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

Area of Science:

• Materials Science
• Archaeometry
• Geochemistry

Background:

• Ancient natural hydraulic mortars are complex binders derived from firing marly or siliceous limestone.
• Their composition and properties vary significantly based on raw materials, preservation conditions, and age.
• Understanding these mortars is crucial for historic building preservation and reconstructing ancient technologies.

Purpose of the Study:

• To characterize the binder composition and mineralogical distribution of ancient natural hydraulic mortars.
• To investigate the stability of calcium carbonate polymorphs and calcium aluminum silicates within the mortar binder.
• To demonstrate the utility of advanced synchrotron-based techniques for analyzing historic binders.

Main Methods:

• Conventional techniques: Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS), Thermogravimetric Analysis (TGA), X-ray Powder Diffraction (XRPD), and Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR).
• Advanced techniques: Micro-X-ray Powder Diffraction (SR-μXRPD) at the European Synchrotron Radiation Facility and Fourier Transform Infrared Microspectroscopy (FPA-FTIR).
• Precise sample preparation enabling high spatial resolution chemical and mineralogical mapping.

Main Results:

• Detailed chemical and mineralogical maps of the mortar binders were generated.
• The distribution and stability of calcium carbonate polymorphs (calcite, aragonite, vaterite) were determined.
• Crystalline and amorphous calcium aluminum silicates within the binder were identified and mapped.

Conclusions:

• SR-μXRPD and FPA-FTIR are powerful, complementary techniques for studying ancient natural hydraulic mortars at high resolution.
• These advanced methods provide fundamental insights into ancient production technologies and the chemical transformations responsible for hydraulic behavior.
• The study successfully mapped compounds in historic binders, advancing the field of archaeometry.