Field Chemical Characterization of Sulfate-Induced Deterioration: A Case Study of Two Auxiliary Shafts in China

Vertical shafts are the lifelines of coal mines, serving as critical conduits for resources and personnel. However, the long-term exposure of shaft walls to groundwater erosion significantly reduces their service life and increases the risk of structural failures. This issue is particularly pressing in Inner Mongolia and Henan Provinces, two of China’s major coal-producing regions, where the challenge of sulfate attack on shafts in deep stratigraphic environments has become a growing concern. This study focused on the corrosion damage observed in these two typical auxiliary shafts: the net diameters and depths of the auxiliary shafts in Shunhe Coal Mine and Mataihao Coal Mine are 6 m and 768.5 m and 9.2 m and 457 m, respectively. The rock section shaft walls in the study range from 5 to 10 m in thickness and are constructed using C40 to C60 grade concrete. To assess the extent of this damage, we conducted a comprehensive analysis of shaft wall samples using water analysis, XRD (X-ray diffraction) analysis, FT-IR (Fourier transform infrared) spectroscopy, and XRF (X-ray fluorescence) analysis. The findings reveal that the identified secondary sulfate reaction products within the shaft wall concrete include calcium sulfate, gypsum, ettringite, and thaumasite. The CaO loss rates in the auxiliary shaft walls of Shunhe Coal Mine and Mataihao Coal Mine are as high as 66% and 47%, respectively. Additionally, the concentrations of SO3 and MgO in both mines exceed normal levels by up to 5 and 11 times, and 13 and 3 times, respectively. Despite this, severe corrosion is primarily confined to the inner surface of the auxiliary shaft walls, without significant penetration into the deeper shaft structure. The corrosion damage is predominantly concentrated in the shaft sections where the geological environment is characterized by bedrock. This study provides field evidence and laboratory analyses to inform the mitigation of sulfate attack in auxiliary shafts.