Multi-scale investigation of the degradation of alkali silicate glasses by unsaturated humidity
Résumé
Unsaturated atmospheric conditions (defined by a relative humidity, RH < 100 %) are considered as mild and not fundamentally differing from the immersion conditions regarding the alteration of glass. The degradation processes induced by unsaturated humidity are nonetheless specific, as notably observed in the cultural heritage. In this work, we addressed the mechanisms of glass atmospheric alteration, by the mean of ageing experiments at 85 RH% as a function of time and temperature of model glass samples. Three alkali lime silicate glass compositions have been studied, two are representative of "unstable" glass compositions of the CH, and one is a soda-lime silicate glass representative of a "stable" glass. The two unstable compositions were characterized by a high speed of hydration that even increased in the timescale of the experiments (up to 9 months at 40°C, 85 RH%), while the stable composition hydrated at a rate two orders of magnitude lower. The dependency of these hydration kinetics on temperature was not arrhenian, revealing a self-acceleration mechanism involving several processes. The multispectroscopic study (EDX, TOF-SIMS, NMR, IR, Raman) of the hydrated layer indicates that unstable compositions retained a significant proportion of alkalis and non-bridging oxygens in the hydrated surface layer, while the stable composition showed a complete chemical transformation of the surface layer into a stack of alkali- and Ca-carbonate salts (on the top) and silica (at the interface with pristine glass).