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Öğe Alkali-activated and geopolymer materials developed using innovative manufacturing techniques: A critical review(Elsevier Sci Ltd, 2021) Gokce, H. S.; Tuyan, M.; Nehdi, M. L.The manufacturing of geopolymers and alkali-activated materials for precast and cast in-situ construction applications could be achieved using suitable production techniques, such as one-part production, pre-setting pressure and hot pressing, two-stage concreting, and 3-dimensional printing. Adequate selection and tailoring of the manufacturing methodology are imperative for overcoming characteristic application problems of these materials and achieving eco-efficiency and superior engineering properties. With distinctive benefits including rapid solidification, lower cost, saving natural resources, less energy consumption, and reduced carbon footprint, alkali-activated and geopolymer composites have emerged as a strong contender for replacing conventional concrete in diverse construction applications. Critical analysis of the literature on innovative production methods indicates that the one-part production technique is promising for the eliminating design complexity of these mixtures. The pre-setting pressure and hot pressing applications would allow the achievement of superior physical and mechanical characteristics in a short period of time, especially for the precast industry. The workability and setting problems of alkali-activated materials could be overcome using the two-stage concreting methodology. To be used in 3D printing of alkali-activated and geopolymer materials, remarkable development is required in terms of rheological aspects. Accordingly, this paper surveys pertinent and recent literature and critically reviews the state-of-the-art of alternative production techniques of alkali-activated and geopolymeric materials, along with its use in recent applications, identifies pertinent knowledge gaps, and defines future research directions.Öğe Development of Eco-Efficient Fly Ash-Based Alkali-Activated and Geopolymer Composites with Reduced Alkaline Activator Dosage(Asce-Amer Soc Civil Engineers, 2020) Gokce, H. S.; Tuyan, M.; Ramyar, K.; Nehdi, M. L.The eco-efficiency and economy of geopolymer composites largely depend on their alkaline activator dosage. In this study, the effect of applying a pre-setting pressure and variation of the aggregate-to-fly ash ratio on the alkaline solution dosage and mechanical strength of fly ash-based geopolymer composites was explored. It is shown that through control of the aggregate-to-ash ratio and application of pre-setting pressure, compressive strength could be increased by 102% and 86% for Class F fly ash-based geopolymer and alkali-activated Class C fly ash-based mixture, respectively. The total alkaline solution consumption could be reduced from 718 to 188 kg/m(3) and from 769 to 262 kg/m3 for Class F fly ash-based geopolymer and alkali-activated Class C fly ash-based mixture, respectively. The proposed method reduced the alkaline solution consumption per compressive strength (6.2 kg.m(-3).MPa-1) by up to 85% compared to that of the reference manually consolidated control geopolymer. The findings demonstrate that fly ash-based geopolymers could be made more sustainable and eco-efficient through tailored production techniques. (C) 2019 American Society of Civil Engineers.Öğe Durability of slag-based alkali-activated materials: A critical review(Springer, 2024) Gokce, H. S.As the world becomes increasingly aware of the devastating effects of climate change, the need for sustainable building materials that are both durable and environmentally friendly increases. Geopolymer and alkali-activated materials formed by a chemical reaction between an alkaline activator solution and an aluminosilicate source have gained popularity in recent years. The alkaline activator solution dissolves the aluminosilicate source, which then undergoes a polycondensation reaction to form a three-dimensional geopolymeric gel network. The development of this network ensures the strength and durability of the material. Today, this phenomenon of durability has been studied in detail to enable the development of superior construction materials, taking into account degradation mechanisms such as carbonation, leaching, shrinkage, fire, freezing and thawing, and exposure to aggressive environments (chlorides, acids, and sulphates). Although there are many unsolved problems in their engineering applications, slag-based alkali-activated materials appear to be more advantageous and are promising as alternative materials to ordinary Portland cement. First of all, it should not be ignored that the cure sensitivity is high in these systems due to compressive strength losses of up to 69%. Loss of strength of