###### Abstract  
This study presents a comprehensive analysis and radiocarbon dating of historical mortar and plaster samples from the San Salvatore--Massino Visconti complex in Piedmont, Northern Italy. Mortar samples and one charcoal sample were collected from various areas within the complex's lower chapels. Samples were selected and characterized by means of a multi-analytical approach in order to draw inferences about their compositional, mineralogical, and microstructural features. The identification of hydromagnesite and magnesite in the mortar samples suggests the usage of magnesian binder mortar, potentially affecting radiocarbon dating due to its slower carbonation kinetics when compared to calcitic mortars. To mitigate this effect, a purification method was developed involving thermal treatment at 550 \({}^{\circ}\)C to isolate datable binding fractions. The results yielded reliable radiocarbon ages consistent with historical context, shedding light on construction materials dating from the 12 th to 16 th centuries. The study also challenges previous notions by demonstrating the feasibility of radiocarbon dating for magnesian mortars, opening new perspectives for dating such materials. These findings offer valuable insights into the construction history and material composition of the complex, corroborating historical information. 14C dating; accelerator mass spectrometry; magnesian mortars; mortar characterization; hydromagnesite; magnesite +
Footnote †: journal: Applied  
**Citation**: [PERSON], [PERSON]; [PERSON], [PERSON]; [PERSON], [PERSON]; [PERSON], [PERSON]; [PERSON], [PERSON], [PERSON] Radiconarbon Dating of Magnesium Mortars: The Case of San Salvatore Church in Massino Visconti, Piedmont, Italy. **Citation**: [PERSON], [PERSON]; [PERSON], [PERSON]; [PERSON], [PERSON]; [PERSON], [PERSON], [PERSON]; [PERSON], [PERSON] Radiconarbon Dating of Magnesium Mortars: The Case of San Salvatore Church in Massino Visconti, Piedmont, Italy. Indeed, these mineral phases in the mortar mix might slightly shift the dating towards more recent dates due to the slower carbonation reaction of magnesium mortars when compared to usual calcite ones. Magnesium oxide (MgO) from the calcination of magnesium carbonates is less reactive with water than is calcium oxide (CaO), and the former has a lower carbonation kinetic. Typically, the carbonation of magnesium lime does not exceed 60% (compared to 95% for calcium lime), so calcium hydroxide transforms into calcite, while most of the magnesium hydroxide either remains uncarbonated or carbonatates slowly and partially to form compounds like hydromagnesite [6; 7; 8; 9]. Furthermore, the identification of LDH (Layer Double Hydroxides) phases like hydrotalcite is crucial for radiocarbon dating. LDH minerals form as a result of a pozzolanic reaction and have a high capacity to fix carbonate anions (CO\({}_{3}\)\({}^{-2}\)) into their crystalline structure, introducing contamination from younger carbon into the system, which would shift the radiocarbon dates [10; 11; 12; 13]. Our study comprised a complete characterization of the binding materials through different analytical techniques, such as X-ray powder diffraction (XRPD), optical microscopy (OM), and scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS), to address these challenges. Furthermore, a wet gravimetric separation for the extraction of a fine fraction (SG) mainly composed of the mortar binder was carried out and the SG was characterized by XRPD to investigate the presence of contaminants. This preliminary characterization is necessary for a reliable radiocarbon dating of samples [12; 14; 15]. Since the analyzed samples exhibited phases considered contaminants for radiocarbon dating purposes (such as LDHs), an additional purification step was carried out through thermal treatment at 550 \({}^{\circ}\)C for 30 min under vacuum conditions, aiming to release the incorporated CO\({}_{3}\)\({}^{-2}\) by collapsing its chemical structure.