FTIR-Based Crystallinity Assessment of Aragonite–Calcite Mixtures in Archaeological Lime Binders Altered by Diagenesis

Michael Toffolo,Lior Regev,Elisabetta Boaretto,Stéphan Dubernet,Yannick Lefrais

doi:10.3390/min9020121

Abstract

Lime plaster and mortar are pyrotechnological materials that have been employed in constructions since prehistoric times. They may nucleate as calcite and/or aragonite under different environmental settings. In nature, aragonite and calcite form through biogenic and geogenic processes that lead to different degrees of atomic order. The latter is a result of defects in the crystal lattice, which affect the properties of crystals, including their interaction with infrared light. Using Fourier transform infrared spectrometry (FTIR) with the KBr pellet method, it is possible to exploit these differences and assess the degree of atomic order of aragonite and calcite crystals and thus their mechanisms of formation. Here we use FTIR to characterize the degree of short-range atomic order of a pyrogenic form of aragonite recently observed in experimental and archaeological lime binders. We show that pyrogenic aragonite has a unique signature that allows its identification in archaeological sediments and lime binders of unknown origin. Based on these results, we developed a new FTIR-based method to assess the integrity and degree of preservation of aragonite and calcite when they occur together in the same material. This method allowed a better assessment of the diagenetic history of an archaeological plaster and finds application in the characterization of present-day conservation materials, such as lime plaster and mortar, where different polymorphs may nucleate and undergo recrystallization processes that can alter the mechanical properties of binders.

Highlights

Anthropogenic carbonates are human-made materials composed of CaCO3 that derive from pyrotechnological processes and include ash, lime plaster, lime mortar and, to a lesser extent, concrete [1,2,3,4]
Pyrogenic aragonite was produced by heating powdered Glycymeris sp. shells to 900 ◦ C for 12 h to obtain quicklime (CaO), which was left to react with the atmosphere at room temperature until it carbonated completely [12]
This is especially the case of archaeological materials such as ash and lime plaster/mortar that undergo burial conditions for thousands of years, and present-day concrete, mortar and plaster exposed to the elements

Summary

Introduction

Anthropogenic carbonates are human-made materials composed of CaCO3 that derive from pyrotechnological processes and include ash, lime plaster, lime mortar and, to a lesser extent, concrete [1,2,3,4]. Lime plaster and mortar are still largely used today in conservation works, with decorative (e.g., coating) or structural (e.g., binder between cobbles) functions [5,6,7] Their physical and chemical properties are determined by the raw materials and lime technology used in their production, as well as by their functional role and environmental conditions upon and after setting [8,9,10]. Their characterization is crucial in the interpretation of the archaeological record, in the study of historic buildings, and in the development of novel construction materials and restoration strategies.

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Conclusion