²⁹Si NMR in Cement: A Theoretical Study on Calcium Silicate Hydrates

The NMR spectra of 29Si in cement-based materials are studied through calculations of the isotropic shielding of silicon atoms within the density functional theory. We focus on the main component of cement, the calcium-silicate-hydrate gel, using widely accepted models based on the observed structures of jennite and tobermorite minerals. The results show that the 29Si chemical shifts are dependent not only on the degree of condensation of the (SiO4) units, as commonly assumed, but also on the local arrangement of the charge compensating H and Ca cations. We find that the NMR spectra for models of the calcium-silicate-hydrate gel based on tobermorite are in better agreement with experiment than those for jennite-based models.本研究通过密度泛函理论 (density functional theory) 下的硅原子各向同性屏蔽计算，对水泥基材料中硅-29的核磁共振谱展开研究。我们聚焦于水泥的主要组分——钙硅水凝胶 (calcium-silicate-hydrate gel)，采用基于已观测到的羟钙硅石 (jennite) 与托勃莫来石 (tobermorite) 矿物结构的公认模型开展相关工作。研究结果表明，硅-29的化学位移不仅如学界普遍假设的那样，依赖于硅氧四面体 (SiO₄) 单元的聚合程度，还与电荷补偿的氢、钙阳离子的局域排布密切相关。我们发现，基于托勃莫来石构建的钙硅水凝胶模型的核磁共振谱，相比基于羟钙硅石的模型，与实验结果的吻合度更高。