Mesoscale texture of cement hydrates
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Mesoscale texture of cement hydrates
Authors
Katerina Ioannidou
aDepartment of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139;
bMultiScale Material Science for Energy and Environment, Massachusetts Institute of Technology–CNRS Joint Laboratory at Massachusetts Institute of Technology, Cambridge, MA 02139;
Konrad J. Krakowiak
aDepartment of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139;
Mathieu Bauchy
cDepartment of Civil and Environmental Engineering, University of California, Los Angeles, CA 90095;
Christian G. Hoover
aDepartment of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139;
Enrico Masoero
dSchool of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne NE1 7RU, United Kingdom;
Sidney Yip
eDepartment of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139;
Franz-Josef Ulm
aDepartment of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139;
Pierre Levitz
fPhysicochimie des Electrolytes et Nanosystèmes interfaciaux Laboratory, CNRS and University Pierre et Marie Curie, 75252 Paris cedex 5, France;
Roland J.-M. Pellenq
aDepartment of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139;
bMultiScale Material Science for Energy and Environment, Massachusetts Institute of Technology–CNRS Joint Laboratory at Massachusetts Institute of Technology, Cambridge, MA 02139;
gCentre Interdisciplinaire de Nanoscience de Marseille, CNRS and Aix-Marseille University, 13288 Marseille cedex 09, France;
Emanuela Del Gado
hDepartment of Physics and Institute for Soft Matter Synthesis and Metrology, Georgetown University, Washington, DC 20057
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.
Upon 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 CO2 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 hundreds of nanometers (12⇓–14). Nanoindentation experiments have highlighted structural and mechanical heterogeneities over the same length scales (15). Their findings suggested that the internal stresses developed over those length scales during setting may be responsible for delayed nonlinear deformations, such as creep, that ultimately lead to major obstacles when designing the material properties and controlling the durability. Despite these advancements, the link between the nanoscale observations and the macroscale models currently used to predict and design cement performance is missing. Hence, to match the experimental observations, those models use ad hoc assumptions that cannot be independently tested or validated. Providing new quantitative information on the mesoscale texture of cement hydrates and how it may impact the material properties is the conundrum.
Here, we use a statistical physics approach to gain insight into the C–S–H at the scale of hundreds of nanometers based on the knowledge developed at the nanoscale. In our model, the complex pore network and the structural heterogeneities naturally emerge from the short-range cohesive interactions typical of nanoscale cement hydrates and the nonequilibrium conditions under which C–S–H densifies during cement setting. The scattering intensity, pore size distribution (PSD), surface area, local volume fractions, indentation modulus, and hardness measured in the simulations are compared with experiments and provide a first, to our knowledge, consistent characterization of the elusive mesoscale structure of C–S–H
