Mesoscale texture of cement hydrates

Abstract

Strength and other mechanical properties of cement and concrete rely upon the formation of calcium-silicate-hydrates (C-S-H) during cement hydration. Controlling structure and properties of the C-S-H phase is a challenge, due to the complexity of this hydration product and of the mechanisms that drive its precipitation from the ionic solution upon dissolution of cement grains in water.Departing from traditional models mostly focused on length scales above the micrometer, recent research addressed the molecular structure of C-S-H. However, small-angle neutron scattering, electron-microscopy imaging, and nanoindentation experiments suggest that its mesoscale organization, extending over hundreds of nanometers, may be more important. Here we unveil the C-S-H mesoscale texture, a crucial step to connect the fundamental scales to the macroscale of engineering properties. We use simulations that combine information of the nanoscale building units of C-S-H and their effective interactions, obtained from atomistic simulations and experiments, into a statistical physics framework for aggregating nanoparticles. We compute small-angle scattering intensities, pore size distributions, specific surface area, local densities, indentation modulus, and hardness of the material, providing quantitative understanding of different experimental investigations. Our results provide insight into how the heterogeneities developed during the early stages of hydration persist in the structure of C-S-H and impact the mechanical performance of the hardened cement paste. Unraveling such links in cement hydrates can be groundbreaking and controlling them can be the key to smarter mix designs of cementitious materials.U pon dissolution of cement powder in water, calcium-silicate-hydrates (C-S-H) precipitate and assemble into a cohesive gel that fills the pore space in the cement paste over hundreds of nanometers and binds the different components of concrete together (1). The mechanics and microstructure are key to concrete performance and durability, but the level of understanding needed to design new, more performant cements and have an impact on the CO 2 footprint of the material is far from being reached (2).Most of the experimental characterization and models used to predict and design cement performance have been developed at a macroscopic level and hardly include any material heterogeneity over length scales smaller than micrometers (3). However, EM imaging, nanoindentation tests, X-rays and neutron scattering, and NMR analysis as well as atomistic simulations have now elucidated several structural and mechanical features concentrated within a few nanometers (4-8). The hygrothermal behavior of cement suggests a hierarchical and complex pore structure that develops during hydration and continues to evolve (1, 9-11). NMR and small-angle neutron scattering (SANS) studies of hardened C-S-H identified distinctive features of the complex pore network and detected significant structural heterogeneities spanning length scales between tens and hu...

The Potential of Mean Force concept for bridging (length and time) scales in the modeling of complex porous materials

“…The experimental tools applied to measure such quantities in cement were electron-microscopy imaging, nanoindentation tests, small angle X-rays and neutron scattering (SAXS and SANS), nuclear magnetic resonance (NMR) spectroscopy and adsorption desorption experiment of N 2 and water. A remarkable agreement with experiment was achieved for both cement and clays [2,7].…”

“…All these quantities that were "measured" in the simulations and compared successfully with experiments provided the first consistent characterization of the complex and elusive meso-scale structure of cement [2]. The experimental tools applied to measure such quantities in cement were electron-microscopy imaging, nanoindentation tests, small angle X-rays and neutron scattering (SAXS and SANS), nuclear magnetic resonance (NMR) spectroscopy and adsorption desorption experiment of N 2 and water.…”

“…[1]. b) Up-scaling to the first mesoscale model of cement hydrates [2]. The color code of the meso-scale model indicates locally dense and loose regions of matter (the density increases from gray, blue, orange to red).…”

“…These are Molecular Dynamics (MD) and Monte Carlo (MC) in different statistical ensembles. New scheme combining MD and Grand Canonical Monte Carlo (GCMC) have been applied with success to simulate the early and late stages of cement hydration, imitating the precipitation of cement particles [2][3][4][5] and the sedimentation of laponite clay mineral [6]. The bottom-up approach therefore provides a rigorous link between the physical chemistry of materials and their overall functional and mechanical properties including elasticity, strength, fracture and permeability, etc… It opens the route to the engineering scale by providing the relevant scale for analytical coarse-graining and homogenization of the properties.…”

Self-Consistent Channel Approach for Upscaling Chloride Diffusivity in Cement Pastes

“…For the time being, we consider prescribing in our model an isotropic pore-scale diffusivity as reasonable model assumption because then the pore-scale diffusion coefficient can be simply back-calculated from the experimental data, not requiring the use of a more expensive optimization algorithm. We may also mention that our approach is fully consistent with the current state of the art in the mathematical modeling of concrete: In fact, our microheterogeneous formulation rests on the famous hydration model of Powers and Brownyard (1948) and Acker (2001), which has not only provided the basis for numerous, experimentally validated, micromechanical descriptions Sanahuja et al 2007;Pichler et al 2009b;Scheiner and Hellmich 2009), but has also been kind of corroborated by very recent statistical physics approaches (Ioannidou et al 2016). In the aforementioned micromechanics approaches, an RVE of cement paste is either composed of water pores, air pores, hydrates, and unhydrated cement (clinker) grains Pichler et al 2009b;Scheiner and Hellmich 2009), or of clinker grains embedded into a hydrate foam matrix, whereby the latter is, at a smaller scale, resolved into hydrates, water pores, and air pores; i.e., a hierarchical system of two RVEs is used to represent cement paste (Pichler and Hellmich 2011; Pichler et al (2009b) and Scheiner and Hellmich (2009), involving only one RVE representing the composite material cement paste.…”