Review
Microscopy of historic mortars—a review

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Abstract

Mortars with mud, gypsum and lime as binder have, since ancient times, been used for very different applications. The characterisation of these historic mortars was until 1970–1980 mostly based on traditional wet chemical analyses but the interpretation of these results is difficult and often impossible without a good knowledge of the nature of the different mortar components. More recently developed mortar characterisation schemes have optical microscopy as a first step in identifying the aggregates, of the various mineral additions (latent hydraulic), binder type, binder-related particles and in describing the pore structure. Optical microscopy is also a valuable aid for damage diagnosis of degraded historic mortars and for the study of the interfacial zone, the bonding and possible reaction rims between aggregates, bricks or stone and the mortar. Automated image analysis techniques or manual point-count/linear traverse methods can be used to determine mix proportions, binder/aggregate ratio, aggregate size distribution and air void system.

Introduction

Mortars with different binder types have been used since ancient times for different applications; masonry mortars between bricks or stones, mortars as wall finishing materials internally (plaster) or externally (render), mortars as foundations for flooring, rubble mortars for the infillings of walls, mortars as casings of water conduits or jointing compounds from terracotta pipes, decoration mortars, etc. The compositional variation in historic mortars is surprisingly large with great differences both geographically and during different time periods. Mud, gypsum and lime had traditionally been the three most common binder types during the construction history of mankind until about two centuries ago, when their use was replaced gradually by different natural cement types and later by Portland cement, which is nowadays the dominant binder type in the construction industry. Mud is probably the oldest binder type in mortars, the use of clay has been identified for example in Catal Hüyük in Turkey, 6000 BC [1]. The use of lime as a binder dates back to the 6th millennium BC. A terrazzo floor excavated in Canjenü in Eastern Turkey laid with a lime mortar has been dated between 12 000 and 5000 BC [2]. A lime mortar used for flooring fishermen's huts excavated at Livinski Vir in Serbia–Montenegro has been dated at about 5600 BC [3]. Although mud and gypsum have been used in Europe during certain time periods and in certain regions, the majority of ancient mortars in Europe are lime-based and most of this review will therefore handle historic lime mortars. Gypsum was used for most applications in Pharaonic Egypt [4] and in other countries in the Middle East, but also in medieval times for masonry mortars in the region around Lübeck in Northern Germany [5], [6] and in the Paris region. Mortar samples from several Gothic cathedrals in and around Paris have been analysed by Adams et al. [7] and their study showed that gypsum mortars were used for the cathedrals of Chartres and Bourges in the early 13th century. A review of the use of gypsum mortar in historic buildings in Europe can be found in Livingston et al. [8].

Until 1970–1980 the characterisation of historic mortars was mostly based on traditional wet chemical analyses [9], [10], [11], [12]. The interpretation of these results however is difficult and often impossible without a good knowledge of the nature of the different mortar components [1], [13], [14]. The majority of later mortar characterisation and/or identification schemes propose optical microscopy and X-ray diffraction techniques as a first step in the qualitative identification of the different components of the mortar. These procedures describe several chemical and other analytical techniques for further qualitative and quantitative analyses like SEM-EDX, microprobe, DSC/DTA/TGA, FTIR, etc. [15], [16], [17], [18]. The choice of the appropriate analytical technique depends mainly on the questions that have to be answered and on the amount of material available. There are at least three distinct fields of interest with different approaches and different requirements; namely the conservation field of historic monuments, the archaeological field and the academic material research field. The conservation field is inter alia interested in specific information about a historic mortar to formulate compositions for compatible repair mortars [19], [20], [21], to know possible causes of degradation of old mortars and to distinguish different building phases through history. The requirements for building conservation regarding formulations for a repair mortar are mainly the hydraulicity of the binder, the mixture proportions (aggregate/binder ratio) and the aggregate grading in order to identify the necessary components to produce a compatible mortar [22]. The archaeological field is interested in the chronology and spatial distribution of the raw materials and of the finished products and can use the information obtained to draw socio-economic conclusions on the provenance of the raw materials and on the production processes (lime burning, mortar mixing, etc.) [10], [14], [23], [24]. More fundamental material research studies use several advanced analytical techniques with the aim of enhancing our knowledge of the burning, the mixing, the hydration and the carbonation processes and of identifying the different mineral phases formed.

Comprehensive information on the historical use of lime-based mortars, about the burning of lime or on the production process of historic mortars can be found in Furlan and Bissegger [1], Sbordoni-Mora [25], Adam [26], Mallinson and Davies [3], Wisser [27], Knöfel and Schubert [28], Middendorf [29], Teutonico et al. [30], Lamprecht [31], Callebaut [32], Pallazzo-Bertholon [14] and Blezard [33].

Section snippets

Polarisation fluorescence microscopy (PFM)

In this paper the attention will be focused on polarised light microscopy of resin-impregnated thin sections although for some purposes other techniques might be better suited. Polarised light microscopy (PLM) is one of the fundamental techniques used in petrology for the study of minerals and rocks. The Scottish geologist, William Nicol, constructed in 1827 the first polarising microscope and Sorby's [34] significant paper on the microscopical structure of crystals started the huge interest in

Other microscopical techniques

The study of polished sections using reflected light microscopy can be very useful for identifying different mineral hydraulic phases (C2S, C3S, C4AF, etc.). This technique is most often used in Portland cement clinker studies and requires the use of selective etching techniques, details of which can be found in the handbook of Campbell [46]. The application of this technique to historic mortars is rather rare [1], [33], [47] and is mostly limited to the study of historic mortars from the 18th

Applications using PFM qualitative analysis

The most straightforward application of PFM for the study of historic mortars is the identification of the inorganic and organic aggregates (inert) and of the various mineral additions (latent hydraulic).

Applications using PFM quantitative analysis

Quantitative image analysis applications using PFM are limited because of the complexity of the composite material. Therefore point-count methods or linear traverse methods (Rosiwal) are often used especially to determine the volume of the different components of the mortar, what is known as a modal analysis. These methods are based on the mathematical field called stereology. Comprehensive references to stereological interpretations can be found in DeHoff and Rhines [87], Underwood [88] and

Discussion and conclusions

The Polarisation–Fluorescence–Microscopy (PFM) technique using thin sections is of primary importance for the characterisation of ancient mortars both because of the complex nature of these composite materials and of the fact that these mortars are dynamic materials. Ancient mortars are complex composite materials and show a very large variation in aggregate and pozzolanic mineral addition contents and the PFM-method is most suitable as a first step to identify these different inorganic and

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