Colloidal Behavior of C-S-H Nanohydrates in Cement Paste
Résumé
Among the hydrates constituting a cement paste, C-S-H is the main component, it represents at least 60 % of the fully hydrated paste. C-S-H has a lamellar structure similar to that of a rare mineral, tobermorite. The germination and growth of the C-S-H particles result in increasing mechanical resistance (strength, cohesion). Mechanical experiments show that the intrinsic properties of the paste remain the same throughout the hydration process. Indeed, despite its very high compression strength properties the hydrated cement paste retains the same very low critical strain (~10 -4) characterizing a small elastic limit of the material. More recently, AFM experiments has revealed that CSH particles, immersed in a solution that contains Ca2+ and OH- ions, attract each other [1] at short range (2 nm) provided that the pH exceeds 11. These two experimental facts suggest that the cohesion is due to short range forces between C-S-H and anhydrous C 3S grains and between C-S-H themselves. Under normal cement conditions, high pH and salt concentrations, C-S-H particles carry a strong negative surface charge density due to the titration of their surface silanol groups. The high surface charge density together with divalent calcium ions in the pore solution makes the cement paste a strongly coupled system. In such systems, it was demonstrated that the DLVO theory based on the mean field Poisson-Boltzmann equation (PB), becomes qualitatively wrong. Indeed, the PB equation predicts a double layer repulsion where it is in fact attractive because it neglects the ion-ion correlations [2]. Delville [3] was the first to suggest that the strong attraction induced by ion-ion correlations could explain the cohesion of early cement paste. In this contribution, we will show, by comparing simulation results to experimental data of titration, electrophoresis and Atomic Force Microscopy, how a simple self consistent electrostatic model solved using a Monte Carlo method in the grand canonical ensemble, can predict the charging process, ion adsorption and the zeta potential reversal [3] of C-S-H as well as the interaction forces [4] between C-S-H particles. [1] C. Plassard, E. Lesniewska, I. Pochard, and A. Nonat, Nanoscale Experimental Investigation of Particle Interactions at the Origin of the Cohesion of Cement, Langmuir 21 (2005) 7263-7270 [2] Guldbrand, L.; Jönsson, B. H. Wennerström, P. Linse, Electrical Double Layer Forces. A Monte Carlo study, J. Chem. Phys. 80 (1984) 2221-2229 [3] A. Delville, R. J. M. Pellenq, A. Caillol, Monte Carlo (N,V,T) Study of the Stability of Charged Interfaces: A Simulation on a Hypersphere J. Chem. Phys. 106 (1997) 7275-7286 [4] C. Labbez, B. Jönsson, I. Pochard, A. Nonat, B. Cabane, Surface Charge Density and Electrokinetic Potential of Highly Charged Minerals: Experiments and Monte Carlo Simulations on Calcium Silicate Hydrate J. Phys. Chem. B 110 (2006) 9219-9230