Sulfate diffusion in coal pillar:experimental data and prediction model

The stability of coal pillar dams is crucial for the long-term service of underground reservoirs storing water or heat.Chemi-cal damage of coal dams induced by ions-atttacking in coal is one of the main reasons for the premature failure of coal dams.However,the diffusion process of harmful ions in coal is far from clear,limiting the reliability and durability of coal dam designs.This paper investigates sulfate diffusion in coal pillar through experimental and analytical methods.Coal specimens are prepared and exposed to sulfate solutions with different concentrations.The sulfate concentrations at different locations and time are measured.Based on experimental data and Fick's law,the time-dependent surface concentration of sulfate and diffusion coefficient are determined and formulated.Further,an analytical model for predicting sulfate diffusion in coal pillar is developed by considering dual time-dependent characteristics and Laplace transformations.Through comparisons with experimental data,the accuracy of the analytical model for predicting sulfate diffusion is verified.Further,sulfate diffusions in coal dams for different concentrations of sulfate in mine water are investigated.It has been found that the sulfate concen-tration of exposure surface and diffusion coefficient in coal are both time-dependent and increase with time.Conventional Fick's law is not able to predict the sulfate diffusion in coal pillar due to the dual time-dependent characteristics.The sulfate attacking makes the coal dam a typical heterogeneous gradient structure.For sulfate concentrations 0.01-0.20 mol/L in mine water,it takes almost 1.5 and 4 years for sulfate ions to diffuse 9.46 and 18.92 m,respectively.The experimental data and developed model provide a practical method for predicting sulfate diffusion in coal pillar,which helps the service life design of coal dams.

Diffusion Characteristic of Sulfate Anion in Surface Region of Cement Mortar at Early Stage

The SO4^（2-）concentration distributions in surface region of cement mortar immersed in sulfate solution at early stage were measured by layered sampling method combined with chemical analysis, and the diffusion coefficients of SO4^（2-）anions in surface region of mortar into internal area were calculated by means of instantaneous plane diffusion theory. The experimental results showed that the SO4^（2-）concentration gradually reduced when the diffusion depth increased in the surface region of mortar. Diffusion coefficient（D） was relevant with the concentration and kind of environmental sulfate solution, which reduced with immersion time at the beginning, and then rose slowly after a period of time. The calculation of initial diffusion coefficient（D0） and starting time of deterioration（t∞） caused by sulfate attack was further attempted based on the data of diffusion coefficient, and it was found that D0 and t∞ were all relevant with concentrations of sulfate and different kind of sulfate as well.