PhD defense: Ayedah Tariq

Title:

Development of optical fiber sensors for in situ measurement of pH in concrete

Resume:

Significant volumes of cementitious materials are used in the nuclear industry to design nuclear power plants containment facilities or surface storage installations for radioactive waste for example. The increasing volume of radioactive waste produced raises the question of a deep geological disposal facility such as Cigéo (industrial center for geological storage of radioactive waste). This type of facility would require various cementitious materials and the ageing of these materials is a major safety issue. The evolution of the mechanical behavior of cementitious structures is mainly the result of physico-chemical disturbances. Although the majority of reactive mechanisms are now understood in a physico-chemically satisfactory manner, the question of monitoring the chemical evolution of cementitious materials and more specifically, of the pore water, remains a very limited field of research in spite of the primordial stakes that it covers. The pH is indeed a reliable and powerful indicator of the state of deterioration of cementitious material structures and its measurement remains of major interest in the continuous monitoring of concrete aging. The majority of methods developed to measure the high pH of concrete (pH = 10.5-13.5) are destructive methods and standard electrochemical probes are not suited for in situ pH measurement, over long periods of time. With regards to optical probes, some systems have been developed in the literature, but they are not applicable for real-time monitoring of high pH variation (> 13). The research work exposed here relates to the development of a pH optode for in situ measurement in cementitious materials, from the hydration to the material ageing. In order to achieve this, a naphthalimide derivative has been selected and perylene derivatives with amine or guanidine functions have been synthesized in order to obtain fluorescent pH probes. The pH probes photophysical and photochemical properties have been characterized in solution. These novel pH probes have been entrapped or bound to various polymer matrices such as hydrogels (polyvinyl alcohol crosslinked with glutaraldehyde or Hydromed D4), or cellulose in order to obtain robust materials, permeable to water and analytes. These materials response to pH variations has been studied. Finally, an optical bench was designed in order to develop a reliable optical pH measurement tool including an internal reference. pH measurements were carried out on the surface of low-pH cement pastes with a short response time of around 100 s and a ±0.1 pH unit precision. Issues regarding the pore water accessibility of cementitious materials and specific interactions between pH probes and solid matrices has been highlighted and outlooks for in situ pH measurement are discussed.

This work is available online.