alkali-activated materials is considered an important indicator of degradation. In binary precursors, the presence of fly ash in slag can result in an improvement of over 10% in compressive strength of the binary-based alkali-activated materials after undergoing carbonation. The binary systems can provide superior resistance to many degradation mechanisms, especially exposure to high-temperature. The partial presence of class F fly ash in the slag-based precursor can overcome the poor ability of alkali-activated materials to withstand high temperatures. Due to the desired pore structure, alkali-activated materials may not be damaged even after 300 freeze-thaw cycles. Their superior permeability compared to cementitious counterparts can extend service life against chloride corrosion by more than 20 times. While traditional (ordinary Portland cement-based) concrete remains the most widely used material in construction, geopolymer concrete's superior performance makes it an increasingly emerging option for sustainable and long-lasting infrastructure.Öğe Effect of elevated temperature on radiation shielding properties of cement and geopolymer mortars including barite aggregate and colemanite powder(Elsevier Sci Ltd, 2023) Kok, S.; Turetken, M. S.; Oksuzer, N.; Gokce, H. S.This study aims to investigate the radiation shielding efficiency of cement and geopolymer-based mortars after exposure to elevated temperatures which may happen during the activation process of nuclear reactors. To contribute to the shielding properties, these binders were modified with the substitution of colemanite powder (5 and 10%), and barite aggregate was used to be an alternative to silica sand, as well. The produced specimens were exposed to 300 and 500 degrees C. While the colemanite powder resulted in significant retardation of setting time up to 183% for cement-based series, a set accelerator effect reaching 67% was found in the geopolymer series. Unlike expectation, the losses in mechanical and shielding properties have become more pronounced in geopolymer mortars at the applied temperatures. In particular, a significant increase reaching 28% in compressive strength of cement series was found after 300 degrees C exposure. It was noted that cement series with reaching a linear attenuation coefficient of 0.162 cm-1 presented more effective shielding against gamma rays when compared to that (0.134 cm-1) of geopolymers. To moderate thermal neutrons, geopolymer series reaching a linear attenuation factor of 2.799 cm-1, were found to be a more promising alternative according to cement series reaching 2.140 cm-1.Öğe Enhancing Burnability Characteristics of Low-Temperature Burnt-Cement Clinker by Recycling Phosphogypsum Wastes(Asce-Amer Soc Civil Engineers, 2021) Aksan, U.; Koseoglu, K.; Gokce, H. S.In the present study, the effect of phosphogypsum, a byproduct from the phosphoric acid production industry, on the clinkerization process was researched in order to improve the burnability of clinkers at lower burning temperatures. Raw meal samples containing phosphogypsum at 0%, 1%, 3%, and 5% by weight were burned at 1,350 degrees C, 1,400 degrees C, 1,450 degrees C, and 1,500 degrees C. The introduction of phosphogypsum reduced the presence of unbounded (free) CaO relatively up to 43%. Thus, the mineralization effect of phosphogypsum promotes lower burning temperatures. The free CaO content of the 5% phosphogypsum containing clinker decreased to 0.94% at 1,400 degrees C after 120 min of burning time. The free CaO content was found to be almost identical (0.75%) to that of industrial clinker burned in a rotary kiln at 1,500 degrees C. Due to the low free CaO content (<1%) and the burnability index (73), the recycling of phosphogypsum wastes was found to be suitable for the production of eco-efficient clinkers. The reduced energy consumption and energy-related CO2 emissions in such modified clinkers can contribute to the economic and ecological goals of the cement industry. (C) 2021 American Society of Civil Engineers.Öğe Gamma ray and neutron shielding characteristics of polypropylene fiber-reinforced heavyweight concrete exposed to high temperatures(Elsevier Sci Ltd, 2020) Demir, I; Gumus, M.; Gokce, H. S.The residual properties of shielding structures need to be redefined after exposure to elevated temperatures due to their probable radiological hazards on biodiversity. In this study, the effect of high temperatures on mechanical, gamma ray and neutron attenuation characteristics was determined for limestone, barite and siderite concrete shields reinforced with polypropylene fiber. The increase in temperature up to 600 degrees C reduced the ultrasonic pulse velocity and compressive strength values of these shields by 59% and 62%, respectively. A good linear correlation (R-2 >= 