Southeast University, Nanjing, China.
Phys Chem Chem Phys. 2018 May 23;20(20):13920-13933. doi: 10.1039/c7cp08640g.
In this paper, first principles and reactive force field molecular dynamics were utilized to study the mechanical properties of tobermorite 9 Å, tobermorite 11 Å and jennite. These are essential minerals in cement chemistry. The mechanical properties calculated by the first-principle method match well with previous experimental results, pointing to the introduction of vdW dispersion-type forces as able to improve the precision. The calculated elastic constants of the three minerals confirm anisotropic mechanical behavior of the layered structures. The crystals jennite and tobermorite 9 demonstrate stronger mechanical behavior in the ab plane than the interlayer direction due to the presence of stable covalent "dreierketten" silicate chains. For tobermorite 11 Å, the Q3 silicate tetrahedrons bridging the neighboring calcium silicate sheets heal the weak interlayer structure and enhance the c-direction stiffness and cohesive strength. Furthermore, analysis of uniaxial tension by reactive force MD elucidated the chemical and mechanical responses of the atomic structures in loading resistance. The stress-strain relation of the layered mineral tensioned along b direction, showing the "strain hardening" region, where stress continues to increase past the yield stage. The strain hardening and ductility enhancement for the minerals is largely due to the ability of the silicate chains to first de-polymerize into short chains or separate tetrahedrons before the broken Q species re-polymerize to form branched networks and ring structures which are able to resist loading. For all three minerals, protons transfer to oxygen and water, resulting in the formation of Si-OH and Ca-OH groups during the strain hardening stage as Si-O-Si or Si-O-Ca bonds start to break and the calcium atoms and silicate morphology is rearranged. Hydrolysis therefore accelerates structural damage and contributes to weakening of mechanical properties in the interlayer direction. Increased tensile stress level in the tobermorite 11 Å can contribute to a greater extent of water damage.
本文利用第一性原理和反应力场分子动力学研究了托贝莫来石 9 Å、托贝莫来石 11 Å 和硅灰石的力学性能。这些是水泥化学中的重要矿物。第一性原理方法计算的力学性能与先前的实验结果吻合良好，表明引入范德华色散型力能够提高精度。三种矿物的计算弹性常数证实了层状结构的各向异性力学行为。由于存在稳定的共价“dreierketten”硅酸盐链，晶体硅灰石和托贝莫来石 9 在 ab 平面上表现出比层间方向更强的力学行为。对于托贝莫来石 11 Å，连接相邻硅酸钙片的 Q3 硅酸盐四面体修复了较弱的层间结构，增强了 c 方向的刚度和内聚强度。此外，通过反应力 MD 对单轴拉伸的分析阐明了原子结构在抵抗加载时的化学和力学响应。沿 b 方向拉伸的层状矿物的应力-应变关系，显示出“应变硬化”区域，其中应力在屈服阶段之后继续增加。矿物的应变硬化和延展性增强主要归因于硅酸盐链在 Q 物种断裂后重新聚合形成支化网络和环状结构之前，首先解聚成短链或分离四面体的能力，这些结构能够抵抗加载。对于所有三种矿物，质子转移到氧和水中，导致在应变硬化阶段形成 Si-OH 和 Ca-OH 基团，因为 Si-O-Si 或 Si-O-Ca 键开始断裂，钙原子和硅酸盐形态发生重排。因此，水解加速了结构损伤，并导致层间方向力学性能的减弱。托贝莫来石 11 Å 中拉伸应力水平的增加可以在更大程度上导致水的破坏。
