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    Compressive Strength and Microstructure of Microwave-Cured Waste Brick Powder-Based Geopolymer Mortars
    (Springer Int Publ Ag, 2024) Gultekin, Adil
    In this study, the compressive strength, microstructure and energy consumption of conventional oven- and microwave-cured waste brick powder-based geopolymers were investigated. For this purpose, the bricks that emerged as defective products during the factory production process were broken, ground, and used as aluminosilicate source. Samples with varying water contents were produced to examine the effect of the water/binder ratio. An alkali solution prepared with sodium silicate and sodium hydroxide was used as activator, and natural river sand was utilized as aggregate. The mixtures were cured in a household microwave oven at 300-W power level for 30, 45 and 60 min. Additionally, oven-cured specimen prepared at 90 degrees C for 1, 3 and 5 days was used for the sake of comparison. Samples with different water contents (ranging from 0.35 to 0.60) were produced to examine the effect of the water/binder ratio. The highest compressive strengths obtained in conventional and microwave curing mortars were 32 MPa and 36 MPa, respectively. The optimum water/binder ratio for oven and microwave curing was 0.4 regarding compressive strength, and lower rates reduced workability and strength. SEM images demonstrated that the microwave-cured paste had a more homogeneous structure and contained fewer cracks and pores than that of the oven-cured specimen at the same water/binder ratio. The results indicated that microwave-cured waste brick powder-based geopolymers with appropriate mixing ratios and curing regimes have higher compressive strengths and more homogeneous microstructures than their conventionally cured counterparts, with 94% energy savings.
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    Effect of Fiber Type and Length on Strength, Fracture Energy, and Durability Properties of Microwave-Cured Fiber-Reinforced Geopolymer Mortars
    (Mdpi, 2024) Gultekin, Adil
    Microwave curing can be an alternative curing method for geopolymer production. Although many properties of microwave-cured geopolymer composites have been investigated, the effect of microwave curing on the strength and durability properties of fiber-reinforced geopolymers remains a topic that requires investigation. In this study, the effect of fiber type and length on the properties of microwave-cured metakaolin-based geopolymers was investigated. For this purpose, PVA (6, 12 mm) and polymer (15, 30 mm) fibers were utilized. Compressive and flexural strength, fracture energy, abrasion resistance, high-temperature resistance, water absorption capacity and rate of capillary water absorption tests were conducted and the microstructure was examined using scanning electron microscopy. For curing, a household microwave oven was used at a power level of 300 watts. With the fibers' inclusion, fracture energies could be increased by up to 1150%, ductility was enhanced, flexural strengths were increased and compressive strengths decreased. Moreover, the rate of capillary water absorption decreased by up to 13%, while water absorption values increased by between 5% and 12%. The results suggested that microwave curing could be an alternative curing method for the production of fiber-reinforced geopolymer composites, offering shorter curing times and lower energy consumption.
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    Effect of water-reducing admixtures water content on rheology, workability, and mechanical properties of fly ash-based geopolymer and slag-based alkali-activated mixtures
    (Ernst & Sohn, 2023) Keser, Hasan Erinc; Ramyar, Kambiz; Gultekin, Adil
    Admixtures used in concrete have been produced for conventional concrete and the behavior of these additives in geopolymer/alkali-activated systems is still complicated. In this study, the effects of different types of water-reducing admixtures on the fresh and mechanical properties of Class F fly ash-based geopolymer mortars and alkali-activated slag mortars were investigated. For this purpose, flow diameter, rheological examination, compressive and flexural strength tests were carried out. The data obtained showed that the admixtures used in the study did not have a positive effect on the fresh properties of geopolymer/alkali-activated mortars, and the improvements in the flow properties were due to the extra water contributed by the admixture. The flow diameters of the mixtures containing the same amount of water with the mixture containing plasticizer were generally higher than those of the mixtures prepared with the admixture up to 10.3% in the fly ash-based mortars and 15.4% in the slag-based mortars. Addition of plasticizers reduced the compressive strength of fly ash-based geopolymer and slag-based alkali-activated mortars up to 31.7% and 29.7%, respectively.