0.97) was found between these values. While the gamma ray linear attenuation coefficients increased by 9% for barite concrete, and reduced by 15% for normal concrete and by 17% for siderite concrete at 600 degrees C, trivial fluctuations (from -1% to +7%) were observed at 300 degrees C. The neutron attenuation factors of these shields gradually reduced up to 31% with the increase of temperature. The performance of siderite concrete was found to be slightly better than that of barite concrete in terms of strength and neutron shielding characteristics. With the satisfactory linear attenuation coefficients at all energy levels of gamma rays, barite concrete seems to be the most suitable concrete type under high temperature risks. (C) 2020 Elsevier Ltd. All rights reserved.Öğe Gamma-ray and neutron shielding capability of blended cement-stabilized barite fillers developed by maximum density method(Pergamon-Elsevier Science Ltd, 2023) Gokce, H. S.; Oksuzer, N.; Kamiloglu, H. A.; Yilmaz, F.The disposal of radiological wastes in the deep geological repositories has become an important issue for the reduction of hazardous risks on the human and biological environments. In this study, blended cement-stabilized barite fillers against gamma rays and neutrons have been developed with the use of the maximum density method by considering various amounts of water (100-300 kg/m3), binder (4-10%), and compaction energy (500-2000 kJ/m3). According to the experimental series of these filler materials, satisfactory response surface results have been achieved in the study. The experimental results revealed that compacted filling material can ensure an efficient shielding capability for gamma rays and neutrons when designed with a density of 3.064 g/ cm3. An increase in binder content, water content, and compaction energy has improved the compressive strength of these materials up to 37 times according to that of the poorest series. Neutron and gamma-ray shielding capabilities were found to be 119 and 14% higher than those of the poorest filler material, respec-tively when theoretical and experimental results are considered in the study.Öğe Improvement of mechanical and transport properties of reactive powder concrete using graphene nanoplatelet and waste glass aggregate(Elsevier Sci Ltd, 2022) Arslan, S.; Oksuzer, N.; Gokce, H. S.In this study, the use of graphene nanoplatelet (GNP) and waste glass aggregate (WGA) was investigated to improve the mechanical and transport properties of reactive powder concrete (RPC), which is an ultra-high strength concrete type. The co-addition of 0.14% GNP and 30% WGA resulted in a significant increase in the compressive strength of RPC from 192.9 to 258.9 MPa and flexural strength from 16.2 to 38.7 MPa. In the study, the most significant improvement (138.6%) was obtained for the flexural strength values of RPC. Considering the strength aspects, the relevant amounts of GNP and WGA enable production of RPC ensuring a conducted electrical charge of less than 100 Coulombs; that is, with negligible chloride ion permeability. Accordingly, the highest improvement (59.8%) after flexural strength was found for the resistance of the specimens against chloride ion penetrability. In conclusion, the findings of this study show that eco-efficient production of RPC with superior mechanical and durability characteristics can be achieved using GNP and WGA, with lower cement consumption per unit value of these studied characteristics, reaching 59%.Öğe Nanomaterial and fiber-reinforced sustainable geopolymers: A systematic critical review(Elsevier Sci Ltd, 2023) Unal, M. T.; Gokce, H. S.; Ayough, P.; Alnahhal, A. M.; Simsek, O.; Nehdi, M. L.Global cement production is associated with a colossal environmental footprint due to its considerable carbon emissions, energy consumption, depletion of natural resources, and waste accumulation. Geopolymer based concretes (GCs) have emerged as revolutionary alternatives to ordinary Portland cement concrete with enormous potential ecological benefits. However, they suffer from deficiencies, such as their brittle behavior under flexural and tensile loads. Using different types of fiber reinforcement and nanomaterial additions in geopolymer composites (GCs) has recently gained considerable attention to enhance various engineering properties, improve crack resistance, toughness, and ductility of the geopolymer matrices. This systematic and critical review analyses the effects of different reinforcing fibers and nanomaterials on the compressive and tensile strengths, modulus of elasticity, and impact resistance of GCs, and highlights the associated microstructural features. It is shown that carbon, basalt, and steel fibers can impart considerable improvement in mechanical strength, modulus of elasticity, and impact