@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 {72, title = {Combinatorial molecular optimization of cement hydrates}, journal = {Nat Commun}, volume = {5}, year = {2014}, month = {Sep-24-2014}, pages = {Article Number: 4960}, abstract = {

Despite its ubiquitous presence in the built environment, concrete\â\€$(1s (Bmolecular-level properties are only recently being explored using experimental and simulation studies. Increasing societal concerns about concrete\â\€$(1s (Benvironmental footprint have provided strong motivation to develop new concrete with greater specific stiffness or strength (for structures with less material). Herein, a combinatorial approach is described to optimize properties of cement hydrates. The method entails screening a computationally generated database of atomic structures of calcium-silicate-hydrate, the binding phase of concrete, against a set of three defect attributes: calcium-to-silicon ratio as compositional index and two correlation distances describing medium-range silicon-oxygen and calcium-oxygen environments. Although structural and mechanical properties correlate well with calcium-to-silicon ratio, the cross-correlation between all three defect attributes reveals an indentation modulus-to-hardness ratio extremum, analogous to identifying optimum network connectivity in glass rheology. We also comment on implications of the present findings for a novel route to optimize the nanoscale mechanical properties of cement hydrate.

Figure 1

}, doi = {10.1038/ncomms5960}, author = {Mohammad Javad Abdolhosseini Qomi and Konrad J. Krakowiak and Mathieu Bauchy and Stewart, K.L. and Rouzbeh Shahsavari and Jagannathan, D. and Brommer, D.B. and Alain Baronnet and Markus J Buehler and Sidney Yip and Franz-Josef Ulm and Krystyn J. Van Vliet and Roland Jean-Marc Pellenq} }