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    High-Temperature Resistance of Industrial Waste-Based Lightweight Geopolymer Mortars Produced Using Microwave Technology
    (Springer Heidelberg, 2024) Tokdemir, Merve; Ramyar, Kambiz; Gultekin, Adil
    The production of microwave-cured lightweight geopolymers has been investigated; however, the durability properties of these materials, particularly the high-temperature resistance, remain an area requiring further investigation. This study examined the high-temperature resistance of fly ash- and brick powder-based lightweight geopolymer mortars. For this purpose, hydrogen peroxide (Hp) was used as a foaming agent at dosages of 0.5%, 1.0%, and 1.5% by the weight of aluminosilicate, and expanded polystyrene (EPS) beads were used as lightweight aggregate at dosages of 0.2%, 0.4%, and 0.8% by the weight of aluminosilicate. Curing was carried out using a conventional laboratory oven (90 degrees C for 48 h) and a household microwave oven at a power level of 300 watts (20 min for fly ash and 30 min for brick powder). Sodium silicate and sodium hydroxide were used as alkali activators, and pumice aggregate with a maximum particle size of 4.75 mm was utilized for mortar mixtures. The results showed that unit weights and compressive strengths decreased with the inclusion of Hp and EPS. While the oven-cured reference mortars lost strength after exposure to 1000 degrees C, the compressive strengths of mortars containing EPS and Hp increased. The compressive strength of all microwave-cured mortar series increased after high-temperature exposure. With the use of Hp and EPS, compressive strength increases of up to 373% were observed after exposure to 1000 degrees C. These strength increases were attributed to continued geopolymerization under the influence of high temperature and new crystal formations as evidenced by XRD analysis.
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    Mechanical and porosity properties of recycled pervious concrete aggregate-bearing pervious concretes
    (Taylor & Francis Ltd, 2024) Yavuz, Demet; Gultekin, Adil
    The recycling of construction wastes holds significant importance both environmentally and economically. While extensive research has been conducted on aggregates derived from various wastes, the recycling of pervious concretes (PC) has been largely overlooked. This study addresses this gap by examining the porosity and mechanical properties of PC manufactured using aggregates obtained from recycled PC. The investigation focuses on three key factors: aggregate size (5/15, 10/15, and 15/25 mm size fractions), fiber inclusion (dosages of 0.1%, 0.2%, and 0.3% by volume), and aggregate type (limestone aggregate and recycled aggregate). Through image processing techniques, void characteristics including amount, structure, and homogeneity were quantified. Results indicate that the use of recycled aggregate led to a decrease in compressive strength ranging from 29% to 65%, depending on aggregate size fraction and fiber content. Porosity assessments revealed higher porosity in concrete utilizing recycled aggregate, with computer-based methods yielding values closely aligned with volumetric results.
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    Properties of Basalt Fiber-reinforced Lightweight Geopolymer Mortars Produced with Expanded Glass Aggregate
    (2024) Gultekin, Adil
    Geopolymers are new-generation construction materials that have attracted attention recently and can be an alternative to cement. In the production of these materials, aluminosilicate powder materials are used together with alkali or acid solutions. Geopolymers have different types of superiorities, such as rapid strength gain, high mechanical properties and good durability. This experimental study investigated the properties of expanded glass aggregate-bearing Class F fly ash-based lightweight geopolymer mortars. The fresh unit weight, water absorption capacity, compressive strength and high-temperature resistance (upon exposure to 900°C) of the mortars were determined. In addition, basalt fiber addition's effects on these properties were investigated. The inclusion ratios of basalt fiber were 0.1%, 0.2% and 0.4% by volume. The compressive strengths of fiber-free lightweight mixture and mixtures, including 0.1%, 0.2% and 0.4% basalt fiber, were found to be 8.2, 8.9, 9.0 and 8.0 MPa, respectively. The compressive strength of all lightweight mortars increased between 61.3% and 76.4% after the high-temperature effect. The results proved that it is possible to produce expanded glass aggregate-bearing lightweight geopolymer mortars with acceptable mechanical properties.