Natural Stone, directly extracted from Nature, is one of the oldest building materials used by Humans, both because of its great resistance and because of its ornamental characteristics.
In the group of ornamental rocks, granite stands out because of its strength and resistance to alterations, which make it become a very appreciated material in building field from antiquity, as it is proved by the big amount of archeological monuments in optimal state of preservation belonging to different ages in History, scattered around the world. Serve as examples some of them located in Extremadura, which on the other hand confirm the granite deposits wealth we have from old ages.
Photos: ‘Corredor del Dolmen del Lácara’. Aljucén – Bronze Age / ‘Columnas graníticas del templo romano de Augustobriga’. Talavera la Vieja – VII Century BC©Copyright JUAN GIL MONTES 2008
These characteristics of granite are established by its belonging to the group of igneous origin rocks, formed by the crystallization of different materials coming from terrestrial magma which got cold deep inside and very slowly, creating its granular texture. Between the different materials that make it up, quartz, feldspar and mica are the main ones and they appear in variable proportion, forming quartz, feldspar and mica according to their origins. Apart from these ones, other mineral we can find in granite are apatite, tourmalines or garnet.
This variation in the granite composition, despite there are some baselines physical-mechanical characteristics: apparent density 2,6-3 T/m3, real density 2,6-3,2 T/m3, water absorption coefficient 0,1-0,7 % of its weight, compression resistance 80-270 MPa, tensile strength around 3 MPa, cutting resistance 8 Mpa, erosion by rubbing 4-7 cm3; allows it to have significant variations in its attributes according to its type and origin.
Between the main uses of granite as ornamental rock, we could emphasise the following: wall covering and cladding, paving and covering for outdoor use (approx. 50% of demand), ornamentation and funerary art (approx. 30% of demand), urban furniture (approx. 10% of demand) and complementary components for building and indoor design (10% remaining).
Each one of the abovementioned entail a series of specific physical-mechanical requirements, determined by uses and environmental conditions that stone is bearing. This way, a granite that is being used outdoor must present a higher resistance to temperature variations and shorter porosity than some other that is being used indoor; and still in the first example, the material which is being used in wet and cold areas must have higher water and freezing resistance than other one with dry and hot destination. This implies a determining factor when choosing the type of stone.
Because of all this, it is imperative to have detailed analysis of each type of granite, analysis made by authorized petrographic laboratories that make standardised tests (UNE-EN) to determine certain number of physical-mechanical attributes through a standard test protocol, to obtain the technical file card of each type of stone. The importance of having these analyses when commercialising the product is essential, for example, when granites with civil projects destination, as a granite that doesn’t meet the normative requirements is refused for using.
Between the main studied technical characteristics in an analysis we find the following:
Petrographic analysis (UNE-EN 12407:01)
Elastic modulus (UNE-EN 22177:85)
Compressive strenght (UNE-EN 1926:99)
Flexural strenght under concentrated load (UNE-EN 12372:99)
Water absorption coefficient by capilarity (UNE-EN 1925:99)
Frost resistance (UNE-EN 12371:02)
Water absorption by atmospheric pressure (UNE-EN 13755:02)
Knoop strenght (UNE-EN 14205:04)
Abrasion resistance (UNE-EN 14157:05)
Depending on its use, then the different technical characteristics of the granite will gain more or less relevance, giving room for a natural discrimination of the different varieties of granite.
As a proof of the abovementioned showed, the following sheets which explain in detail the relevance of some test, according to the expected use:
TABLE 2: IMPORTANCE OF TESTS REGARDING USES
Following below, as a brief comparison, we’ll detail some of the different physical-mechanical characteristics that the most popular varieties of grey granite have:
As Gris Robledo granite producers, we want to demonstrate that our granite presents the best results between these which are found in a similar range regarding appearance, so we can claim that its quality is above the others.
The importance of a deep understanding is frequently made complicated because of the opacity of the suppliers when giving them, turning it very difficult to get detailed technical sheets, for example when talking about Chinese origin granites.
In conclusion, despite it could seem that similar appearance granites are the same, we can check it is not true, and quality should never be less important than price when choosing, in order to avoid undesirable results in a medium-long-term. It is preferable then to bet for a suitable quality-price relationship , as although it could be more expensive at the beginning, it will be cheaper.