The growth of charged platelets.

Growth models of charged nanoplatelets are investigated with Monte Carlo simulations and simple theory. In a first model, 2-dimensional simulations in the canonical ensemble are used to demonstrate that the growth of a single weakly charged platelet could be limited by its own internal repulsion. The short range attractive interaction in the crystal is modeled with a square well potential while the electrostatic interactions are described with a screened Coulomb potential. The qualitative behavior of this case can also be described by simply balancing the attractive crystal energy with the screened Coulomb repulsion between the crystal sites. This repulsion is a free energy term dominated by counterion entropy and of course reduced by added salt. For a strongly coupled system, that is with high charge density and divalent counterions as in calcium silicate hydrate, the main product of cement hydration, the screened Coulomb approximation becomes inadequate and the growth behavior has to be described with the full primitive model. In this case, the energetic interactions become relatively more important and the entropy of the system plays a minor role. As a consequence, the electrostatic interactions gradually become less of a hindrance for aggregation and in extreme cases electrostatics actually promote the growth. This is manifested as an increased aggregation with, for example, increasing surface charge density. In the presence of divalent calcium ions and at the high negative surface charge density typical for calcium silicate hydrate, electrostatic interactions are not a hindrance for an infinite growth of the particles. By combining experimental and simulated data we can show that the limited sized platelets found in cement paste is due to a very fast nucleation rate compared to the growth rate.

Controlling the cohesion of cement paste.

The main source of cohesion in cement paste is the nanoparticles of calcium silicate hydrate (C-S-H), which are formed upon the dissolution of the original tricalcium silicate (C(3)S). The interaction between highly charged C-S-H particles in the presence of divalent calcium counterions is strongly attractive because of ion-ion correlations and a negligible entropic repulsion. Traditional double-layer theory based on the Poisson-Boltzmann equation becomes qualitatively incorrect in these systems. Monte Carlo (MC) simulations in the framework of the primitive model of electrolyte solution is then an alternative, where ion-ion correlations are properly included. In addition to divalent calcium counterions, commercial Portland cement contains a variety of other ions (sodium, potassium, sulfate, etc.). The influence of high concentrations of these ionic additives as well as pH on the stability of the final concrete construction is investigated through MC simulations in a grand canonical ensemble. The results show that calcium ions have a strong physical affinity (in opposition to specific chemical adsorption) to the negatively charged silicate particles of interest (C-S-H, C(3)S). This gives concrete surprisingly robust properties, and the cement cohesion is unaffected by the addition of a large variety of additives provided that the calcium concentration and the C-S-H surface charge are high enough. This general phenomenon is also semiquantitatively reproduced from a simple analytical model. The simulations also predict that the affinity of divalent counterions for a highly and oppositely charged surface sometimes is high enough to cause a "charge reversal" of the apparent surface charge in agreement with electrophoretic measurements on both C(3)S and C-S-H particles.

Onset of cohesion in cement paste.

It is generally agreed that the cohesion of cement paste occurs through the formation of a network of nanoparticles of a calcium-silicate-hydrate ("C-S-H"). However, the mechanism by which these particles develop this cohesion has not been established. Here we propose a dielectric continuum model which includes all ionic interactions within a dispersion of C-S-H particles. It takes into account all co-ions and counterions explicitly (with pure Coulomb interactions between ions and between ions and the surfaces) and makes no further assumptions concerning their hydration or their interactions with the surface sites. At high surface charge densities, the model shows that the surface charge of C-S-H particles is overcompensated by Ca2+ ions, giving a reversal of the apparent particle charge. Also, at high surface charge densities, the model predicts that the correlations of ions located around neighboring particles causes an attraction between the particle surfaces. This attraction has a range of approximately 3 nm and a magnitude of 1 nN, values that are in good agreement with recent AFM experiments. These predictions are stable with respect to small changes in surface-surface separation, hydrated ion radius, and dielectric constant of the solution. The model also describes the effect of changes in cement composition through the introduction of other ions, either monovalent (Na) or multivalent (aluminum or iron hydroxide).

Investigation by atomic force microscopy of forces at the origin of cement cohesion.

In cement paste, the cohesion results of the interactions between calcium silicate hydrate (CSH) surfaces in an interstitial ionic solution. (N, V, T) Monte Carlo simulations show that the interactions are due to the ion correlation forces influenced by the surface charge density, the ionic concentration and the ion valence. This paper deals with the direct measurement in solutions by atomic force microscopy (AFM) of the forces and the interaction ranges between a probe and an atomically smooth substrate covered by CSH nanoparticles. Different electrolytic solutions (Ca(OH)2, CaCl2, NaCl, NaOH) have been used in order to determine influent parameters permitting to identify the nature of acting forces. Investigations have been rendered possible by selecting appropriate experimental setup and solutions. The selected probe and substrate on which CSH nanoparticles have previously grown are neutral regarding the reactivity during experiments permitting the exchange of solutions. Results show that a force originates from electrostatic nature and differs from Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Agreement is found between experiments and (N,V,T) Monte Carlo simulations of ionic correlation forces. These forces are at the origin of the cohesion of cement paste.

Characterization of Calcium Aluminate Hydrates and Related Hydrates of Cement Pastes by (27)Al MQ-MAS NMR.

27Al multi quantum (MQ) MAS NMR spectroscopy was used for the first time to characterize calcium aluminate hydrates, which are of importance in the chemistry of high alumina and Portland cements. Substitution sites of silicon by aluminum in the calcium silicate hydrates (C-S-H) which are the main component of Portland cement paste were studied too. Synthetic samples of Ca(3)Al(OH)(12), [CaAl(OH)(4)][OH(H(2)O)(1.5)], [Ca(2)Al(OH)(6)](OH).3H(2)O, [Ca(2)Al(OH)(6)](2)(CO(3)).5H(2)O, [Mg(2)Al(OH)(6)](CO(3))(0.5).3H(2)O, Al(OH)(3), and C-S-H substituted by aluminum were prepared. In most of the samples, the two dimension 3Q-MAS NMR spectra allow one, more easily than the MAS-only NMR spectra, to obtain the chemical shift, delta(iso), and the quadrupolar parameters nu(Q) and eta, which label each site and bring information on its symmetry and environment. The distributions of the aluminum environments were observed for each site. In [Ca(2)Al(OH)](6)(OH).3H(2)O, (27)Al MAS spectrum demonstrates the presence of two octahedral aluminum sites. In the C-S-H substituted with Al, tetrahedral aluminum is observed, in bridging and nonbridging sites of the silicate chains, mostly in the bridging sites for the sample investigated.

[Inside and outside particulate pollution of a locality]. / Pollution particulaire a l'intérieur et à l'extéreur d'un local

The present study of the suspended matter concentration within and outside premises has shown: that inside premises, the filtrating effect of the walls during non-occupation periods (week-end) is apparent, thus the inside concentration remains below the outside concentration; that during the hours and the days when the premises are occupied, the inside concentration is significantly above the outside concentration; that the increase in concentration may be expressed by an "occupation index" depending on the number of people within the premises; that chemical composition and grain size spectrum undergo changes which seem to indicate the appearance of less acid particles which contain more ammonia and nitrates and less SO4, Ca, Fe and Pb than those to be found outside. These particles seem to originate from condensation and recent abrasion. This final point of the conclusions, based on preliminary experimental evidence, has not the pretention to be definite.