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    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.
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    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.