| In this paper,taking the limestone tunnel in Chongqing area as the research object,the present situation in a large number of tunnel drainage pipeline is investigated and then the crystals samples and water samples are collected and the indoor test is carry out.Secondly,according to the analysis results and related theories,the crystallization conditions,the process and the chemical reaction in the drainage pipeline are analyzed.Finally,the solid-liquid interface model and the solid-solid interface model are established to carry out the molecular dynamics simulation to analyze the microscopic mechanism of the pipe wall crystallization of the tunnel drain pipe.The main research results are as follows:(1)Through the analysis of the composition of the crystal block,we can see that the main chemical composition is CaCO3,the mineral form is calcite.Water analysis shows that the main ions are Ca2+,Mg2+,Na+,K+,Cl-,SO42-,CO32-,HCO3-and OH-.The water have a high total hardness,drainage channels may occur crystal phenomenon.The main form of the C element in the aqueous solution is CO32-when the pH is greater than 11,and when the pH is less than 8,the main form of the C element in the aqueous solution is HCO3-.(2)From the conditions,process and chemical reaction of crystallization,it is found that the main chemical composition of the tunnel drainage channel in the limestone area is CaCO3.The crystallization rate at the tube wall is mainly affected by the temperature,Ca2+concentration and CO32-concentration or HCO3-concentration and impurity ions(Mg2+and SO42-).The deposition rate of the crystals in the aqueous solution is affected by the flow rate of the water,and the faster the flow rate of the water is,the higher the rate of calcite deposition is,in addition to the influence of temperature and scale ion concentration.(3)With the increase of temperature,the self-diffusion coefficient of water molecules increases.The activity of water molecules is enhanced,and the contact probability of calcium ions and carbonate ions is reduced.Therefore,to a certain extent,an increase in temperature can suppress the growth of crystals.When the temperature is in the range of 283K～308K,the self-diffusion coefficient of calcium ion and carbonate decreases first and then decreases with the increase of temperature,and then decreases.The self-difusion coefficient of Ca2+and carbonate ions reaches the maximum at T=303K.At this time,the activity of the two ions is large and the reaction is easy to produce ions.Under certain conditions,the crystal is formed.The chemical bond between Ca2+and O atoms in the water solution may affect the reaction of Ca2+and CO32-Combined to make the crystal slow.When the temperature is 308K,the maximum peak value of the radial distribution function is the largest.At this time,Ca2+has the highest probability of forming ionic bond in O,and Ca2+has the highest degree of hydration.The interaction between PVC and scale-forming ionic solution or calcite is weak.(4)The interface model of calcite crystal face and PVC is established.The molecular dynamics simulation showed that the binding energy between pure calcite crystal and PVC tube was very small.The binding energy between the calcite(104)face and the PVC is larger than that of the calcite(1-10)face and the PVC,that is to say,calcite(104)face is easier to combine with PVC. |
