The mortar samples of the Castle of Cannero (Lake Maggiore, Italy) have been characterized and radiocarbon (14C) dated. The presence of LDH phases was identified. The hydraulic reaction was evaluated by a multi-analytical approach. Careful extraction, preparation and purification of the binder fraction have been performed. Contaminations due to LDH phases have been removed allowing reliable absolute dating of the structures. Non-hydraulic lime-based mortars represent only part of the binding materials found in archaeological and historical structures, and a new challenge is the application of 14C dating techniques on mortars that feature hydraulic reactions. This research work aims at 14C dating a series of Mg-rich hydraulic mortars from the Castle of Cannero (Lake Maggiore, Italy), from which both charcoals and mortar samples were collected. A multi-analytical approach employing X-ray powder diffraction (XRPD), optical microscopy (OM), and scanning electron microscopy/energy-dispersive microanalysis (SEM-EDS) was adopted in order to carefully characterize the samples. A wet gravimetric separation for the extraction of the fine fraction mainly composed by the mortar binder was carried out and the binder fraction was characterized by XRPD in order to investigate the presence of contaminants. The binding fractions are characterized by the widespread occurrence of hydrotalcite-type minerals, considered contaminants in 14C dating of mortars because of their capability to exchange carbonate anions even after the hardening process. A further purification treatment by thermal decomposition was performed before 14C dating by AMS. The obtained dates were consistent with archaeological expectations, confirming the potential of the developed purification methodology for hydraulic mortars dating.

In nature, calcium carbonate (CaCO3) in the form of calcite and aragonite nucleates through different pathways including geogenic and biogenic processes. It may also occur as pyrogenic lime plaster and laboratory-precipitated crystals. All of these formation processes are conducive to different degrees of local structural order in CaCO3 crystals, with the pyrogenic and precipitated forms being the least ordered. These variations affect the manner in which crystals interact with electromagnetic radiation, and thus formation processes may be tracked using methods such as X-ray diffraction and infrared spectroscopy. Here we show that defects in the crystal structure of CaCO3 may be detected by looking at the luminescence of crystals. Using cathodoluminescence by scanning electron microscopy (SEM-CL) and laser-induced fluorescence (LIF), it is possible to discern different polymorphs and their mechanism of formation. We were thus able to determine that pyrogenic calcite and aragonite exhibit blue luminescence due to the incorporation of distortions in the crystal lattice caused by heat and rapid precipitation, in agreement with infrared spectroscopy assessments of local structural order. These results provide the first detailed reference database of SEM-CL and LIF spectra of CaCO3 standards, and find application in the characterization of optical, archaeological and construction materials.

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Accurate radiocarbon (14C) dating of lime mortars requires a thorough mineralogical characterization of binders in order to verify the presence of carbon-bearing contaminants. In the last 20 years, cathodoluminescence (CL) has been widely used for the identification of geologic calcium carbonate (CaCO3) aggregates and unreacted lime lumps within the particle size fraction selected for carbon recovery. These components are major sources of older and younger carbon, respectively, and should be removed to obtain accurate age determinations. More recently, laser-induced fluorescence (LIF) has provided another means of investigating the preservation state and composition of CaCO3 binders. Considered the growing interest of the mortar dating community in the latest advancements of these analytical methods, here we review the principles of CL and LIF of CaCO3, their instrument setup, and their application to the characterization of ancient lime mortars used for 14C dating. In addition, we provide examples of SEM-CL and LIF analyses using high-resolution instrumentation, we discuss current issues and propose future lines of research.

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