Effects Of Black Shale–water Interactions On Concrete

Cement concrete is a building material widely used in civil engineering construction today,and the quality of construction engineering is greatly affected by the performance of concrete.The acid-containing solution can cause dissolution,voids and cracks in the surface hardened cement of non-metallic building materials mainly composed of mortar,concrete and lime sand bricks,resulting in a decrease in the mechanical strength of the material,which in turn leads to the destruction of the building.Black shale is easy to react with aqueous solution in an oxidizing environment due to its sulfide minerals and organic matter,forming a highly aggressive acidic solution.There have been many cases of acid corrosion of engineering building materials caused by chemical weathering of black shale.In severe cases,it may even affect the safety of engineering buildings and the safety of life and property.Therefore,this project intends to study the chemical characteristics of black shale water rock and its acid erosion effect on cement concrete.In this paper,the black shale of the Qingxi Formation in the Sanjiang area of Guangxi were used as the research objects,and the chemical composition characteristics of black shale minerals in the study areas was selected.The Qingxi River in Sanjiang area was selected.The black shale in the study area was subjected to the interaction experiment of different rock mass reaction solution and concrete under the condition of semi-soaking,and the water quality change,mineral and concrete composition change,fracture distribution and mechanical strength of black shale and concrete interacted with water were obtained.The characteristics of variation were analyzed by thermodynamics theory.The thermodynamic characteristics of black shale water and rock were analyzed for the oxidation and acid etching of concrete.The Rhreeqc software was used to simulate the mineral saturation index of concrete etched by different immersion height black shale water and rock.Through research,the following main results and understandings have been achieved:(1)The engineering geological characteristics of the Sanjiang black shale research area in Guangxi was investigated respectively.The sample minerals and weathering products in the study area were determined by X-ray diffractometry(XRD).The results show that some minerals contained in black shale are easily dissolved and decomposed in acidic water environment(mainly including illite,muscovite,kaolinite,calcite and quartz),resulting in the release of internal structure of black shale.And black shale will also form different weathering productsat different stages of oxidative acid etching(mainly including white iron sputum,chlorophyll,goethite,jarosite,and high-speed iron chlorophyll),and the resulting product will produce volume.The expansion causes the black shale to expand and break along the structural plane,which eventually leads to a sharp decline in the strength of the black shale.The main chemical composition of the rock sample in the Sanjiang black shale research area in Guangxi is: quartz(67.0%-74.5)%),calcite(12.0%-14.0%),dolomite(4.2%-5.1%)and pyrite(0.5%-2.6%),the chemical content of other substances is low;Detecting the environmental water characteristics of the study area,the environmental water value is lower than the pH value,which is strongly acidic and is sulfate-acidic water.(2)The experimental analysis of the interaction between black shale reaction solution and concrete with different rock mass volume under semi-immersion conditions shows that the chemical action of black shale water rock is mainly affected by the reaction area of water rock,water and air,and capillary Water plays an important role in this process;while waterrock interaction is most obvious at the interface of water,gas and solid three-phase.The acidic solution produced by the action of water and rock will cause serious corrosion on the concrete surface.At the same time,the formed sulfate is the main factor affecting the physical and mechanical properties of the concrete,which can cause the internal expansion and corrosion of the concrete,but in the short term,the acid corrosion of the concrete surface is formed.After the sulphate material is filled with internal pore cracks,the overall compressive strength of the concrete is slightly improved.(3)Selecting the main chemical reactions in the oxidation-etching process of water and rock as the research object,using the thermal theory such as enthalpy,entropy and Gibbs free energy to analyze the related reactants in the oxidation-etching process of water and rock interaction And the thermodynamics of the product.The results show that the oxidative acid corrosion of pyrite in black shale is greatly affected by temperature,and the product of oxidative acid etching is beneficial to promote the further oxidative decomposition of rock samples.The oxidation etch rate of black shale in cold regions is more Faster;the higher the temperature,the more severe the corrosion of the concrete on the concrete,and the lower the temperature,the corrosion of the concrete to the concrete.The secondary corrosion of the secondary production of dihydrate gypsum is more serious,and the two promote each other,ie corrosion The reaction endothermicity of dihydrate gypsum will bring a more favorable temperature environment to the secondary corrosion of concrete;the secondary production of black shale oxidativeacid etching,white iron sputum,goethite,jarosite and high iron leaf The formation of green crickets is susceptible to external conditions.(4)Using the Phreeqc software to simulate the mineral saturation index of concrete etched by different immersion height black shale water rocks.The results show that the pyrite saturation index in black shale is affected by pH.Larger,the stronger the acidity of the solution,the better the dissolution of pyrite;the goethite is more likely to occur in an environment with a lower pH value,indicating that pyrite is more susceptible to oxidative decomposition in an acidic environment,ie The acidic environment plays a progressive role in the oxidative decomposition of black shale;the hydration of concrete in an acidic environment is more intense,the physical effect of sulfate on black shale on corrosion(ie,expansion corrosion),in acidic conditions It is not obvious,that is,its expansion effect is low in the short term,and the salt-forming effect of the salt(water gypsum and ettringite)generated by the acid etching is the filling of the internal pores of the concrete,resulting in the short-term uniaxial compressive strength does not fall.In the long run,as the acid etching effect is reduced(the black shale consumption that produces the acid etching effect is reduced),the environment where the concrete is dissolved is increased by the pH value,and the concrete is in the acidic condition.The saturation index of the salt(shui gypsum and ettringite)which is formed by the sulfate corrosion is increasing continuously,and the salt having the swelling effect is continuously precipitated,and the concrete will expand and crack.