Tests of Decorative Stones

Petrographic examination

The test is based on visual assessment of the rock appearance and microscopic appearance of rock slices and polished sections. It consists of macroscopic description (associations of minerals and clasts), determination of the size of quartz grains, grains and fractions of stable rocks, types of feldspars and micas, description of roundness of clastic grains, forms of coal mass and types of groundmass (material of gaps), and forms and uprise of binder. Microphotography of the rock, and/or infrared spectrum used to identify clay mass or fragments of fine-grained rocks are also presented. If there are pebbles over ca. 5 mm, identification of pebbles on the sample slice is performed as well. Rock type, maximum size of pebbles and roundness are determined. Results of XRF and XRD analysis were also used for a more detailed description. The test was performed at the Institute of Geonics of the Czech Academy of Sciences, Department of Laboratory Research on Geomaterials according to ČSN EN 12407 – Natural stone test methods – Petrographic examination.

Apparent density and real density; total, open and apparent porosity

Bulk density and open porosity are calculated from the known weight of the dried specimen, weight of the same specimen saturated with water under vacuum (pressure lower than 2 kPa) and weight of the specimen immersed under the water surface, i.e. based on the Archimedes law. Open porosity thus represents the ratio percentage of pores in the specimen volume, to which water penetrates under specified negative pressure.

The specific density is determined for the finely ground test bodies based on the determination of their weight and volume of water displaced by the ground samples (pycnometric determination) during the test. It is expected that pores inaccessible to water are removed during grinding. The total porosity is calculated from the specific density and bulk density, and thus includes also closed pores, i.e. pores inaccessible to water during soaking of the stone (contrary to open or apparent porosity).

The tests described above were performed at the Research Institute of Inorganic Chemistry according to ČSN EN 1936: Natural stone test methods – Determination of real density and apparent density, and of total and open porosity.

The apparent porosity was calculated as the product of bulk density and water absorption determined at atmospheric pressure (see below) divided by water density (998 kg/m3 at 20C). The apparent porosity thus represents percentage of the volume ratio of pores penetrated by water under atmospheric pressure. The determination of apparent porosity is not described in the standard mentioned above; the values of apparent porosity are, however, often published in academic literature.

Water absorption

The test is based on the determination of weight of the dried test specimen and of the test specimen soaked with water under specified (standard) conditions. The result indicates the percentage of the weight increase of the dried stone as a result of its absorption of water under atmospheric pressure.

The test was performed at the Research Institute of Inorganic Chemistry according to ČSN EN 13755: Natural stone test methods – Determination of water absorption at atmospheric pressure.

Water absorbtion coefficient

The test is based on the measurement of the rate of weight increase of test bodies immersed instantly with one plane surface into water. The rate of increase is governed by the laws of capillary action in porous bodies, according to which the increase is ideally proportional to the square root of the absorption time. The value of the slope in these coordinates, referred to as water absorption coefficient, and is the result of the test. The higher the value is, the more rapidly the stone saturates with water. The action stops when the saturated state has been reached, characterized by the value of water absorption.

The test was performed at the Research Institute of Inorganic Chemistry according to ČSN EN 1925: Natural stone test methods – Determination of water absorption coefficient by capillarity.

Linear thermal expansion

The length change of the test specimen caused by the change of its temperature from 20 C to 80 C is measured. The result is expressed as the linear thermal expansion, which is the ratio of increase in the length of the test specimen to the product of its total length and temperature increase (i.e. 60 K). The result is usually formatted in so-called engineering notation. The value of 8.5×10-6 K-1 thus means that the stone with a length of 1 m heated by 1 K is extended by 8.5 µm.

The test was performed at the Research Institute of Inorganic Chemistry according to ČSN EN 14581: Natural stone test methods – Determination of linear thermal expansion coefficient.

Compressive strength

The cube- or cylinder-shaped test specimen is placed with its front planes between steel plates of the press loaded with increasing force, which is recorded. The compressive strength is calculated as the ratio of the maximum applied force before the sample destruction and the cross-area of the test specimen.

The determination of compressive strength was performed according to ČSN EN 1926: Natural stone test methods – Determination of compressive strength, at the Testing Laboratory of the Rock Research Centre of VŠB – Technical University Ostrava, Faculty of Mining and Geology. It is a test laboratory accredited by the Czech Accreditation Institute. The test was performed with dried samples using MTS 816 Rock Test System.

