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.
Keywords: ettringite; gypsum; microstructure; powdery precipitated substances; sulfuration mechanism.