Micro-CT and mercury intrusion porosimetry characterization of the fabric of roofing slate

The petrological fabric of roofing slate determines the main properties of the slate as a construction material, such as durability and waterproofing. Roofing slates are rocks derived from the low-grade metamorphism of lutites, with a characteristic lepidoblastic fabric. However, little attention by petrologists has been paid to the role of this fabric in the behaviour of roofin slates. This work characterizes the fabric and pore system of two roofing slate varieties from Spain, using two different techniques, each with its advantages and disadvantages: X-ray microtomography, useful to find heterogeneities and mineral inclusions on the slate bulk,and mercury intrusion porosimetry, which defines the pore system. The differentiation of mineral inclusions is very useful for predicting the weathering of a slate, while the definition of the pore system may help to understand how the slate will behave during its service life

A morphological and size-based study of the changes of iron sulfides in the Caples and Torlesse Terranes (Otago Schist, New zealand) during prograde metamorphic evolution

It is widely accepted that metamorphism induces a remobilization of iron sulfides, sweeping away original ones while creating new ones. This paper analyzes size distributions of iron sulfides in several samples from the Caples and Torlesse terranes from the Otago Schist (New Zealand) using high-resolution X-ray computed tomography, which allows all iron sulfides larger than the resolution at which X-ray scans were performed to be characterized. Framboids and clusters of framboids are common in unmetamorphosed samples, but disappear in greenschist/amphibolite facies samples, where iron sulfides have anhedral habits. By contrast, the size and standard deviation of the new iron sulfides both remain within the same range. The results illuminate the evolution of iron sulfides throughout metamorphism, proposing boundaries for the metamorphic processes based on the shape of these iron sulfides

Petrography of roofing slate for quality assessment

The commercial term roofing slates refers to slate and stone for discontinuous roofing and external cladding, according to the European Norm EN-12326. This group comprises stones such as slate s.s., shale, phyllite, and carbonate slate, but even schist, cinerite, sandstone, meta-arkose, schistose quartzite, quartzitic flagstone, gneiss and mica schist. However, the main lithologies used for roofing are the first four. All these rocks have a well-developed and layered structure in common, allowing them to be split into large, thin and regular tiles. There are several places in the world where roofing stone could be quarried, but nowadays the main exploitations are located in Spain, China and Brazil. The difference in nature (petrology and characteristics) of the roofing stones from different parts of the world plays a major role in the final use, although builders often think that all stones are equally suited for the same purpose. Generally speaking, almost all of the World’s roofing slates are mainly composed of quartz, mica and chlorite, together with some accessory minerals such as iron sulfides, iron oxides and carbonates. The weathering of these two minerals creates characteristic pathologies, oxidation and gypsification, respectively. Some iron sulfides alteration causes oxidation, which can be seen as brown to red stain marks over the surface of the tile. Oxidation is far the main cause of complaints and rejections, even when is just a aesthetical issue. On the other hand, gypsification can lead to the tile destruction, but it has less impact on the complaints. Since weathering is related to the mineralogical composition and so indirectly to the origin of the roofing slate, identifying and quantifying that mineralogy becomes critical to secure a proper use of the slate. In this sense, optical examination with Polarized Optical Light (POL) using both transmitted and reflected light and X-ray Diffraction are powerful methods, especially when combined with other techniques, such as Micro Computed X-ray Tomography (MCT). POL and XRD give information about the type of the rock, its internal structure and mineral components, while MCT gives information about the occurrence, size and distribution of the metallic minerals, specially the iron sulfides. This works revises the main types of roofing slates found nowadays in the market, together with their main characteristics found by POL, XRD and MCT

Iberian roofing slate as a Global Heritage Stone Province Resource

The Iberian Peninsula is, nowadays, the main producer of roofing slate of the world. Most of the outcrops are located in the NW of the Iberian Peninsula, in the regions of Galicia, Leon, and in Portugal. The technique of working and roofing with slate was brought from Flanders by King Felipe II by the second half of the XVI century. The most representative building from this period is the Monastery of El Escorial, N Madrid. However, the Spanish slate industry remained incipient until the 1960s, when Galicia and Leon suffered an accelerated industrialization process which greatly enhanced the volume of production. Additionally, the Portuguese slate industry was well developed by the second half of the XIX century. Most of the Portuguese production was exported, mainly to the United Kingdom. By the second half of the XX century, the Spanish, and in a lesser extent, the Portuguese roofing slate spread all over Europe, forcing most of the existing European quarries to close. Nowadays, different varieties of roofing slates are quarried, mainly in Spain, being used indistinctly in new residential construction and for restoration of historical buildings. The main importing countries are France, Germany and the United Kingdom. This work presents an overview of the history and main varieties of the Iberian roofing slate, in order to propose its inclusion as a Global Heritage Stone Province

Analysis of the correlations between freeze–thaw and salt crystallization tests

Two of the most popular weathering tests used for dimension stone are EN 12371 freeze-thaw (FT) and EN 12370 Salt Crystallization (SC). These tests are used to assign a durability value to the rocks. Both tests are based on the cyclical growth of crystals in the porous system of the rock, which causes structural stress on the rock matrix that may affect its integrity. The weathering mechanism is the same in both tests although the rate of volume increase is higher for the salt crystals. Due to this similarity, the two tests provide complementary information. The EN (European Norms) recommend evaluating the results together, but do not establish correlations between the results of these two tests for different types of rocks. Knowing these relationships would improve the understanding of the results and the response to weathering of the rock once placed in a building. In this work, several ornamental rocks (sandstones, limestones, dolostones, and a travertine) from the Iberian Peninsula were submitted to FT and SC tests. Rocks were mineralogically and petrographically characterized, and their porous systems were defined by a combination of techniques (scanning electronic microscopy, digital images analysis, and mercury porosimetry). The results of the tests were quantified numerically to compare them. The velocity of propagation of ultrasonic waves was measured before and after the tests. After the statistical analysis, significant correlations were found between the FT and SC tests, and between the connected porosity and the durability index of the rocks