Calcium silicate hydrate (CSH) is the main binding phase of Portland cement, the single most important structural material in use worldwide. Due to the complex structure and chemistry of CSH at various length scales, the focus has progressively turned towards its atomic level comprehension. We study electronic structure and bonding of a large subset of the known CSH minerals. Our results reveal a wide range of contributions from each type of bonding, especially hydrogen bonding, which should enable critical analysis of spectroscopic measurements and construction of realistic C-S-H models. We find the total bond order density (TBOD) as the ideal overall metric for assessing crystal cohesion of these complex materials and should replace conventional measures such as Ca:Si ratio. A rarely known orthorhombic phase Suolunite is found to have higher cohesion (TBOD) in comparison to Jennite and Tobermorite, which are considered the backbone of hydrated Portland cement.

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Calcium silicate hydrate (C-S-H) is the most important phase of hydrated cement gel which is the key material in construction industry. It is well accepted that hardened cement paste consists of either poorly crystalline or completely disordered phases. Although a myriad of speculative atomistic models of disordered C-S-H have been proposed, the fundamental basis of structure-property relationships remain elusive. This study focuses upon the correlations between mechanical properties and electronic structure based on well-defined quantum mechanical parameters. We use 20 CSH minerals with known structure to gain fundamental understanding of structure-property relationship. The results indicate Si-O bond order density, which represents the cumulative bond strength of SiO bonds, has no direct correlation with bulk mechanical properties which is counterintuitive and against conventional wisdom. The variations are determined more precisely by the overall atomic and electronic structure dictated by bond order density of the Ca-O and hydrogen bonds (HB). Most importantly, there is a multifaceted balance between different types of interatomic bonds including the HBs in controlling mechanical properties. HBs categorized in relation to next nearest neighbor (NNN) enable us to identify specific types of HBs that are prevalent in CSH. In certain crystals such as suolunite, the HB network is organized in such a unique way that enhances its mechanical properties. The approach and findings presented in this paper points to a broad roadmap for the developing next-generation cements.

Amorphous germania (a-GeO 2 ) is an excellent glass former of great industrial and scientific importance. However, in comparison with a-SiO 2 , its structure and fundamental properties were less well studied. Using a large near-perfect continuous random network (CRN) model with 1296 atoms and no over-or under-coordinated atoms, we have investigated the structural, electronic, and optical properties of a-GeO 2 glass. Our results show that the bond length and bond angle distributions in a-GeO 2 are larger than in a-SiO 2 . The gross features of the electronic density of states in a-GeO 2 are similar to a-SiO 2 , but the Ge-O bonds are weaker than Si-O bonds as reflected in the lower calculated total bond order density. The average tetrahedral angle (θ) and bridging angle (φ) is smaller in a-GeO 2 than in a-SiO 2 . The calculated optical absorption spectrum shows two distinctive peaks in excellent agreement with experiment. The calculated refractive index of a-GeO 2 (n=1.69) is also in close agreement with the measured value. In contrast to a-SiO 2 , there is no clear evidence of excitonic peak in a-GeO 2 . These a-GeO 2 and a-SiO 2 models could be used as a prototype for other investigations of these glasses or their mixtures containing defects, substitutional impurities and in the form of vitreous nano-particles.