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New uses for X-ray Tomography in natural building stones: characterization, pathologies and restoration of historical and recent roofing slates

Final Report Summary - TOMOSLATE (New uses for X-ray Tomography in natural building stones: characterization, pathologies and restoration of historical and recent roofing slates)

TOMOSLATE. New uses for X-ray Tomography in natural building stones: Characterization, Pathologies and Restoration of historical and recent roofing slates.
The TOMOSLATE project was funded by the ERC through an IEF Marie Skłodowska-Curie action running during the years 2015 and 2016. This project was focused on the study of the characteristics of roofing slate using X-ray Micro Computed Tomography (MCT), together with other analysis techniques. Roofing slate is a common dimension stone with an important presence in historical heritage buildings in the European continent. Europe is the first producer (Spain) and consumer (France, UK and Germany). Roofing slate industry has then a special importance in Europe, since an important part of its architectural heritage is built with this special type of rock. In spite of its importance, there are few studies about roofing slate, especially compared with the abundant scientific literature regarding other dimension rocks such as marble or granite. This project was conceived to fill these knowledge voids, and to propose solutions for the main problems of the European roofing slate industry (competence from other roofing materials, pathologies and durability), always thinking on the direct application of the obtained results.
The project was proposed based on the following main objectives:
- Petrological characterization of each variety of roofing slate. Roofing slate is the commercial term used for the building rocks used for roofing. Most of these rocks are slates s.s. but other rocks can be found such as argillites, phyllites and mica-schists. The properties and aspect of these different rocks differ from one to another. So far, there was not a formal classification for roofing slates. The roofing slate industry uses commercial names, instead of the petrological denominations for each rock. The result is a general confusion about the nature and characteristics of roofing slates. There is a clear need for a concise and scientific classification for roofing slates.
- Characterization of the main pathologies on roofing slates and proposal of a methodology to mitigate the incidence of weathering on roofing slates. The most important pathologies affecting roofing slates are iron sulphides oxidation and gypsification, causing important losses to the industry. The mechanisms and effects of these pathologies were studied and characterized. For the first pathology, a solution was proposed, consisting in the application of a special protective paint using a spray can.
- Analysis of the standardized test methods applied in roofing slates. These test methods evaluate the performance of roofing slates. Nowadays, there are four roofing slate standards in the world (EU, USA, China and India), each of them with their own test methods. Since roofing slate is a global stone, it is necessary to compare among the different test methods in order to interpret different results in a common framework.
An important part of this project was the dissemination of the results by different activities. Technical talks were given in the main forums of roofing slate in Europe and the USA (International Federation for the Roofing Trade and National Slate Association, respectively). Additionally, the existence and objectives of the project were explained during visits to slate quarries, companies and thematic museums. During this project, a web of collaborators was developed.
The application of MCT determined, with high accuracy, the occurrence of iron sulphides on roofing slates. As pointed before, iron sulphides are responsible of the main pathology on roofing slates, oxidation. With MCT, the abundance and size of small (3-50 µm) iron sulphides (micropyrites), was determined, allowing inferring the oxidation potential of a given roofing slate. An unexpected result at this point was the geological meaning of these iron sulphides populations. The data provided by MCT, and analyzed with a proper statistical technique, opened a new research line with several applications in other fields of Geosciences, such as paleoecology, ore mining or metamorphic petrology. The first results of this new research line were published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology.
A new classification was proposed using two distinctive features of roofing slates, petrology and colour. This classification was first presented during the 35th International Geological Congress (2016) in Cape Town. There are four types of rock used as roofing slate: low-grade slate, slate s.s. phyllite and mica-schist. These rocks correspond to a continuous sequence of low-grade metamorphism of pelitic rocks. They share a common characteristic, a continuous slaty cleavage that allows obtaining regular and thick tiles, but their constructive properties differ, especially the mechanical behaviour. Besides petrology, colour plays also an important role, since it is the result of the mineralogy of the rock. There are three colour families in roofing slates, purple-red, green and black. The combination of these features defines twelve lithotypes. Each of these lithotypes generally describes the aspect and petrology of the roofing slate. This classification allows inferring the general technical characteristics of each slate lithotype concerning weathering and mechanical behavior as well as indicating the correct test methods for each lithotype.
The review and analysis of the roofing slate standard methods highlighted some facts which could lead to an improvement in the standards themselves, especially the European Standard EN 12326 and the American Standard ASTM C406. The results of this review were published in the journal Construction and Building Materials.
Regarding the oxidation of iron sulphides, a method based on the application of a protective coating paint using a spray can was proposed to avoid or reduce it. This product was already available on the market but never used for this purpose. However, it needed some modifications (e.g. colour) to be applicable for slates.
Several companies were contacted in order to ask them to produce a prototype of the desired product according to the provided specifications, such as colour. Finally, a company from Barcelona, Spain (Montana Colors) agreed to produce the first prototypes. In order to test the viability of this product, a set of experiments were performed at the facilities of the Geology Department from Ghent University. These experiments consisted in Thermal Cycles according to EN 12326-2. However, this test proved to be inefficient, since it is designed to measure the oxidation of iron sulphides in natural slate, not the protection against oxidation given by a product. However, the data provided were the best approximation for the behaviour of the product available. The only reliable data would be field testing. The effectiveness of the protection of this prototype will be tested by installing an experimental slate cover at the Laboratory of the Slate Technological Center in Galicia facilities, Spain, by the first half of the year 2017. These facilities have the technicians and instruments to completely fulfill the requirements of the final development of this method.
The activities held during the TOMOSLATE project allowed a deeper understanding on many facts related with roofing slates. The outcome of this project can be separated into two parts, one of industrial interest (analysis of pathologies and the product to prevent these pathologies) and one of academic interest (analysis of micropyrite populations with MCT, classification of roofing slates). The results of the project are still being disseminated among the roofing slate industry, and in the following months more scientific publications will be published.