Atomistic modeling of crystal structure of Ca 1.67 SiH x

Abstract

15The atomic structure of calcium-silicate-hydrate (C1,67-S-Hx) has been investigated by 16 theoretical methods in order to establish a better insight into its structure. Three models for C-17 S-H all derived from tobermorite are proposed and a large number of structures were created 18 within each model by making a random distribution of silica oligomers of different size 19 within each structure. These structures were subjected to structural relaxation by geometry 20 optimization and molecular dynamics steps. That resulted in a set of energies within each 21 model. Despite an energy distribution between individual structures within each model, 22 significant energy differences are observed between the three models. The C-S-H model 1 related to the lowest energy is considered as the most probable. It turns out to be characterized 2 by the distribution of dimeric and pentameric silicate and the absence of monomers. This 3 model has mass density which is the closest to the experimental one. 4 5

Revised Atomistic Models of the Crystal Structure of C–S–H with high C/S Ratio

“…The atomic composition of an unit cell of each structure is identical, thus the total energies can be compared directly, without any additional approximations and assumptions. The previous results for obtained for the structures in [15] we will refer in the paper as 1 † , 2 † and 3 † .…”

“…In contrary to previously suggested in [15] structures, models 1, 2 and 3 have the identical composition, and thus the energies can be compared directly and without any additional assumptions. In all structures, the kinetic energy has a minor contribution in the enthalpy (≈ 0.6%) at 300 K. The low contribution of the kinetic energy is a consequence of tightly bound molecules and ions.…”

“…There are many ways to select crystallographic rules for building structures that can reproduce known amount of calcium atoms and ratio of silicon atoms in Q1 and Q2 positions [5,13,14]. Based on sparse experimental knowledge about C−S−H structure, we proposed [15] three distinctive crystallographic models, that differ in the statistical distribution of oligomeric silica units and the location of calcium ions. Within each crystallographic model, we created a large number of structures in order to determine relative stability of models in terms of energy and their density that can be related to experimental results.…”

“…Revised Atomistic Models of the Crystal Structure of C-S-H | 3 TS a in details in [15]. An inbuilt property for all these models is a random character of created defects.…”

“…Since the composition of C−S−H (Ca x Si y H z O x+2y+z/2 ) contains two independent parameters, the comparison between different geometries require additional assumptions and can not be performed directly. Therefore, in the current study we have modified the algorithms used in [15], and applied additional condition, so all structures have an identical chemical composition per unit cell (both inside one model and across different models). The atomic composition of an unit cell of each structure is identical, thus the total energies can be compared directly, without any additional approximations and assumptions.…”