Michele Secco

Crystal chemistry of cement-asbestos

Abstract A study of a representative number of cement-asbestos (CA) samples removed from different localities in Italy has been accomplished with a combination of analytical techniques, including XRF, XRPD, SEM/EDS, micro-Raman, and electron backscattered diffraction (EBSD), to elucidate the mineralogical and chemical variability of this class of building materials on a large scale. We describe a complex mineralogy including phases of cement hydration, residual non-hydrated components, and a relevant fraction attributed to various processes of deterioration. With the aid of the CaO-MgOSiO2 compositional diagram, three groups of CAs have been identified on the basis of their chemical parameters. This result is important for environmental and waste management issues

Mortar Dating Methodology: Assessing Recurrent Issues and Needs for Further Research

Absolute dating of mortars is crucial when trying to pin down construction phases of archaeological sites and historic stone buildings to a certain point in time or to confirm, but possibly also challenge, existing chronologies. To evaluate various sample preparation methods for radiocarbon (14C) dating of mortars as well as to compare different dating methods, i.e. 14C and optically stimulated luminescence (OSL), a mortar dating intercomparison study (MODIS) was set up, exploring existing limits and needs for further research. Four mortar samples were selected and distributed among the participating laboratories: one of which was expected not to present any problem related to the sample preparation methodologies for anthropogenic lime extraction, whereas all others addressed specific known sample preparation issues. Data obtained from the various mortar dating approaches are evaluated relative to the historical framework of the mortar samples and any deviation observed is contextualized to the composition and specific mineralogy of the sampled material.

Preparation and Dating of Mortar Samples-Mortar Dating Inter-Comparison Study (MODIS)

Seven radiocarbon laboratories: Abo/Aarhus, CIRCE, CIRCe, ETHZ, Poznan, RICH, and Milano-Bicocca performed separation of carbonaceous fractions suitable for 14C dating of four mortar samples selected for the MOrtar Dating Inter-comparison Study (MODIS). In addition, optically stimulated luminescence (OSL) analyses were completed by Milano-Bicocca and IRAMAT-CRP2A Bordeaux. Each laboratory performed separation according to laboratory protocol. Results of this first intercomparison show that even though consistent 14C ages were obtained by different laboratories, two mortars yielded ages different than expected from the archaeological context.

Red/yellow pigments in Pompeii and Herculaneum: which is which?

In Pompeii, Herculaneum, Oplontis, and other archaeological sites destroyed by the 79 AD eruption of Mt. Vesuvius, in large areas of the wall paintings, the original paint has suffered from the high temperature reached because of the hot lapilli fall, the hot gases, or the hot pyroclastic/mud flow, depending on the distance from the volcano. This has produced visible changes in the color of the pigments. The most well-known case is the conversion of yellow goethite to red hematite, causing extensive yellow background panels of many frescoes to turn red [1]. This transformation has been studied as an effect of heat, both as an unintentional result of fire, and as traditionally practiced by artists and craftsmen to intentionally calcine yellow ochre to achieve a broader palette. The induced phase transition can actually be observed directly under the Raman laser beam if the laser energy is not properly controlled [2]. This poses the interesting question of recognizing what was originally red and what turned red during the eruption. A number of measurable parameters were adopted for the identification of yellows and reds from specific locations affected by high temperatures, including the mineral composition of the pigment mixture, the broadening of the XRPD and RS peaks of hematite due to the high temperature transition, and the trace element impurities contained in the mineral pigments. Yellow and red pigments from sites subject to the Vesuvian eruption effects (Pompeii, Hercolaneum) were characterized and compared to similar pigments from other archaeological sites (Nora, Aquileia). The samples from the tablinum of the Casa del Bicentenario in Herculaneum were investigated through a joint collaboration between GCI and the Parco Archeologico di Ercolano in the frame of the Herculaneum Conservation Project (HCP). The puzzling and almost ubiquitous presence of Pb in the red/yellow pigments will also be discussed.

Crystal chemistry of clinker relicts from aged cementitious materials

Despite the general tendency to consider Portland cement virtually fully hydrated within a few years, the occurrence of non-reacted clinker phases in cementitious materials that are several decades old is rather common. In this work, the integration of microstructural analysis by scanning electron microscopy and quantitative microchemical and micromineralogical characterization techniques, such as electron microprobe analysis and electron backscatter diffraction, allowed the definition of the crystal-chemical properties of partially hydrated cement residuals within different types of aged cementitious materials. The results on several clinker relicts show that the calcium silicate phases are transformed systematically and pervasively by hydration reactions, whereas the aluminate and ferrite phases do frequently persist in the anhydrous state. These relict phases may be distinguished through their peculiar chemical, mineralogical and textural features. These observations raise interesting questions concerning the long-term hydration kinetics of clinker phases and the durability behaviour of cementitious materials in operating conditions.