Decoding ancient mortars: complementary strengths of SR-μXRPD and FPA-FTIR in high-resolution binder analysis

The binder of the ancient natural hydraulic mortar is obtained by firing marly or siliceous limestone, which is naturally rich in clay minerals or silica. The setting and hardening process of these mortars is complex, as is their composition, which varies depending on the raw materials, the environment of preservation, and the age of the material. Natural hydraulic mortars from important religious and historic buildings in Florence (i.e. Giotto's Bell Tower, Medici Riccardi Palace and Trebbio Castle) were characterized using conventional techniques (SEM-EDS, TGA, XRPD and ATR-FTIR). In-depth analyses of the binder were performed on the same sample portions through precise sample preparation and the high spatial resolution of advanced techniques, producing high-quality chemical and mineralogical maps. Micro-X-ray powder Diffraction at the European Synchrotron Radiation Facility (SR-μXRPD) and Fourier transform infrared microspectroscopic technique (FPA-FTIR) allow the determination of the distribution and stability of calcium carbonate polymorphs (calcite, aragonite, vaterite) as well as crystalline and amorphous calcium aluminum silicate in the binder of ancient mortars. These results are of fundamental importance for the understanding of ancient production technologies and the chemical transformations that can give these ancient materials their hydraulic behavior. For the first time, SR-μXRPD and FPA-FTIR have been used to study ancient natural hydraulic mortars, demonstrating the complementarity of these high-resolution techniques for mapping compounds in the historic binders.