resistance of GCs. Furthermore, the introduction of polyethylene fibers has been shown to induce complex cracking behaviors, thereby contributing to the strain-hardening capability of geopolymer matrices. The integration of nanomaterials into GCs has emerged as a powerful strategy for achieving substantial enhancements in mechanical performance. Optimal nanomaterial dosages, typically around 2%, have been identified, with the specific surface area of nanoparticles proving to be a crucial determinant of the resulting mechanical properties. Notably, nano-silica has exhibited pronounced macro-scale reinforcement effects, whereas nano-titanium has displayed the potential to significantly enhance gel micromechanical characteristics. Additionally, synergistic combinations of nanomaterials and fiber reinforcements have led to the development of novel and distinctive mechanical properties within GCs. The synthesis of these findings underscores current best practices, highlights areas requiring further investigation, and emphasizes the need for concerted research efforts to advance the knowledge and implementation of sustainable geopolymers. This review offers a comprehensive analysis of the effects of fiber reinforcements and nanomaterials on geopolymer composites, providing valuable insights for researchers and practitioners.Öğe Natural radioactivity of barite concrete shields containing commonly used supplementary materials(Elsevier Sci Ltd, 2020) Gokce, H. S.; Ozturk, B. Canbaz; Cam, N. F.; Andic-Cakir, O.The recycling of hazardous materials within new composites has a sustainable importance as it contributes on the reduction of high radionuclide concentrations. In this study, the effect of cementitious materials i.e. viscosity modifier, silica fume and fly ash on the Ra-226, Th-232 and K-40 activity levels of barite concrete shields was researched by using different mixing compositions in terms of binder content, w/b ratio and the amount of supplementary cementitious materials. In comparison to cement, silica fume and fly ash used in the production of concrete shields were found to have significantly higher activity levels of K-40, as well as Ra-226, Th-232 and K-40, respectively. The Ra-226, Th-232 and K-40 activity levels of concrete shields range between 2.2 and 20.7 Bq kg(-1); 2.6 and 7.0 Bq kg(-1); 51.5 and 89.3 Bq kg(-1), respectively. These activity values were found similar or less than those of building materials in the world. These satisfactory results are mostly caused by the barite source which has lower radioactivity levels. The concrete mixture details used in the study caused significant variation in the natural radioactivity of barite concrete shields. The introduction of silica fume which has high K-40 activity levels and fly ash with high Ra-226, Th-232 and K-40 activity levels compared to cement and barite aggregate, significantly increased the radiological hazard parameters, although all results were found to be below the limits recommended by international reports. In conclusion. Ra-226 activity levels of the concrete shields were significantly increased (up to 8.4 times) by the variation of mix design parameters used in comparison with concrete mixtures that have the least Ra-226 activity level. The Th-232 and K-40 activity levels were as well increased in relatively less amounts (1.7 and 0.73 times higher, respectively). (C) 2019 Elsevier Ltd. All rights reserved.Öğe A novel internal curing method for 3D-printed geopolymer structures reinforced with a steel cable: Electro-heating(Elsevier, 2022) Gokce, H. S.; Gungor, O.; Oksuzer, N.Buildability and structural integrity are the inherent challenges of 3D printed geopolymer structures needed to be overcome. Reinforcing with steel cable ensuring a higher structural integrity of the printed structure can simultaneously improve the buildability due to the heating capability of this component. The controllable instant heating capability of steel cable was achieved by the use of a specific circuit system in the study. As a novel internal curing technique, electro-heating is recommended in the additive manufacturing of steel cable reinforced geopolymer structures by the authors.Öğe An online software to simulate the shielding properties of materials for neutrons and photons: NGCal(Pergamon-Elsevier Science Ltd, 2021) Gokce, H. S.; Gungor, O.; Yilmaz, H.NGCal, a free online computation software (http://ngcal.com), has been developed for narrow beam transmission properties of photons (for energies of X rays and gamma rays between 0.002 MeV and 20 MeV), thermal (25.4 meV) and fast neutrons (4 MeV), in this regard it theoretically calculates the mass and linear attenuation coefficient/factor, mean free path, half- and tenth-value layers of materials formed by elements, compounds and composites. Its calculation assumptions, user guide as well as a comparative example of its results to those of other online computation platforms (XCOM, NCNR, WinXCOM, MCNPX, and Phy-X/PSD) are presented in this study. The developed software allows for more data input consisting of elements/oxides/compound and their fractions, and presents a number of shielding results required by researchers and scientists working on radiation shielding properties of multi-component materials.