Giovanni Pesce

Dating of Old Lime Based Mixtures with the "Pure Lime Lumps" Technique

A number of studies carried out over the last forty years describe the application of radiocarbon dating of lime mixtures such as mortars, plasters and renders. Despite the fact that this method is very simple in principle, several studies have highlighted various practical challenges and factors that must be considered. These arise mainly from the contamination of samples with carbonaceous substances such as incompletely burnt limestone and aggregates of fossil origin including limestone sand. However, recently studies have shown that accurate sample processing allow a significant reduction of these error sources and moreover adoption of a special sampling procedure based on the careful selection of lumps of incompletely mixed lime, provides an interesting alternative that avoids problems associated with contamination. The founding principle underlying this technique is the use of the pure lime lumps. These are thought to originate from imperfect mixing and are most prevalent in mortars, renders and plasters predating mechanical mixing. Previous sampling methods for radiocarbon dating did not discriminate between pure and contaminated lime lumps. As pure lumps contain the same lime as that used in other parts of the mixtures but importantly are free of contaminants such as sand grains or under burned pieces of limestone, they can dramatically reduce the errors in the radiocarbon dating.

Monitoring hydration in lime-metakaolin composites using electrochemical impedance spectroscopy and nuclear magnetic resonance spectroscopy

Abstract This paper describes a study of the hydraulic reactions between metakaolin (MK) and air lime using electrochemical impedance spectroscopy (EIS) and nuclear magnetic resonance spectroscopy (NMR). Tests were carried out at 20, 25 and 30°C on lime-MK pastes with 10:1 w/w ratio. Tests over 28 days allowed identification of relevant changes in the EIS signals and characterization of pastes using thermal analysis (TGA/DSC), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP) and uni-axial compressive tests. Tests over shorter periods of time (up to 42 h) allowed more detailed studies of the hydraulic phases formed at the very beginning of the reactions. Results of thermal analyses demonstrate formation of hydraulic compounds such as CSH, C4AH13 and C3ASH6 and show their evolution over time. MIP analysis demonstrates changes in pore size distribution related to the formation and trasformation of hydraulic phases. Variations of impedance response with time are shown to be associated with reaction kinetics. Changes in the NMR signal within the first 42 h of reaction are shown to be associated with the dissolution of calcium hydroxide in the pore solution. Overall, this paper demonstrates the importance of NMR in the study of hydraulic reactions in lime based materials and the ability of EIS to detect the formation of hydraulic compounds and the end of the calcium hydroxide dissolution process.

Nanostructured Pd hydride microelectrodes: in situ monitoring of pH variations in a porous medium

In this study we report the exceptional potentiometric properties of pH microprobes made with nanostructured palladium hydride microelectrodes and demonstrate their application by monitoring pH variations resulting from a reaction confined in a porous medium. Their potentiometric response was found to be reproducible and stable over several hours but primarily Nernstian over a remarkably wide pH range, including alkaline conditions up to pH 14. Continuous operation was demonstrated by reloading hydrogen at regular intervals to maintain the correct hydride composition thereby alleviating the need for calibration. These properties were validated by detecting pH transients during the carbonation of Ca(OH)2 within a fibrous mesh. Experimental pHs recorded in situ were in excellent agreement with theoretical calculations for the CO2 partial pressures considered. Results also showed that the electrodes were sufficiently sensitive to differentiate between the formation of vaterite and calcite, two polymorphs of CaCO3. These nanostructured microelectrodes are uniquely suited to the determination of pH in highly alkaline solutions, particularly those arising from interfacial reactions at solid and porous surfaces and are thus highly appropriate as pH sensing tips in scanning electrochemical microscopy.

Scanning electrochemical microscopy: using the potentiometric mode of SECM to study the mixed potential arising from two independent redox processes

This study demonstrates how the potentiometric mode of the scanning electrochemical microscope (SECM) can be used to sensitively probe and alter the mixed potential due to two independent redox processes provided that the transport of one of the species involved is controlled by diffusion. This is illustrated with the discharge of hydrogen from nanostructured Pd hydride films deposited on the SECM tip. In deareated buffered solutions the open circuit potential of the PdH in equilibrium between its β and α phases (OCP(β→α)) does not depend on the tip-substrate distance while in aerated conditions it is found to be controlled by hindered diffusion of oxygen. Chronopotentiometric and amperometric measurements at several tip-substrate distances reveal how the flux of oxygen toward the Pd hydride film determines its potential. Linear sweep voltammetry shows that the polarization resistance increases when the tip approaches an inert substrate. The SECM methodology also demonstrates how dissolved oxygen affects the rate of hydrogen extraction from the Pd lattice. Over a wide potential window, the highly reactive nanostructure promotes the reduction of oxygen which rapidly discharges hydrogen from the PdH. The flux of oxygen toward the tip can be adjusted via hindered diffusion. Approaching the substrate decreases the flux of oxygen, lengthens the hydrogen discharge, and shifts OCP(β→α) negatively. The results are consistent with a mixed potential due to the rate of oxygen reduction balancing that of the hydride oxidation. The methodology is generic and applicable to other mixed potential processes in corrosion or catalysis.

An experimental and computational study to resolve the composition of dolomitic lime.

Lowering the environmental impact, and moving away from a reliance on cement based binders, is a key challenge of the construction industry. Dolomitic lime binders are produced at lower temperatures than cement, re-adsorb released CO2 during strengthening, and are recognised for their superior permeability, flexibility and resilience. While dolomite consists of alternating layers of magnesium and calcium the distribution in dolomitic lime is not yet fully understood. Here we combine experimental and computational methods to confirm that dolomite phase separates into lime and periclase during thermal decomposition. Raman inactivity of decomposed dolomite agrees with XRD studies suggesting phase separation. Our results rule out the formation of mixed phase oxides and predict an upper bound for bulk and surface substitution defect concentrations. Transferred to study macroscopic models of lime mortars these findings indicate that only the pure phases need be considered and that for the construction industry superior artificial mortars should be obtained from mixing fine powders of pure magnesium and calcium hydroxide.

Characterisation of Lime/Metakaolin Paste Using Impedance Spectroscopy

This paper describes the study of lime/metakaolin pastes using impedance spectroscopy during the first three weeks after mixing. Changes in the composition, structure and morphology were obtained using scanning electron microscopy, energy dispersive analysis of X-rays, differential scanning calorimetry, thermal gravimetric analysis and mercury intrusion porosimetry. The impedance data was studied with reference to changes in ac conductivity, relative permittivity and bulk resistance. Electrical behaviour was found consistent with structural and compositional changes. This study highlights the importance of impedance spectroscopy as a method for the non destructive monitoring and evaluation of lime based hydraulic binders.

Provenance Study of Wood Found in Archaeological and Architectural Objects

In this paper we would like to address the study of the provenance of wood used for artistic objects (e.g., statues, panels), architectural structures (e.g., rafters, beams) and archaeological findings, (e.g., foundation piles) through the analysis of its growing curves. The comparison of these curves (sample curves) with different standard curves that originated from different places can in fact be used for discovering the provenance of the wood used in the respective objects. The method that we emphasize here is based on the analysis of two values: the “correlation coefficient” value and the “coincidence rate” value, developed by Eckstein (2007) in a study of a few statues and panels of the Lubeck Cathedral. However, our study is also based on a graphic comparison of the trend curves of samples with the trend curves of the references. In fact, by following this methodology, it has been possible to study the provenance of a large number of samples gathered by the Dendrochronological Laboratory of the Institute for the History of Material Culture (ISCUM – Genoa, Italy) during the past years.