Ancient Analogues of Modern Cement: Calcium Hydrosilicates in Mortars and Concretes from Gallo–Roman Thermal Baths of Western France

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Abstract
The long‐term durability of cementbased materials over periods of time not directly accessible to laboratory experiments has to be thoroughly assessed to guarantee the safety of a radioactive waste repository. In this work, archaeological mortars and concretes have been studied to check whether their use as analogues of actual technological cements used for waste disposal is possible. Samples of mortar and concrete have been collected from several Gallo‐Roman thermae of western France; they were examined as petrographically thin sections with optical microscopy and were analyzed by X‐ray diffraction of microsamples (10−3 mg) and by electron probe microanalysis. The masonry cement appears to be entirely carbonated with large voids and fissures. In contrast, the use of lime with additional crushed brick or tile for the bath structure concrete and coating mortar induced pozzuolanic reactions. Despite the fact that the matrices are essentially composed of calcium carbonate, calcium aluminosilicates with compositions close to those of hydrogrossular, hydrated calcium silicates with composition and a structure comparable with those of calcium hydrosilicate formed in modern cement, and ettringite deposits in voids of concretes could also be identified. It is suggestea that these ancient artifacts can help identify very slow processes that rule cement alteration over long periods of time. The artifacts may be considered as possible analogues of present‐day materials whenever sufficient care is taken in interpreting and extrapolating data.