基于分子动力学水分和离子在水化硅酸钙纳米孔道中的传输特性研究

水化硅酸钙凝胶是水泥水化产物中最基本的粘结相,水分子和离子在凝胶孔中的传输从根本上决定着水泥混凝土材料的服役寿命.采用分子动力学方法系统地研究了水分子、氯离子和钠离子在1nm、2nm、3nm和4nm的水化硅酸钙凝胶孔中的传输过程.基于径向分布函数和均方位移的离子轨迹分析发现在纳米孔道中离子和水分子展现出异于毛细水的分子结构和动力学特性:水分子有序性排布、离子大量在界面吸附和扩散速度急剧下降.这种分子结构与动力学的特性是因为水化硅酸钙界面处硅链中的非桥接氧会与水分子形成稳定的氢键连接,而钠离子可以形成Na-O化学键,同时表面的钙离子也可以与氯离子形成CaCl2团簇体.此外,随着孔径的增大,离子和水分子的扩散系数逐渐由0.15×10-9m2/s、0.7×10-9m2/s增大到1.3×10-9m2/s、3×10-9m2/s,这很接近于实验测得的毛细水的扩散系数,说明在纳米尺度上,孔径的约束和界面化学键作用是决定离子和水分子传输的关键因素.

Transport Characteristics of Water and Ions in Nano Channels of Calcium Silicate Hydrate Based on Molecular Dynamics

Calcium silicate hydrate (C-S-H) gel is the most basic binder phase in cement hydration products.The transport of water and ions in gel pores fundamentally determines the service life of the cement concrete materials.Molecular dynamics method is used to study the transport process of water molecules, chloride ions and sodium ions in hydrated calcium silicate gel pores of 1 nm, 2 nm, 3 nm and 4 nm.Based on the analysis of radial distribution function and mean square displacement, it is found that the molecular structure and dynamics characteristics of the ions and water are different from those of the capillary water.The water molecules order arrangement, ions adsorption at the interface and diffusion rate decreases sharply.These characteristics of the structure and dynamics of molecules are because that non bridging oxygen at the silicate chains located at the surface of C-S-H can form stable hydrogen bonds with water molecules, and sodium ions can form Na-O chemical bonds.Meanwhile, calcium ions in the surface can also form CaCl2 clusters with chloride ions.In addition, with the diameter increasing, the diffusion coefficient of ions and water molecules gradually increases from 0.15×10-9 m2/s, 0.7×10-9 m2/s to 1.3×10-9 m2/s, 3×10-9 m2/s, which is very close to the value of capillary water measured by experiment.On the nano scale, pore size constraint and interfacial chemical bonding are the key factors in determining ions and water molecules transport.