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    Effect of waste travertine powder on properties of rhyolitic tuff-based geopolymer
    (Elsevier, 2024) Tekin, Ilker; Pekgoz, Mahfuz; Dirikolu, Irem; Kiamahalleh, Mohammad Valizadeh; Gholampour, Aliakbar; Gencel, Osman; Ozbakkaloglu, Togay
    This investigation explores the potential of geopolymer technology as an eco-friendly substitute for conventional construction materials by emphasizing the innovative use of naturally occurring green rhyolitic tuff and repurposed travertine powder in geopolymer paste formulations. Replacement levels of tuff with travertine at 40 %, 45 %, and 50 %, along with an alkaline solution with a NaOH molarity range of 8.2 M-22.1 M, have been analyzed for their impact on flowability, water absorption, porosity, compressive strength, and unit weight of the geopolymers over curing periods of 2, 28, and 90 days. The flowability measurements show that adding 40 %, 45 %, and 50 % travertine leads to approximately 7 %, 31 %, and 31 % reductions in the flowability of the geopolymer, respectively. It is demonstrated that adding 40 % travertine significantly improves the geopolymers' compressive strength with an 11.5 M NaOH concentration, showing substantial increases of approximately 15.5, 9.0, and 2.4 times at the curing duration of 2, 28, and 90 days, respectively, relative to the geopolymer without travertine. As an optimum points, an 18.5 M NaOH and an 0.7 solution-to-powder ratio decrease the long-term apparent porosity and water absorption of the geopolymer without travertine by about 14 % and 19 % compared to those at the same solution-to-powder ratio with 11.5 M NaOH, respectively. Microscopic examinations were employed to validate the development of sodium aluminosilicate hydrate, calcium aluminosilicate hydrate, and calcium silicate hydrate gels, underscoring the advantageous contribution of the elevated CaO content in the travertine to the geopolymer matrix. This research not only highlights the environmental benefits of repurposing waste materials but also contributes to the development of more sustainable and durable geopolymer pastes, offering a promising approach to enhancing environmental stewardship in material science practices.
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    The effects of ZnO powder and different fiber types used in tuff/calcite-based geopolymers on electromagnetic shielding effectiveness
    (Elsevier, 2024) Tekin, Ilker; Pekgoz, Mahfuz
    Electromagnetic (EM) shielding has become increasingly important in our modern world due to the growing use of electronic devices, which poses risks of EM pollution and its adverse effects on humans. Many studies have explored the incorporation of various additives into geopolymer mixes to enhance their EM shielding properties. This study investigates the impact of tuff/calcitebased geopolymers reinforced with three different types of fibers (Polypropylene, Steel, and Carbon) and varying percentages (2.5 %, 5 %, and 10 %) of ZnO powder on EM shielding effectiveness. Compressive strength, flexural strength, water absorption, length change tests, and EM shielding (900-6000 MHz at every 100MHz step) measurements were performed on geopolymer samples. Optimal shielding performance was achieved by the addition of 10 % ZnO by volume alongside carbon fiber. This combination resulted in a total shielding value of -78.92dB at a bandwidth of 2800MHz.
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    An investigation of concrete stress-strain behavior by the image analysis method
    (Pontificia Univ Catolica Chile, Escuela Construccion Civil, 2021) Pekgoz, Mahfuz; Gunaydin, Osman; Gucluer, Kadir
    Concrete is a composite load-bearing building material. The deformation behavior of load-bearing materials under load is vital for the building system. Investigation of these brittle and quasi-brittle behavior patterns at various load levels provides an advantage in the evaluation of mechanical properties. In this study, the deformations occurring within the concrete samples in different stress-strain regions were investigated using an image analysis technique. The experimental samples experienced elastic-limit loading for two hours to clearly monitor the deformations at elastic, plastic, and breaking points. For the microstructure studies, the samples were prepared with epoxy for image analysis. Thin-sections were taken from each series of epoxy-impregnated concrete test samples, examined under a microscope, and photographed. Deformation studies on the digital photographs were carried out by the image analysis method. The results show that crack formation and crack types change because of increased stress and deformations. Crack formations within the concrete are parallel to the loading direction and occurred mainly in the aggregate-cementpaste interface. At 85% of the ultimate stress, crack length was measured as 0.665-29.505 mm and crack width 0.180-4.128 mm, while the crack length was 0.305-32.688 mm and crack width were 0.106-2.906 mm at fracture stress.
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    Microstructural investigation and strength properties of structural lightweight concrete produced with Zeolitic tuff aggregate
    (Elsevier, 2021) Pekgoz, Mahfuz; Tekin, Ilker
    In this study, physical, mechanical, and microscopic investigations were carried out on structural lightweight concretes (LWC) produced with zeolitic aggregate. Zeolitic tuff aggregate, 300 kg/m(3), and 400 kg/m(3) dosages OPC and 0.70 and 0.45 water/cement ratios were used in LWC productions. An optical microscope method was used to determine the aggregate-cement interface zone (ITZ) on the hardened LWC. SEM analyses were performed on the specimens to determine microstructural properties as well. Moreover, water absorption, capillary suction, and compressive strength tests were carried out. The experimental results showed that ITZ thickness and compressive strength are significantly depend on the cement dosage and w/c ratio in structural LWC. ITZ thicknesses were measured 20-33% less in LWC produced with a 0.45 w/c ratio and the 400 kg/m(3) cement dosage. On the SEM analyses, dense CH and ettringite formations were observed at the early age of all LWC in high w/c ratios. However, on the 28th day, well-formed CSH structures were observed in large areas with lower w/c ratios.