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This paper presents an experimental investigation of the sulfuration mechanism of concrete. The microstructure, mineral phase composition, substance content, and pH of the concrete were determined using scanning electron microscopy, X-ray diffraction, comprehensive thermal analysis, and pore-solution pH test. It was observed that light-grey spots appeared on the surface of the specimen, and a large amount of powdery precipitated substances appeared. At the initial stage of sulfuration reaction, the formation of ettringite blocked the concrete pores and densified its cracks and voids. Subsequently, ettringite reacted with H⁺ to form gypsum, and the continuous increase in gypsum in the pores increased the number of cracks and broadened their width. Gypsum was the final product of the sulfuration reaction, and the mass percentage of gypsum in the powdery precipitated substances at different water–cement ratios was more than 50%. When the water–cement ratios was 0.37, 0.47, and 0.57, the highest Ca(OH)₂ content was found for the lowest water–cement ratio. As the water–cement ratios increased, the amount of powdery precipitated substances decreased and the CaCO₃ content and pH increased. Full article

The c-C₄F₈ gas is considered to have great potential as a gaseous medium for gas-insulated equipment, due to its good insulation properties and its relatively low greenhouse gas potential (GWP) relative to SF₆. However, the decomposition is an important indicator of its use in equipment. In this paper, the decomposition characteristics of c-C₄F₈ and the influence by oxygen have been explored through experiments and theoretical calculations. Firstly, the breakdown test of mixed gas was carried out and the precipitated elements of the electrodes and breakdown products of gas mixture were analyzed by X-ray photoelectron spectroscopy (XPS) and gas chromatography mass spectrometry (GC-MS). At the same time, the differences in decomposition products have also been studied when a small amount of O₂ was present. The path and mechanism of c-C₄F₈ decomposition is then discussed, based on density functional theory (DFT). The results show that the black powdery substance descends on the electrode surface after the breakdown of the mixture of c-C₄F₈/N₂ gas containing O₂, and its main constituent elements are C, O and F. O₂ can promote the decomposition of c-C₄F₈. The mixture with O₂ produced a large number of additional toxic and corrosive COF₂ in addition to generating more CF₄, C₂F₄, C₂F₆, C₃F₆ and C₃F₈. The GWP values of the products are lower than SF₆. Comprehensive insulation properties and decomposition characteristics, c-C₄F₈ should not be mixed with dry air for use, and the oxygen content should be strictly controlled in c-C₄F₈ mixed gas. Full article