature of 20 ± 2°C. Then, 3D‐printed geopolymer (3DPG) samples were subjected to the F‐T in accordance with the TS EN 15177 standard. Finally, the weight loss (WL), compressive strength (CS), flexural strength (FS), and ultrasonic pulse velocity (UPV) of 3DPG specimens exposed to F‐T were determined. Additionally, scanning electron microscopy (SEM), x‐ray powder diffraction (XRD), and energy dispersive x‐ray spectroscopy (EDX) analyses were carried out on specimens. The results of this investigation showed that 3D‐printed GPM containing 0.25% MWCNT had the highest FS, UPV, and CS. Furthermore, this study demonstrated that the 3D‐printed GPM with 0.25% MWCNT has a strong resistance to F‐T with a CS retention rate of 95.86%.

In order to popularize the use of geopolymers, it is necessary to investigate the usability of geopolymers in water structures in or on the shores of seas and rivers. For this purpose, underwater geopolymer mortar with different percentages of polypropylene (PP) fiber was produced in this study. In the first stage, the effect of PP fiber on fresh underwater geopolymer mortar was investigated by performing slump and pH tests. In the next stage, mechanical tests were performed on the hardened mortars that were poured and waited underwater for 3, 7, and 28 days. Finally, microstructural analyses were carried out to verify the mechanical experiments and to examine the microstructural state of the underwater geopolymer mortar. As a result of this study, after 28 days, the compressive and flexural strengths of underwater geopolymer specimens containing 0.5% PP fiber were boosted by 3.90% and 210.75%, respectively, compared to the fiber‐free underwater geopolymer specimens. In addition, the 28‐day compressive strength achieved 53.05 MPa, demonstrating the geopolymer mortar's potential to achieve sufficient strength in a submerged condition. These findings underscore the viability of underwater geopolymer mortar for marine structures and other submerged building applications.

This research aims to study the bacterial healing process of cement mortar samples exposed to durability effects using bacteria characterized by their ability to precipitate calcium carbonate. Sporosarcina pasteurii is widely used for bacterial healing. This research investigates the performance of S. pasteurii with five types of bacteria isolated from caves (Viridibacillus arenosi, Streptomyces spororaveus, Bacillus zhangzhouensis, Stenotrophomonas rhizophila, and Serratia quinivorans). Cement mortar samples were exposed to high temperatures and freeze–thaw effects to create microcracks. Microcracks were also induced by applying flexural strength loads. Then, the formed microcracks were healed using spraying and injection of the bacterial solution for the first group for 90 days. The control and healed samples were assessed using flexural and compressive strength, water absorption, capillary water absorption, and ultrasonic pulse velocity (UPV) tests. Microstructural analysis was also conducted to evaluate the bacterial healing products. Subsequently, statistical analysis was performed using the results of these tests to compare the various bacterial efficiencies. As a result of the statistical analysis, the total efficiency scores obtained in the statistical analysis were 119 for Stenotrophomonas rhizophila, 112 for Viridibacillus arenosi, and 105 for S. pasteurii. Thus, Stenotrophomonas rhizophila and Viridibacillus arenosi showed the best healing performance compared to the other types of bacteria.