On the formation of cementitious C–S–H nanoparticles

Multi-step nucleation pathway of C-S-H during cement hydration from atomistic simulations

The Calcium Silicate Hydrate (C-S-H) nucleation is a crucial step during cement hydration and determines to a great extent the rheology, microstructure, and properties of the cement paste. Recent evidence indicates that the C-S-H nucleation involves at least two steps, yet the underlying atomic scale mechanism, the nature of the primary particles and their stability, or how they merge/aggregate to form larger structures is unknown. In this work, we use atomistic simulation methods, specifically DFT, evolutionary algorithms (EA), and Molecular Dynamics (MD), to investigate the structure and formation of C-S-H primary particles (PPs) from the ions in solution, and then discuss a possible formation pathway for the C-S-H nucleation. Our simulations indicate that even for small sizes the most stable clusters encode C-S-H structural motifs, and we identified a C4S4H2 cluster candidate to be the C-S-H basic building block. We suggest a formation path in which small clusters formed by silicate dimers merge into large elongated aggregates. Upon dehydration, the C-S-H basic building blocks can be formed within the aggregates, and eventually crystallize.

3D Multi-Ion Corrosion Model in Hierarchically Structured Cementitious Materials Obtained from Nano-XCT Data

Corrosion of steel reinforcements in concrete constructions is a worldwide problem. To assess the degradation of rebars in reinforced concrete, an accurate description of electric current, potential and concentrations of various species present in the concrete matrix is necessary. Although the concrete matrix is a heterogeneous porous material with intricate microstructure, mass transport has been treated in a homogeneous material so far, modifying bulk transport coefficients by additional factors (porosity, constrictivity, tortuosity), which led to so-called effective coefficients (e.g., diffusivity). This study presents an approach where the real 3D microstructure of concrete is obtained from high-resolution X-ray computed tomography (XCT), processed to generate a mesh for finite element method (FEM) computations, and finally combined with a multi-species system of transport and electric potential equations. This methodology allows for a more realistic description of ion movements and reactions in the bulk concrete and on the rebar surface and, consequently, a better evaluation of anodic and cathodic currents, ultimately responsible for the loss of reinforcement mass and its location. The results of this study are compared with a state-of-the-art model and numerical calculations for 2D and 3D geometries.

Thermodynamics of water confined in porous calcium-silicate-hydrates

Water within pores of cementitious materials plays a crucial role in the damage processes of cement pastes, particularly in the binding material comprising calcium-silicate-hydrates (C-S-H). Here, we employed Grand Canonical Monte Carlo simulations to investigate the properties of water confined at ambient temperature within and between C-S-H nanoparticles or "grains" as a function of the relative humidity (%RH). We address the effect of water on the cohesion of cement pastes by computing fluid internal pressures within and between grains as a function of %RH and intergranular separation distance, from 1 to 10 Å. We found that, within a C-S-H grain and between C-S-H grains, pores are completely filled with water for %RH larger than 20%. While the cohesion of the cement paste is mainly driven by the calcium ions in the C-S-H, water facilitates a disjoining behavior inside a C-S-H grain. Between C-S-H grains, confined water diminishes or enhances the cohesion of the material depending on the intergranular distance. At very low %RH, the loss of water increases the cohesion within a C-S-H grain and reduces the cohesion between C-S-H grains. These findings provide insights into the behavior of C-S-H in dry or high-temperature environments, with a loss of cohesion between C-S-H grains due to the loss of water content. Such quantification provides the necessary baseline to understand cement paste damaging upon extreme thermal, mechanical, and salt-rich environments.

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 axb 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 Si-O bonds are stiffer than iono-covalent Ca-O 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 Si-O-Si bridging angles. Polyhedra with the seven-fold coordinated Ca(2+) cations undergo large structural changes. Especially, axial Ca-O bonds perpendicular to the axb plane are significantly shortened. Besides, it was shown that structural parameters, more or less in parallel orientation to the axb 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 Ca-O distances.

Nanostructure of calcium silicate hydrates in cements

Calcium silicate hydrate (CSH) is the major volume phase in the matrix of Portland cement concrete. Total x-ray scattering measurements with synchrotron x rays on synthetic CSH(I) shows nanocrystalline ordering with a particle diameter of 3.5(5) nm, similar to a size-broadened 1.1 nm tobermorite crystal structure. The CSH component in hydrated tricalcium silicate is found to be similar to CSH(I). Only a slight bend and additional disorder within the CaO sheets is required to explain its nanocrystalline structure.