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The oxidation behaviour of Incoloy 909 superalloy subjected to diffusion boronizing treatments was studied at 750 and 850°C in atmospheres of dry air and a mixture of dry air + water vapour for times up to 10 h. The oxidation behaviour of the coated samples was described by the parabolic rate law with activation energies of 26 and 21 kJ mol−1 when oxidation was conducted in dry air or in the mixture dry air + water vapour respectively. Oxidation kinetics of the boronized coatings were sensitive to temperature and the oxidation environment promoting the development of a thick SiO2 layer on top of the coatings which also included minor additions of other oxides that acted as a barrier that reduced oxidation. However, in the conditions that included water vapour boron-rich oxides were also formed and a different oxidation behaviour was experienced. [ABSTRACT FROM AUTHOR]

An alternative solution to reduce environmental pollution using aluminum waste foundry sand (AWFS) and fly ash (FA) to produce sustainable construction materials was studied. New mortars were prepared by partially replacing ordinary Portland cement with fly ash at 5, 10 and 15 % mass. and a total replace of Ottawa sand (OS) with AWFS. The specimens were cured at 25°C with a 100% relative humidity. The mechanical behavior was evaluated by compression test at the ages of 7, 14 and 28 days. The microstructural characteristics were analyzed by scanning electron microscopy (SEM). The results indicate that the addition of milling fly ash in AWFS mortars increases the mechanical resistance, mostly at 5% mass reaching the maximum value of 10 MPa at 28 days of age. Microstructurally, it was found a porous cement matrix with some cracking caused by the reaction of portland cement with the metallic aluminum remaining in the waste sand, which is correlated to the low mechanical resistance obtained. The final mechanical characteristic makes this new product a serious candidate to be used as a sustainable building material working at low load. [ABSTRACT FROM AUTHOR]

In this study, it was evaluated the chemical interaction between iron ore (Fe2O3) and a refractory concrete based on Al2O3- MgAl2O4 at 1650 °C for 4 h using an air atmosphere, in order to determine the chemical behavior and possible reactions between these materials. After testing, the refractory concrete was microstructurally characterized by optical microscopy and scanning electron microscopy (SEM-EDS), as well as X-ray diffraction and Raman spectroscopy. The results showed that magnesium aluminate phase incorporates in solid solution Fe+2 ions, resulting in the formation of (Fe, Mg) (Al2O4) spinels and after their saturation, the rest of Fe+2 ions tends to precipitate at the grain boundaries of alumina and magnesium aluminate phases. This fact promotes during the concrete cooling, the formation of wustite, hematite and magnetite phases (detected in the microstructure as a dendritic and widmanstätten morphologies). The presence of hematite phase was corroborated by X-ray diffraction; likewise the presence of modified spinel phase whose hkl reflections width is higher compared with the original spinel, due to the incorporation of Fe+2 in the initial MgAl2O4 phase. For Raman spectroscopy, it was confirmed the presence of corundum (alumina) phase at characteristic frequencies of 378, 416 and 644 cm-1, hematite phase at 226, 292, 411 and 612 cm-1, magnetite phase at 520 and 668 cm-1 and spinel MgAl2O4 and (Fe, Mg) (Al2O4) ) with its main frequencies at 406 and 666 major cm-1. [ABSTRACT FROM AUTHOR]

The partial replacement effect of Portland cement by geothermal nano-SiO 2 waste (GNSW) for sustainable Portland-cement-based concrete was investigated to improve the properties of concrete exposed at high critical temperatures. Portland cement was partially replaced by 20 and 30 wt.% of GNSW. The partial replacement effect on Portland-cement-based concrete subjected to 350, 550, and 750 °C was evaluated by measuring the weight changes, ultrasonic pulse velocity, thermogravimetric and differential thermal analysis, X-ray diffraction, surface inspection, and scanning electron microscopy under residual conditions. The ultrasonic pulse velocity results showed that the GNSW specimens maintained suitable stability after being heated to 350 °C. The SEM analysis revealed a denser microstructure for the 20 wt.% of partial replacement of Portland cement by GNSW specimen compared to the reference concrete when exposed to temperatures up to 400 °C, maintaining stability in its microstructure. The weight losses were higher for the specimens with partial replacements of GNSW than the reference concrete at 550 °C, which can be attributed to the pozzolanic activity presented by the GNSW, which increases the amounts of CSH gel, leading to a much denser cementitious matrix, causing a higher weight loss compared to the reference concrete. GNSW is a viable supplementary cementitious material, enhancing thermal properties up to 400 °C due to its high pozzolanic activity and filler effect while offering environmental benefits by reducing industrial waste.