Study on laboratory acid erosion characteristics of carbonate rocks in different formations under hydrodynamic action

ZHANG Jiaxin; CHU Xuewei; FU Hai; ZHANG Qilin; ZONG Shaokang

doi:10.11932/karst20250307

Abstract：In Southwest China,carbonate rocks are extensively distributed,and their unique dissolution features pose significant challenges to engineering construction in this region.This study focused on Guizhou Province,located in the core area of the karst regions in Southwest China.A phosphogypsum stacking site with potential leakage risks and intense karst development in the surrounding area was selected as the study area.The pH values of the phosphogypsum leachate,measured in the field at the landfill,ranged from 1.12 and 2.64.The dissolution characteristics of carbonate rocks are expected to change in such an acidic environment.While existing studies have mainly investigated the dissolution effects on carbonate rocks by altering the single factor,the combined influence of lithology,fluid state,and acid concentration on carbonate rocks dissolution remains underexplored.In order to study the dissolution mechanism of carbonate rocks after the original environmental change,five strata samples in the area of Fuquan City,Guizhou Province were collected in this experiment.These samples included limestone of the Lower Triassic Maocaopu Formation(T1m),dolomite of the Anshun Formation(T1a),limestone of the Middle Triassic Qingyan Formation(T2q2),limestone of the Falang Formation(T2f2),and dolomite of the Late Aurora Dengying Formation(Z2dy).Indoor dynamic simulation experiments on dissolution were carried out under different acid concentrations and hydrodynamic conditions. The research results show:(1)Compared with the rock samples of the Maocaopu Formation,which have a relatively uniform mineral composition with a calcite content of 96.72％,the Falang Formation contains 47.22％calcite and 51.93％dolomite.Differences in the connectivity of the contact surfaces between these minerals result in the easy detachment of the surface material from the rocks. (2)At a pH value of 1,the difference in the amount of dissolution per unit area between limestone and dolomite is significant.The amount of dissolution per unit area of the dolomite from the Qingyan Formation,the Maocaopu Formation,and the Falang Formation increases by 12％to 32％,compared with that of dolomite from the Anshun Formation and the Dengying Formation.When the pH value is raised to 2,increasing the rotational speed of the disk does not result in a significant increase in the amount of dissolution per unit area for either limestone or dolomite. (3)Under hydrodynamic conditions of 200 rpm,the CaO/MgO ratio of the dolomite from the Falang Formation is 17.83,with an acid-rock reaction rate constant of 0.964× 10-6.In contrast,the limestone from the Maocaopu Formation has a CaO/MgO ratio of 65.59 and an acid-rock reaction rate constant of 1.023×10-6.Under hydrodynamic conditions of 400 rpm,the difference between the acid-rock reaction rate constants of the Falang Formation and the Maocaopu Formation is 5.38× 10-6.The difference in CaO/MgO ratios between the two formations is significant;however,the difference in acid-rock reaction rate constants of the Falang Formation and the Maocaopu Formation is not substantial under high hydrodynamic force.In conclusion,in acidic solutions,the chemical reaction rates(R)of the five rock groups follow the order:the Qingyan Formation＞the Maocaopu Formation＞the Falang Formation＞the Anshun Formation＞the Dengying Formation.During the dissolution of carbonate rocks,three factors interact:hydrodynamics,acid concentration,and lithology.Higher acid concentrations amplify the differences in dissolution caused by variations in hydrodynamic conditions.Conversely,increased hydrodynamic activities accentuate the influence of differences in tuff rock properties on dissolution.Under the erosive action of acidic water,the uneven erosion arises from differences in the properties of both the acid and the rock-such as compound content,mineral composition,and crystal morphology-resulting in the formation of irregular erosion channels.