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    Comparative analysis of long-term and high temperature performances of OPC based high strength mortar and silica fume based high strength geopolymer mortars
    (Elsevier Science Inc, 2024) Saridemir, Mustafa; Celikten, Serhat; Bulut, Metehan; Deniz, Suvat
    The effects of Class C fly ash (FA) contents on the performance of silica fume (SF) based high strength geopolymer mortars (HSGM) subjected to high temperatures up to 1000 degrees C are investigated. The percentages of FA substitution by SF are 10%, 15%, 20% and 25% by weight. The alkali activators used consist of sodium silicate (SS) and sodium hydroxide (SH) and are used in mixtures with SH/SS ratios of 0.3, 0.4, 0.5 and 0.6. In order to compare SF based HSGMs, ordinary Portland cement (OPC) based high strength mortar (HSM) as a control mortar is also produced with the same dosage and water content. The results at environmental temperature show that higher mechanical properties are obtained from SF based HSGMs compared to OPC based HSM. The optimum replacements of Class C FA and SH/SS ratios are 15 % and 0.3 or 0.4 in terms of mechanical properties. At 28 days, SF based HSGMs with flexural strength (ffs) of 15 MPa and compressive strength (fc) of 100 MPa can be produced without thermal curing. High reductions in the mechanical properties are seen on the OPC based HSM and SF based HSGMs subjected to high temperatures. In addition, SF based HSGMs with fc values above 25 MPa can also be obtained after exposure to 1000 degrees C. Alterations in the microstructure of OPC based HSM and SF based HSGMs under the influence of high temperatures are also examined with XRD, FTIR, SM, and FESEM/EDX analyses. Particularly, a spongy structure with volumetric expansion is seen with the formation of the glassy phase in the matrix of SF based HSGMs subjected to a temperature of 1000 degrees C.
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    Long-term properties of steel fiber reinforced silica fume based AAMs at ambient and high temperatures
    (Elsevier Sci Ltd, 2024) Saridemir, Mustafa; Bulut, Metehan; Deniz, Suvat; Deneme, Ibrahim Ozguer
    In this study, the influences of Class C fly ash (FA), steel fiber (STF) and high temperature are researched on the mechanical and microstructural properties of STF reinforced silica fume (SF) based alkali activated mortars (AAMs) cured at the ambient (25 degrees C) temperature. The STF reinforced SF based AAMs are produced with 10%, 15%, 20% and 25% Class C FA by weight in place of SF, 0.5%, 1% and 1.5% STF by volume, liquid sodium silicate (Na2SiO3) and solid sodium hydroxide (NaOH). The apparent density (rho(a)), ultrasonic pulse velocity (U-pv), flexural strength (f(fs)) and compressive strength (f(c)) results of the STF reinforced control mortars (CMs) and SF based AAMs subjected to the ambient and elevated temperatures are investigated. The microstructural investigations are conducted with X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM)/energy dispersive X-ray (EDAX). Moreover, color changes, pores and interfacial transition zones (ITZs) in samples exposed to the ambient and high temperatures are examined by a stereoscopic microscope (SM). The results have shown that the high strengths can be obtained from STF reinforced SF based AAMs at ambient temperature (25 degrees C) and the optimum Class C FA and STF contents are 15% and 1%. Additionally, when STF reinforced SF based AAMs are exposed to high temperatures (especially 750 degrees C and 1000 degrees C), a serious decrease in the mechanical properties is observed due to the formation of a porous and spongy structure in the matrix and the STFs completely oxidizing and losing their properties.