Scanning electron microscopy and X-ray microanalysis of the condition of concrete structures of a chemical enterprise with long-term corrosion in a sulfate environment

Long-term operation of premises in aggressive environments of chemical enterprises affects the surfaces of concrete structures and leads to the formation of destructive processes. The aim of the study was to determine changes in the structure and mineral composition of concrete during long-term operation in an aggressive sulfate environment at a chemical plant for the production of titanium dioxide using the sulfate method to predict the service life of concrete structures. It has been established that during the production of TiO2 pigment by the sulfate method, the ore is decomposed by sulfate acid at high temperature, accompanied by the release of H2SO4 vapor, sulfur dioxide SO2, hydrogen sulfide H2S and elemental sulfur. Studies have established that sulfate corrosion leads to the formation of sulfur and iron oxides on the concrete surface. Studies have revealed vast areas of the surface covered with crystals of elemental sulfur, the contents of which are confirmed by the results of X-ray microanalysis. The sizes of sulfur crystals in the image range from 12 to 180 µm, the shape corresponds to the rhombic allotropic modification S8. The penetration depth of iron sulfates into the thickness of concrete is about 50 microns. The microstructure of the surface is loose, with signs of corrosion. Exposure to a high concentration of sulphate acid vapors in the workshop of the enterprise led to the destruction of calcium carbonate and the formation of gypsum crystals in concrete. The formation of gypsum crystalline hydrates provokes sulfate corrosion of concrete, resulting in the formation of pores and microcracks in concrete with the formation of calcium hydrosulfoaluminate. The studies carried out using scanning electron microscopy and X-ray microanalysis make it possible to investigate the structural changes and mineral composition of concrete under the influence of long-term sulfate corrosion.