Host laboratory: Research Laboratory for Migration and Interactions in the Geosphere (LETIS)
Beginning of the thesis: October 2018
Student name: Ayedah TARIQ
The ageing of cementitious materials, mainly their mechanical durability under the various environmental constraints encountered in the context of Cigéo (Centre industriel de stockage géologique des déchets radioactifs) is a major safety issue. Changes in the mechanical behaviour of cementitious structures are mainly the result of physico-chemical disturbances. Since 2010, numerous studies have been carried out to understand these changes and try to extrapolate them to the longer term. Although most of the reactive mechanisms are now satisfactorily understood from a physico-chemical point of view, there are still uncertainties regarding the chemical evolution of the pore solution contained in concrete. There is little experimental data to validate the hydration or ageing models found in the literature. The developments proposed in this study are not intended to lead to an industrial monitoring application.
The pH is a reliable and powerful indicator of the state of alteration of cement structures and its measurement remains of major interest in the framework of continuous monitoring of their ageing. Optical probes are an interesting option for monitoring the pH variation of concrete over long periods of time. However, the systems described in the literature are not applicable for real-time monitoring of high pH variations (> 13).
In this context, the objectives are to design and implement a pH optode in order to monitor in situ the ageing of cementitious materials from setting to several months/years, with the final objectives being to
- the comparison of experimental results with hydration models, and
- the presentation of an operational demonstrator on a real site at the LRS of Tournemire.
For this, an optical bench was designed and used to measure the pH of alkaline solutions. These first tests were carried out using a fluorescent molecular probe with a low pKa (12.5) for the targeted applications. A guanidinium probe with a higher pKa (13-13.5) is being synthesized. Tests will be carried out in climatic chambers at the LUTECE laboratory in samples of cementitious materials with humidity and temperature control to verify the validity of the measurements in relation to the saturation state of the material. The polymer will also be tested to evaluate the pH value in the concretes of the existing works by injecting it into the porosity of the material.