Microstructural changes in limestone after treatment with Na2CO3 solution: Implications for eliminating H2S in tunnels

Abstract

Research on the effects of the pre-injected lye on the microstructure of the surrounding rock is indispensable for H2S treatment during tunnel construction. In this study, limestone samples from the Huangjiagou tunnel in Guizhou Province in southwest China were pre-adsorbed with 500 ppm of H2S and soaked in 1 %, 3 %, and 5 % Na2CO3 solution for 12, 24, and 36 h. The changes in the pore structure and mineral composition, and the secondary precipitates formed on the rock surface after Na2CO3 solution injection were studied by using N2 gas adsorption, X-ray diffraction, and field-emission scanning electron microscopy. It is clearly shown that the average volumes of micro- and meso-pores in the samples after treated decreased by 66.17 % and 16.63 %, respectively, of the original values. This resulted in a significant decrease in the specific surface area and total pore volume. The average pore width increased linearly with increasing Na2CO3 concentration or soaking time, whereas the curve for the relationship with the specific surface area was parabolic, with a minimum at a concentration of approximately 3 % and soaking time of 24 h. Clay (e.g., illite and kaolinite) and feldspar are more easily dissolved than other minerals, and the released metal ions easily form carbonate secondary precipitates with CO32− ions. The mass dissolution of clay minerals and the blocking effect of secondary precipitation decreases the amounts of micropores and mesopores of diameter less than 10 nm, therefore the proportion of macropores increases and the fractal dimensions are simpler. The results of this study will be useful in strategy development and parameter selection for Na2CO3 solution injection for H2S treatment in tunnel engineering.