@article {244, title = {Crystal-chemistry control of the mechanical properties of 2:1 clay minerals}, journal = {Applied Clay Science}, volume = {143}, year = {2017}, month = {Jul-2017}, pages = {387 - 398}, abstract = {

Clay minerals are the main constituents of the clay matrix of a wide variety of sedimentary deposits. When subjected to burial, some of these minerals undergo phase transitions accompanied by atomic substitutions which ultimately impact the cohesive interactions between their constitutive layers. The most common among such transitions is smectite illitization, which is also highly relevant for oil and gas exploration and production from source rocks. The impact of this transition on the mechanical properties of clay minerals as well as clay bearing rocks remains, however, to be properly addressed. To this end, a set of macroscopic single 2:1 clay minerals (pyrophyllite, talc, vermiculite, phlogopite, muscovite, and clintonite) representative of the two octahedral fillings and the variation in surface charge densities was investigated. A hybrid experimental-modeling approach is proposed, which combines nanoindentation in orthogonal directions (x1 and x3-directions) with analytical derivations of the cohesive energy (Ui) using XRD and TEM-EDS measurements. The results highlight that the interlayer energy defines the elasticity (modulus M), strength (hardness H) and ductility behavior (M/H) of these materials; and not the bond energy stored into the constituent layers. This finding permits the derivation of predictive stiffness and strength functional relations as derivatives of the interlayer energy that account for the arrangement of nanoscale layers and the interlayer composition. These relations suggest that as the cohesion increases with the coulombic interactions through progressive smectite illitization, the system loses its capacity to dissipate applied energy by dislocation mechanisms in between the layers, entailing an increase in brittleness of the clay particles with burial. By way of conclusion, the geophysics and geochemistry implications of these results for predicting the macroscopic mechanical performance of clay bearing geomaterials, such as economically valuable source rocks, are discussed.

}, issn = {01691317}, doi = {10.1016/j.clay.2017.04.010}, author = {Berthonneau, Jeremie and Christian G. Hoover and Grauby, Olivier and Alain Baronnet and Roland Jean-Marc Pellenq and Franz-Josef Ulm} } @article {196, title = {Mesoscale texture of cement hydrates}, journal = {Proceedings of the National Academy of Sciences}, volume = {113}, year = {2016}, month = {Feb-23-2016}, pages = {2029 - 2034}, 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.

Fig. 1.

}, issn = {0027-8424}, doi = {10.1073/pnas.1520487113}, author = {Katerina Ioannidou and Konrad J. Krakowiak and Mathieu Bauchy and Christian G. Hoover and Enrico Masoero and Sidney Yip and Franz-Josef Ulm and Pierre E. Levitz and Roland Jean-Marc Pellenq and Emanuela Del Gado} } @article {148, title = {Experimental chemo-mechanics of early-age fracture properties of cement paste}, journal = {Cement and Concrete Research}, volume = {75}, year = {2015}, month = {Sep-2015}, pages = {42 - 52}, abstract = {

The risk of early-age fracture of cementitious materials in ever more challenging environments provides a unique opportunity to employ an experimental chemo-mechanical platform to develop functional relations between hydration degree, fracture and strength properties, assessed by isothermal calorimetry, micro-scratching, splitting and microindentation on white cement paste at various curing ages from 7\ h to 28\ days. We show that the modulus, tensile strength, fracture toughness and energy all evolve with a natural logarithmic dependence on the hydration degree. These trends are linked to the densification of the material during the hydration process, explained by compaction mechanics and free volume theory. We show that while the fracture process zone size is essentially constant during the hydration process, the ductility of the material, quantified by M/H, decreases, and is consistent with the evolution of Kc/H. Both quantities provide a convenient way to experimentally assess the fracture sensitivity of early-age cement-based materials.

}, issn = {00088846}, doi = {10.1016/j.cemconres.2015.04.004}, author = {Christian G. Hoover and Franz-Josef Ulm} } @article {137, title = {Fracture toughness of calcium{\textendash}silicate{\textendash}hydrate from molecular dynamics simulations}, journal = {Journal of Non-Crystalline Solids}, volume = {419}, year = {2015}, month = {Jul-01-2015}, pages = {58 - 64}, abstract = {

Concrete is the most widely manufactured material in the world. Its binding phase, calcium\–silicate\–hydrate (C\–S\–H), is responsible for its mechanical properties and has an atomic structure fairly similar to that of usual calcium silicate glasses, which makes it appealing to study this material with tools and theories traditionally used for non-crystalline solids. Here, following this idea, we use molecular dynamics simulations to evaluate the fracture toughness of C\–S\–H, inaccessible experimentally. This allows us to discuss the brittleness of the material at the atomic scale. We show that, at this scale, C\–S\–H breaks in a ductile way, which prevents one from using methods based on linear elastic fracture mechanics. Knowledge of the fracture properties of C\–S\–H at the atomic scale opens the way for an upscaling approach to the design of tougher cement paste, which would allow for the design of slender environment-friendly infrastructures, requiring less material.

}, issn = {00223093}, doi = {10.1016/j.jnoncrysol.2015.03.031}, author = {Mathieu Bauchy and Hadrien Laubie and Mohammad Javad Abdolhosseini Qomi and Christian G. Hoover and Franz-Josef Ulm and Roland Jean-Marc Pellenq} }