Données de la diffusion centrale des rayons X sur la texture d'alites hydratées à court terme et à deux températures

Abstract

La diffusion centrale des rayons X est appliquée ici à l'examen des microtextures d'alites hydratées à court terme. On montre en premier lieu que l'obtention de courbes de diffusion significatives est subordonnées à un état de désorption défini de l'échantillon, les résultats conduisent à se représenter la phase (CSH) comme un système évolutif tendant vers des limites plus nettes à l'état désorbé. Ces particularités sont sans doute à rapprocher du comportement en retrait/fluage. Les déterminations de surface spécifique, déduites de la loi de Porod, conduisent à des valeurs de 3 à 400 m²g⁻¹ à 24 h 25°C et sont donc analogues à celles obtenues par adsorption de vapeur d'eau. Ces surfaces sont plus faibles pour les échantillons hydratés à 60°C ainsi que, probablement, la microporosité.

Summary

The small-angle scattering of X-rays, which provides data on very small-scale discontinuous textures, is here applied to samples of alite hydrated for a short period, at temperatures of 25 and 60°C. The situation is complicated, however, by the fact that the scattering phase (CSH) cannot be isolated, and does not necessarily correspond to the models on which the scattering theory is built.

It is shown in the first place that as the hydrated system is in exchange with the ambiance and the intensity of the scattering proportional to the square of the deviations of density between the pores and the dense phase, the obtaining of reproducible and significant data is subordinated to a defined state of desorption of the sample. This condition being realized, the main conclusions deduced from this work are the following:

• a) Desorption at 1.3% of relative humidity entails a considerable increase in scattered intensity, which can be entirely interpreted only by assuming an evolution of the (CSH) phase in the direction of a clearer definition of discontinuities. This evolution is partially irreversible and the return to 96% of humidity does not restore the initial conditions.

• b) The order of magnitude of the heterogeneities that can be deduced from the scatter curves falls between some ten to some hundreds of Å. These heterogeneities, statistically, do not appear to be dominated by bidimensional forms.

• c) The curves obtained in the desorbed state make it possible to consider determining the absolute specific surfaces by recourse to the Porod methods. We then find, with reference to the CSH phase, values of 3 to 400 m²g⁻¹ for samples hydrated 24 hours at 25°C, but appreciably lower values for samples hydrated at 50°C. These values are close to those obtained by adsorption of water vapour.

The (CSH) phase, according to these tests, does not appear as a particular whole defined once for all, but as a system that is an essentially evolving one in terms of the hygrometry. It woudl certainly be interesting to know to what extent these data can be compared to the physical behaviour of hardened pastes.