Journal of Nanomaterials

Volume 2017, Article ID 8476258, 6 pages

https://doi.org/10.1155/2017/8476258

## Temperature Effects on Tensile and Compressive Mechanical Behaviors of C-S-H Structure via Atomic Simulation

^{1}Shanxi Key Lab. of Material Strength & Structural Impact, Taiyuan University of Technology, Taiyuan 030024, China^{2}College of Mechanics, Taiyuan University of Technology, Taiyuan 030024, China^{3}LGCGM, Institut National des Sciences Appliquées de Rennes, 35708 Rennes, France^{4}Institute of Applied Mathematics, Taiyuan University of Technology, Taiyuan 030024, China

Correspondence should be addressed to Hao Xin; nc.ude.tuyt@oahnix and Zhihua Wang; nc.ude.tuyt@hzgnaw

Received 27 July 2017; Accepted 5 November 2017; Published 29 November 2017

Academic Editor: Ajayan Vinu

Copyright © 2017 Hao Xin et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

An atomic scale model of amorphous calcium silicate hydrate (C-S-H) with Ca/Si ratio of 1.67 is constructed. Effects of temperature on mechanical properties of C-S-H structure under tensile and compressive loading in the layered direction are investigated via molecular dynamics simulations. Results from present simulations show that (1) the tensile strength and Young’s modulus of C-S-H structure significantly decrease with the increase of the temperature; (2) the water layer plays an important role in the mechanical properties of C-S-H structure; (3) the compressive strength is stronger than tensile strength, which corresponds with the characteristic of cement paste.

#### 1. Introduction

The hydration products of Portland cement are mainly calcium silicate hydrate (C-S-H) and calcium hydroxide (CH), of which C-S-H accounts for about 60%–70% and CH about 20%. C-S-H is the most important binding phase of cement paste, which has a great influence on the mechanical properties of the cement paste. The investigation of the C-S-H structure model and its mechanical properties for the C-S-H gel may help to improve the performance of concrete and to understand the constitutive relationship of the concrete, thus to design the high strength concrete with good durability. There have been many investigations of C-S-H since recent decades, though an accurate enough model is still under seeking so far to describe the atomic structure of C-S-H.

The atomic structure of C-S-H is commonly modeled from the tobermorite or jennite structure. Ye et al. [1] have built a long-range disordered and short-range ordered amorphous C-S-H structure by the annealing process based on Hamid’s tobermorite model [2]. However, two basic characteristics, the calcium-to-silicon ratio (Ca/Si) and the density, of the amorphous C-S-H structure are incompatible with the real C-S-H. Murray et al. [3] have compared the mechanical properties of Hamid’s 11 Å tobermorite [2] and the possible C-S-H structure [4] by removal of the bridging silica tetrahedral of silicon chain in 11 Å tobermorite, finding that the discontinuous silicon chain in C-S-H can lead to a decrease in the tensile strength. In fact, the nuclear magnetic resonance (NMR) experiment has confirmed that the silicon-oxygen tetrahedron in C-S-H has a certain distribution of : ≈ 10%, , and [5]. Besides, research by small-angle neutron scattering measurements also gave out the chemical component as (CaO)_{1.7} (SiO_{2}) (H_{2}O)_{1.8} and determined the Ca/Si ratio and the density of C-S-H particles, which are separately 1.7 and 2.6 g/cm^{3} [6].

Pellenq et al. [7] have proposed a cCSH model based on experimental data [5, 6], with the Ca/Si ratio of 1.7 and the density of 2.45 g/cm^{3}. In accordance with the experimental data, Pellenq et al. [7] have also proposed a bottom-up atomic modeling method of the cCSH molecular structure, which can be described mainly as follows: the supercell of the anhydrous 11 Å tobermorite [2] for the initial configuration is constructed without considering the presence of any OH groups; some of the silicon-oxygen tetrahedrons are deleted to obtain a defected C-S-H structure, where , , , and Ca/Si = 1.65, using the core-shell potential for relaxation at 0 K; the water adsorption of the C-S-H model at 300 K with the Grand Canonical Monte Carlo (GCMC) is simulated, with a density of 2.56 g/cm^{3}, with further relaxation at 0 K. The final model with the chemical composition of (CaO)_{1.65}(SiO_{2})(H_{2}O)_{1.75} is obtained with a density of 2.45 g/cm^{3}, which has a good agreement with the neutron scattering experiments of (CaO)_{1.7}(SiO_{2})(H_{2}O)_{1.8}.

According to the modeling method of Pellenq et al., Hou et al. [8, 9] have investigated the mechanical properties of C-S-H during the axial stretching process in three different directions using ClayFF [10] force field, as well as the influence of the contained water percentage on the mechanical properties of the C-S-H using the CSH-FF [11] force field. For Young’s modulus of C-S-H, Constantinides and Ulm [12] have obtained Young’s modulus of LD C-S-H and HD C-S-H by nanoindentation experiment, which are 21.7 GPa and 29.4 GPa, separately. Hou et al. [9] have also investigated the effects of water/calcium ratio (W/C = 0.0–1.0) on the mechanical properties of the layered C-S-H based on the CSH-FF force field. Their work shows that Young’s modulus decreases from 67 GPa to 47 GPa and the maximum tensile strength decreases from 7.5 GPa to 3.8 GPa with the increase in water.

However, in practical engineering, concrete is often exposed in a variety of environments, where certain parameters (temperature, pressure, and humidity) may have a greater influence on the mechanical properties of the C-S-H. For instance, the extreme temperature in Russia and Canada could be lower than 200 K (where local people still live) and some concrete structures in certain factories (such as nuclear power plants) may suffer as high temperature as 400 K or even higher. However, the influence of the temperature on Young’s modulus of C-S-H structure on the nanometer scale has seldom been considered and investigated yet. In order to understand the macroscopic mechanical properties of concrete, it is necessary to understand the structure and behavior of C-S-H gel at the atomic level. In present work, we mainly focus on the influence of the temperature on mechanical properties of C-S-H structure under tensile/compressive loading. The stress-strain curve, Young’s modulus, density, tensile strength, and tensile deformation process of C-S-H under different temperatures were analyzed.

#### 2. Numerical Simulation of C-S-H

##### 2.1. Model and Force Field

For the real amorphous C-S-H, the main features of the structure need to be considered, such as long-range disorder, short-range order, calcium silicon ratio (Ca/Si), the layered structure, distribution, and density. The construction of the amorphous C-S-H model is referred to as Pellenq’s cCSH model based on the experimental data [5, 6]. The initial configuration is monoclinic as shown in Figures 1(a) and 1(b) showing how to make an orthogonal box conversion of the atomic structure before the simulation. The relative atomic position and model size remain unchanged after the box transformation.