Engineering the bonding scheme in C-S-H: The iono-covalent framework

On the basis of recent molecular simulation and experimental studies, we discuss two possible strategies for tuning the mechanical properties of cememitious materials by modifying the bonding scheme in the hydrates at molecular level. We focus the discussion on the calcium silicate hydrates (C-S-H). A first strategy would be based on the strengthening of the network of cohesion forces acting between the individual C-S-H lamellae or between their crystallites. Atomic scale simulations by ab initio, molecular dynamics and energy minimization techniques show that the iono-covalent forces between individual C-S-H layers or C-S-H layer stacks, separated by strongly localized calcium ions and water molecules, are orders of magnitude larger than the ionic correlation forces acting between C-S-H surfaces separated by nm- or multi-nm-thick layers of mobile water molecules and ions. The elastic properties derived from this iono-covalent bonding scheme are in good agreement with experimental values obtained by ultrasonic or statistical (grid) nanoindentation techniques. The concept picture for C-S-H which follows is that of a crystalline semi-continuum, with dense domains (crystallites or particles) iono-covalently bonded to each other, possibly entangled also, and embedded as long as the mesoscale porosity is water-saturated in a relatively weak attractive stress field due to fluctuating electrostatic forces. Depending on the size, the aspect ratio, and the turbostratic order of the crystallites, and also the composition of the interstitial solution, the relative importance of each contribution could be modified. This provides the basis for a better control of properties such as early age or long term strength development for instance. In this respect, the microstructure-properties relationships in clay minerals provide interesting leads, pointing to the importance of bonding continuity rather than bond strength. A second strategy to tune the mechanical properties of cement systems, akin to a biomimetic approach, is to hybridize the hydrates by grafting organic moieties on the mineral lamellac. This can be achieved by controlled hydrolysis of organo-silane precursor mixtures or by performing the hydration of the anhydrous silicates in silanized polymer solutions. The outcome may be materials with improved fracture energy and larger strain at failure. (C) 2007 Elsevier Ltd. All rights reserved.