Öğe Performance of fly ash-blended Portland cement concrete developed by using fine or coarse recycled concrete aggregate(Elsevier Sci Ltd, 2022) Simsek, O.; Sefidehkhan, H. Pourghadri; Gokce, H. S.The recycling of concrete and industrial wastes in the production of new concrete structures has increased its share due to the growing environmental concerns of the world. In this study, the use of recycled concrete aggregate at various replacement amounts (0-100 %) was investigated to be fine or coarse aggregate sources instead of crushed natural aggregate to understand better its effects on the dimensional stability and durability aspects of fly ash-blended Portland cement concrete. In the presence of recycled concrete aggregate, fly ash was very important for eliminating possible mechanical losses of concrete at 90 days. The use of coarse recycled concrete aggregate resulted in better compressive strength values of concrete exposed to with and without wetting-drying and freezing-thawing cycles when compared to fine ones. While the free drying shrinkage of concrete increased at significant amounts (68 and 79 %) for fine and coarse recycled concrete aggregate, the restrained drying shrinkage crack widths were reduced up to by 13.5 %. In conclusion, it should be noted that the amount and size of recycled concrete aggregate need to be designed by considering the residual quality and dimensional stability of concrete, which exposes weathering actions in place.Öğe Physical and mechanical properties of cement containing regional hazelnut shell ash wastes(Elsevier Sci Ltd, 2020) Baran, Y.; Gokce, H. S.; Durmaz, M.Turkey is ranked in the first place with two-thirds of the world hazelnut production. As the amount of shell obtained from shelled-hazelnut is almost equal to that of hazelnut by weight, the shell is a regionally important waste. The biomass is consumed as a fuel in some homes, workplaces, and public buildings with the help of specific caldrons. In the study, the use of ash was researched in the production of blended cement. For this purpose, blended cement mixtures containing hazelnut shell ash at the ratios of 0, 5, 10, 15, 20, 25 and 30% by weight of CEM I 42.5R ordinary Portland cement were prepared. The binary cementitious systems were assessed in terms of chemical, physical and mechanical properties. Hazelnut shell ash introduction increased the water demand up to 59% for standard consistency and reduced setting time up to 96% according to the ordinary Portland cement. Compressive strength results were remarkably reduced by an increase in the amount of ash. With over 5% of hazelnut shell ash introduction, the 28-d compressive strength value was found unsatisfactory (<42.5 MPa). The insufficient total amount of SiO2+Fe2O3+Al2O3 reduced the usage amount of the biomass ash in cementitious products due to having no pozzolanic characteristic. Hazelnut shell ash can be used as an alkaline set accelerator in concrete products as it is not steel-reinforced due to its high chlorine ion content. (C) 2020 Elsevier Ltd. All rights reserved.Öğe Physical, mechanical, and radiation attenuation properties of serpentine concrete containing boric acid(Elsevier Sci Ltd, 2021) Zayed, A. M.; Masoud, M. A.; Shahien, M. G.; Gokce, H. S.; Sakr, K.; Kansouh, W. A.; El-Khayatt, A. M.Serpentine and boric acid are effective materials to produce the radiation-shielding concrete (RSC) for attenuating the fast and thermal neutrons, respectively. Therefore, this study investigates the effect of boric acid (0, 1, and 3% by cement weight) on the physical, mechanical, microstructural, and radiation shielding properties of serpentine concrete. The results showed that the addition of boric acid hindered the hydration of cement through the generation of amorphous borate material, which encapsulates the cement phases and retards their transformation into hydration products. Such a retardation process negatively influenced the mechanical properties of boric acid-bearing concrete and contributed to the thicker and less condensed interfacial transition zone. Furthermore, unlike total gamma-rays, boric acid enhanced the attenuation properties of serpentine concrete against thermal and fast neutrons favoring the former. There was a satisfying agreement between the theoretical and measured values of fast neutron attenuation parameters. However, negligible variations in the theoretical values of photon interaction parameters showed that porosity was the principal reason for the reduction in gamma-ray attenuation as a result of boric acid addition. (C) 2020 Elsevier Ltd. All rights reserved.