Li Yue, Zhang Guosheng, Hou Dongshuai, Wang Zigeng
Key Laboratory of Urban Security and Disaster Engineering of Ministry of Education, Beijing Key Laboratory of Earthquake Engineering and Structural Retrofit, Beijing University of Technology Beijing 100124 China.
Department of Civil Engineering, Qingdao University of Technology Qingdao China 266033
RSC Adv. 2020 Nov 4;10(66):40180-40195. doi: 10.1039/d0ra07717h. eCollection 2020 Nov 2.
The sustainable green building material magnesium phosphate cement (MPC) is widely used in the fields of solidifying heavy metals and nuclear waste and repair and reinforcement. Magnesium potassium phosphate hexahydrate (MKP) is the main hydration product of MPC. The transport of water and ions in MKP nanochannels determines the mechanical properties and durability of MPC materials. Herein, the interface models of MKP crystals with sodium chloride solution in the [001], [010] and [100] direction were established by molecular dynamics. The interaction of the MKP interface with water and ions was studied and the durability of MPC in sodium chloride solution was explained at the molecular level. The results show that a large number of water molecules are adsorbed on the MKP crystal surface through hydrogen bonds and Coulomb interactions; the surface water molecules have the bigger dipole moment and the dipole vector of most of the water molecules points to the solid matrix, when the crystal surfaces of the three models all show hydrophilicity. In addition, plenty of sodium ions are adsorbed at the MKP interface, and some potassium ions are desorbed from the matrix. In the MKP[001] model, the amount of potassium ions separated from the matrix and diffused into the solution is the highest and the interface crystal is the most disordered. Due to the attack of water and ions, the K-Os bond loses its chemical stability and the order of the MKP crystal is destroyed, which explains the decline of MPC performance after the erosion of sodium chloride solution at the molecular level. Besides, in the three models, the Na-Cl ion bond is more unstable than the K-Cl ion bond due to the smaller radius of the sodium atom. The stability of ionic bonds in the models is as follows: MKP[010] > MKP[100] > MKP[001].
可持续绿色建筑材料磷酸镁水泥（MPC）广泛应用于重金属和核废料固化以及修复与加固领域。六水合磷酸钾镁（MKP）是MPC的主要水化产物。MKP纳米通道中的水和离子传输决定了MPC材料的力学性能和耐久性。在此，通过分子动力学建立了MKP晶体在[001]、[010]和[100]方向与氯化钠溶液的界面模型。研究了MKP界面与水和离子的相互作用，并从分子水平解释了MPC在氯化钠溶液中的耐久性。结果表明，大量水分子通过氢键和库仑相互作用吸附在MKP晶体表面；表面水分子具有较大的偶极矩，且大多数水分子的偶极矢量指向固体基质，此时三种模型的晶体表面均表现出亲水性。此外，大量钠离子吸附在MKP界面，一些钾离子从基质中解吸。在MKP[001]模型中，从基质分离并扩散到溶液中的钾离子数量最多，界面晶体最无序。由于水和离子的侵蚀，K-O键失去化学稳定性，MKP晶体的有序性被破坏，这从分子水平解释了氯化钠溶液侵蚀后MPC性能下降的原因。此外，在三种模型中，由于钠原子半径较小，Na-Cl离子键比K-Cl离子键更不稳定。模型中离子键的稳定性顺序为：MKP[010]＞MKP[100]＞MKP[001]。
