氯离子与硫酸根离子在水化硅酸钙表面竞争吸附的分子动力学研究

Molecular Kinetics of Competitive Adsorption of Chloride and Sulphate Ions on C-S-H Surface

水化硅酸钙凝胶(C-S-H)是水泥基材料中主要起粘结作用的胶体,也是发生氯离子和硫酸根离子物理吸附的主要原因。由于实验方法无法直观表现C-S-H物理吸附的过程以及吸附量随时间的变化规律,本研究采用分子动力学模型模拟C-S-H物理吸附氯离子及硫酸根离子的过程,探究吸附过程中氯离子和硫酸根离子之间的交互作用关系。模拟利用配位数并结合等温吸附法计算了3.5%NaCl溶液及3.5%NaCl+3.5%Na2SO4复合溶液中Cl^(-)和SO_(4)^(2-)的吸附量。研究发现:物理吸附作用包括离子间的近程库伦(Coulomb)作用以及长程范德华(VDW)作用,SO_(4)^(2-)抑制了C-S-H对Cl^(-)的吸附,减弱了C-S-H对Cl^(-)的长程VDW作用,但并没有减少位点对Cl^(-)的吸附量,只是延缓了吸附发生的时间;SiOCa+是Cl^(-)及SO_(4)^(2-)的主要吸附位点,SiOH可以吸附少量Cl^(-),但对SO_(4)^(2-)无明显吸附作用,且SiOCa+与SiOH位点存在竞争关系。两种溶液中C-S-H凝胶对Cl^(-)的吸附量相差不大,约为0.06 mmol/g,对SO_(4)^(2-)的吸附量约为0.02 mmol/g。由分子动力学计算得出的C-S-H的离子吸附量,与通过测试实验合成的C-S-H及水泥浆体中的C-S-H的离子吸附量相符,由此可以证明分子动力学模拟可准确、可靠地表征侵蚀离子在水泥基材料内的复杂交互作用。

Hydrated calcium silicate(C-S-H)gel is the main bonding colloid in cement-based materials,and it is also the main source of physical adsorption of chloride and sulphate ions.Due to that the process of C-S-H physical adsorption and the change of the adsorption amount with time cannot be reflected directly by experimental methods.In this paper,a molecular dynamics model was used to simulate the process of C-S-H phy-sical adsorption of chloride and sulphate ions.The interaction between chloride and sulfate ions during the adsorption process was investigated.The coordination number and the isothermal adsorption method were used to calculate the adsorption amount of Cl^(-)and SO_(4)^(2-)in 3.5%NaCl and 3.5%NaCl+3.5%Na2SO4 solution.The research results show that the adsorption capacity of C-S-H gel for Cl^(-)in the two solutions is almost the same,about 0.1 mmol/g,and the adsorption capacity for SO_(4)^(2-)is about 0.04 mmol/g in compound salts solution.The physical adsorption includes short-range Coulomb interaction between ions and long-range van der Waals(VDW)interaction.The adsorption of C-S-H to Cl^(-)is delayed and the long-range VDW effect of C-S-H on Cl^(-)is weakened by SO_(4)^(2-).However,the presence of SO_(4)^(2-)doesn’t reduce the amount of Cl^(-)adsorption at the site,it only delays the time of adsorption.SiOCa+is the main adsorption site for Cl^(-)and SO_(4)^(2-).The SiOH sites which have no obvious adsorption effect on SO_(4)^(2-)have ability to adsorb Cl^(-),but the amount is negligible.Moreover,there is also a competitive relationship between SiOCa+and SiOH sites.The ions adsorption of C-S-H calculated by molecular dynamic is consistent with the ion adsorption of the C-S-H synthesized by experimental methods or the C-S-H in cement slurry.It can be proved that molecular dynamics simulations can be used to accurately and reliably characterize the complex interactions of aggressive ions in cement-based materials.