Understanding of bonding and mechanical characteristics of cementitious mineral tobermorite from first principles

This paper reports density functional theory study of the structural and mechanical properties of tobermorite mineral (9 Å phase) as one of the main component of cementitious materials in a concrete chemistry. Calculated bulk modulus and elastic constants reflect a relatively high resistance of the tobermorite structure with respect to external isostatic compression. Moreover, the elastic constants proved the anisotropic character of the tobermorite structure. The directions parallel to the a x b plane are more resistant to the compression than the perpendicular direction. The largest contribution to this resistance comes from the “dreierketten” silicate chains. The bonding analysis linked macroscopic mechanical properties and the atomic structure of the tobermorite. It was found that polar covalent SiO bonds are stiffer than iono‐covalent CaO bonds. The SiO 4 tetrahedra are resistant with respect to the compression and the effect of external pressure is reflected by the large mutual tilting of these tetrahedra as it is shown by changes of the SiOSi bridging angles. Polyhedra with the seven‐fold coordinated Ca 2+ cations undergo large structural changes. Especially, axial CaO bonds perpendicular to the a x b plane are significantly shortened. Besides, it was shown that structural parameters, more or less in parallel orientation to the a x b plane, are mainly responsible for the high resistance of the tobermorite structure to external pressure. The main mechanism of a dissipation of energy entered to the structure through the compression is proceeded by the tilting of the tetrahedra of the silicate chains and by large shortening of the axial CaO distances. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2011