Öğe A regional supplementary cementitious material for the cement industry: Pistachio shell ash(Elsevier Sci Ltd, 2021) Tekin, I; Dirikolu, I; Gokce, H. S.The recycling of agricultural wastes in cementitious products has recently been a trendy approach for the sustainability goals of the world as it ensures economic and ecological advantages in addition to the safe disposal of these wastes. In this regard, the potentiality of a biomass ash obtained from the incineration of pistachio shell to serve as a supplementary cementitious material is documented in the present study. An increase of up to 30% in the amount of pistachio shell ash resulted in the proportional increase of both water requirement and setting time of the ordinary Portland cement by up to 54% and 300%, respectively. The incorporation of up to 20% pistachio shell ash into the cement yielded specimens of comparable or higher compressive strength values relatively varying between 98% and 117% at the 28 days and later curing ages. Beside the cementitious property, it was observed that a presence of graphitic structures in the pistachio shell ash remarkably improved the mechanical and the microstructural properties of cement mortars. In conclusion, at least 10% incorporation of this ash into the cement is highly recommended for the cement industry as it is beneficial in achieving the highest ultimate (400-day) compressive strength and promotes no loss on the early (2-day) compressive strength. (C) 2020 Elsevier Ltd. All rights reserved.Öğe Simulation and optimization of gamma-ray linear attenuation coefficients of barite concrete shields(Elsevier Sci Ltd, 2020) Sensoy, A. T.; Gokce, H. S.The remarkable increment in radioactivity risks of today's industrial world is required to the efficient design of shielding structures. In the present study, the optimal design parameters of barite concrete shields for providing the highest gamma radiation shielding capability were researched by using a hybrid technique of the Box-Behnken Design based response surface method and Particle Swarm Optimization. Barite aggregate fraction, water-to-cement ratio and cement content of the shields were limited as 0:100 (by weight), 0.30:0.70 (by weight) and 200:600 (kg/m(3)) in the optimization problem, respectively. The regression equation derived from the regression learning process presented an excellent correlation to define the XCOM linear attenuation coefficients. A nonlinear response surface fitted to the regression equation was used as the validation of the PSO results obtained from MATLAB. The results of this study have shown that the proposed method provides satisfactory predictions for the barite concrete radiation-shielding capability depending on the design parameters. In addition, it was found that the linear attenuation coefficient could be increased by 102% by the design of the best shield according to the poorest mixture. (C) 2020 Elsevier Ltd. All rights reserved.Öğe The Toughness of Polypropylene Fiber-Reinforced Foam Concrete under Various Uni- and Tri-Axial Compression Loads(Korean Society Of Civil Engineers-Ksce, 2023) Gokce, H. S.; Oksuzer, N.; Kamiloglu, H. A.; Eyuboglu, M.; Yilmaz, F.Foam concrete has recently become a key construction material in terms of meeting the special needs of modern engineering applications such as thermal insulation, absorption of static and dynamic loads. In this study, the effect of polypropylene fiber content and various uni- and tri-axial compression loads on the toughness response of polypropylene fiber-reinforced foam concrete was investigated. Up to a certain strain level (0.1 mm/mm), the ultimate compression stress of specimens under uni- and tri-axial loading increased from about 1 MPa to 16 MPa with the increased target densities of foam concrete. There was a strain-hardening capability of low-density foam concrete while the specimens failed by strain-softening in the high-density series. The optimum fiber amounts were found to be 3.9%, 4.6%, and 6.4% for low, medium, and high target densities of foam concrete, respectively. At low-density series, the bubbles were observed to be relatively bigger and mostly merged with each other. A reduction in foam content (vice versa, increasing target density of mixture) and the presence of fiber resulted in smaller pore size and a more homogenous distribution of them in the matrix. In conclusion, the desired pore structure and efficient bridging of fibers in the matrix allowed the production of favorable foam concrete with higher toughness.
