Molecular Dynamics to Understand the Mechanical Behavior of Cement Paste

Abstract

The tensile strength of cement paste is one of the most important mechanical properties that influence shrinkage cracks in cementitious materials. Cement pastes that exhibit low tensile strength tend to exhibit greater shrinkage crack potential and reduced durability. Increasing the tensile strength in cement paste can minimize the shrinkage cracking potential. It is believed that the strength and cohesion of cement paste are controlled by the formation of calcium silicate hydrate (C-S-H) gel. To enhance macroscopic mechanical properties (tensile strength), it is necessary to understand the structure and behavior of C-S-H gel at the atomic level. Previously, molecular statics was used to determine minimal potential energy and the mechanical properties of crystalline C-S-H structures. From this study, a plausible atomic structure of C-S-H gel is proposed. This research effort builds on the aforementioned work by using molecular dynamics to derive tensile and compressive strengths of C-S-H structures from uniaxial stress–strain data. The results from the molecular dynamics simulations showed that the maximum strengths (i.e., compressive and tensile) for the proposed C-S-H structures are three orders of magnitude higher than the strength at the macrolevel. However, the tensile strength of the proposed C-S-H gel is 23% of the compressive strength. This research also concludes that electrostatic forces and bond forces in the silicate chains are the main contributors to cement strength at the atomic level and that breakage in silicate chains leads to low tensile strength in C-S-H gel.

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

“…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 11Å tobermorite [2] and the possible C-S-H structure [4] by removal of the bridging silica tetrahedral of silicon chain in 11Å 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 siliconoxygen tetrahedron in C-S-H has a certain distribution of : 0 ≈ 10%, 1 ≈ 67%, and 2 ≈ 23% [5].…”

Strength and Some Durability Properties of Concrete Containing Rice Husk Ash Produced in a Charcoal Incinerator at Low Specific Surface

“…Though the high compressive strength of concrete is known to be due to the strength and cohesion of CSH gels, it is also known to be weak in tension. The work of Murray et al (Murray et al 2010) suggests that electrostatic and bond forces at the atomic level of silicate chains as the main reason for the compressive strength of concrete and that its weak tensile strength is due to breaks in silicate chains at the atomic level. The growth of more CSH gels resulting from the pozzolanic reactions of the RHA would have contributed to the tensile strength increase recorded for cylinders containing RHA.…”

Nano–micro modelling of mechanical properties of cement paste based on molecular dynamics

“…As shown in Figure 3, silicate chains went through deformation and distortion, quite different from the continuous silicate chains, and several tetrahedrons became separated. Because the bond forces in the silicate chains are the main contributors to cement strength at the atomic level (Murray et al, 2010), breaks in the silicate chains lead to a reduction in elastic properties. The structure obtained after annealing was considered to have the characteristics of an amorphous structure, and the mechanical properties from the simulation can be regarded as parameters with zero porosity.…”

“…However, the ultimate strengths (tension) of C-S-H gels were reported to be of magnitude 10 3 -10 4 MPa (Murray et al, 2010) while the tensile strength of cement paste determined from the empirical model (Ghebrab and Soroushian, 2010) was considered to be 1-10 MPa. The tensile strength of cement paste calculated using Griffith theory was within this range, which indicates that the gap in tensile strength by orders of magnitude was bridged through the multiscale coupling methods used in this paper.…”

“…By contrast, at the nanoscale, the strength of the contents of components represented by C-S-H can be as high as 10 4 MPa (Murray et al, 2010). An explanation for the difference in mechanical strength at different scales was the starting point of this study.…”