Flexural strength

The beam-shaped test specimen is placed on two bearers. In the middle of their distance, the specimen is loaded against the bearers with a gradually increasing force. The flexural strength is calculated from the known force causing breakage of the test specimen, the cross-section of the body and the distance of the bearers.

The determination of flexural strength was performed according to ČSN EN 12372: Natural stone test methods – Determination of flexural strength under concentrated load at the Testing Laboratory of the Rock Research Centre of VŠB – Technical University Ostrava, Faculty of Mining and Geology. It is a test laboratory accredited by the Czech Accreditation Institute. The test was performed with dried samples using MTS 816 Rock Test System.

Abrasion resistance

The stone slab is abraded in the prescribed way using a grinding wheel while adding standardized abrasive. After the specified time, the thickness of the ground groove is measured.

The test was performed according to ČSN EN 1341: Slabs of natural stone for external paving – Requirements and test methods at the Testing Laboratory of the Rock Research Centre of VŠB – Technical University Ostrava, Faculty of Mining and Geology. It is a test laboratory accredited by the Czech Accreditation Institute. The system of FORM + TEST Seidner + Co. GmbH was used to perform the test.

Frost resistance

Test bodies soaked with water are cyclically frozen in the air and defrosted in water in the prescribed manner. Changes in their appearance and mechanical properties due to this stress are assessed. The flexural frost resistance coefficient represents a percentage decrease of flexural strength and, by analogy, the compressive frost resistance coefficient is a percentage decrease of flexural strength, both after 50 freezing cycles. After 100 freezing cycles the samples are visually evaluated according to the following scale: 0 – Specimen intact,
1 – Very minor damage (minor rounding of corners and edges) with no loss of integrity,
2 – One or several minor cracks (≤0.1 mm wide), or detachment of small fragments (≤0.1 mm2 per fragment),
3 – One or several minor cracks, larger than those defined for grade 2; alteration of materials in veins,
4 – Specimen broken in two pieces; or major cracks,
5 – Specimen broken in multiple pieces or disintegrated.

The test was performed according to ČSN EN 12371: Natural stone test methods – Determination of frost resistance at the Testing Laboratory of the Rock Research Centre of VŠB – Technical University Ostrava, Faculty of Mining and Geology. It is a test laboratory accredited by the Czech Accreditation Institute.

Particle size based on image analysis

The orientation particle size curve of the rock is determined by computer analysis of an image taken by optical microscope from rock slices using transmitted polarized light. The measured values of grain lengths are arranged and sorted in categories (fractions) corresponding to sieves with mesh sizes of 0.063; 0.1; 0.2; 0.5; 1; 2 and 4 mm. The lengths of all measured grains and the grain lengths in individual dimensional fractions are added up. The total length of grains in each fraction is expressed as a percentage of the total length of all measured grains. The measurement is performed with stereological correction to the size of quartz grains in slices according to Krumbein, i.e. all measured lengths of grains are multiplied by the empirical coefficient of 1.23.

The test was performed at the Institute of Geonics of the Czech Academy of Sciences, Studentská 1768, 708 00 Ostrava-Poruba, Department of Laboratory Research on Geomaterials.

X-ray flourescence analysis (XRF)

X-ray fluorescence analysis is a physical method where atoms of the analyzed sample are excited by X-ray radiation. During deexcitation, secondary X-ray radiation is produced – when electrons move from higher to lower electron shells – with the line spectrum of wavelengths characteristic for each element. Measured wavelengths and intensities of secondary X-ray radiation can be used to determine the content of chemical elements in the analyzed sample. As a rule, the result is expressed in weight percentage of oxides of identified elements.

X-ray fluorescence analysis was performed at the Research Institute of Inorganic Chemistry using Philips PW 1404 spectrometer equipped with UniQuant analytical program yielding semi-quantitative determination of the content of 74 elements (from fluorine to uranium).

X-ray diffraction phase analysis (XRD)

X-ray diffraction phase analysis can be used to determine phase composition of the samples. X-ray radiation interacts with atoms of crystals producing diffraction interference waves that are detected, while each crystalline structure exhibits a typical and unambiguous spectrum. The wave intensity is proportional to the content of the given structure in the sample.

X-ray diffraction phase analysis was performed at the Research Institute of Inorganic Chemistry using Philips MPD 1880 diffractometer, by evaluation of the diffraction data using X´Pert programs (X´Pert HighScore Plus Software version 2.1b and X´Pert Industry Software version 1.1g). Phases were identified using ICCD PDF2 database of diffraction data containing about 107,000 of inorganic standards.