- A LITTLE HISTORY
The history of natural stone runs parallel to the history of the human being. The oldest dwellings erected by humans that have managed to survive are made of stone. Sometimes they are crammed with messages and built with formidable slabs and support columns. Humans erected sarcophagi and large pyramids, with stone, which is an enduring and safe material. The sepulcher rock partnership still stands today.
The first cobbled pavements, which were laid in order to meet the demands of new means of transport, were made of stone, which contributed to their dramatic development. The Roman roads are an example of this. Also, important public works such as the aqueduct in Segovia - the most important one in the Roman world – were made out of stone. With 813m in length and 163 arches, it supports a channel carrying water from the Fuenfría Range to the Caserón Tower, from which water was supplied all over the city. Humans used stone to build places of worship as a symbol of their spirituality as well as castles and city walls as defences against external attacks. However, the greatest development of stone as a building material took place during the second half of the 20th century. It was then that the traditional architectural use of blocks of rock as structural elements in buildings was transformed into a superficial one, as thin decorative sheets of stone with no structural function started to be employed in buildings.
This new application of stone is still developing and has allowed current manufacturers to obtain increasingly reduced thicknesses, reaching dimensions of as little as 5mm, which constitutes the authentic layer of skin of architectural elements.
- FEATURES AND PROPERTIES
The most relevant characteristics of natural rock are hardness, resistance to different stress types, composition, porosity, colour, and durability. The latter is perhaps the most important technological feature. There are numerous architectural remains where stone has been the only remaining material, which demonstrates it stability at real scale.
The best uses of natural stone require good knowledge of its properties, which are determined through testing.
- Petrographic analysis to determine composition and structure.
- Resistance to bending stress.
- Resistance to compressive stress.
- Resistance to impacts.
- Slip resistance.
- Resistance to thermal changes.
- Frost resistance.
- Resistance to SO2 (sulphur dioxide) pollution.
- THE OPERATION OF NATURAL STONE
ResearchThe same techniques used in other types of mining research are generally used in the research of natural stone, though specific components related to the peculiarities of natural rock deposits are also considered. Indeed, in the case of natural stone, apart from establishing both mineral mass and physical and chemical properties, it is necessary to determine the minimum volume of non-altered mineral removable. This is what is known as "block size", whose characteristics determine, quite definitely, the profitability of a given operation. This requires exhaustive studies on the fracturing conditions of the massif rock using very detailed geomechanical mapping studies in order to assess this highly important parameter as thoroughly as possible.
Rock blastingOnce the exploitation front is reached, rock blasting is usually carried out with diamond wire or other devices with cutting elements, which allow for a wellsquared prismatic block called primary block to be obtained. Subsequently, the primary block is divided into smaller pieces at the quarry. Their sizes are generally between 2-10m3 although exceptionally, and for special jobs, blocks of up to 80-90 t can be obtained. The price of the larger blocks is proportionately higher, due to the fact that it is difficult to obtain them intact. In the particular case of roofing slates, the primary block, which is called “rachón”, is a completely irregular shape, is smaller in size and it weighs about 5 t. Traditional quarrying
- TRANSFORMATION PROCESSES
The blocks obtained at the quarry blocks are sent to factories for the manufacturing of the finished products that are used in building sites. The block is delivered at the factory and cut into thin layers called slabs or boards. These are typically 2-4 cm thick, but any thickness is possible. When the block is large enough, the cutting is usually done on machines called "weavers". When the blocks are smaller, the cutting is performed with rock cuttings.
The boards of finished products are further divided into smaller pieces, usually on request. The most common examples are plates for cladding, paving tiles, pavers, kitchen worktops, washbasins etc. Diamond blades are normally used although, for special formats, water jet cutting can be used. This is an innovative technology that has been recently providing excellent results.
WATER JET CUTTING
This cutting method requires a highly pressurized water flow which passes through a very small-diameter hole (nozzle). This extremely strong water jet impacts on a very small area of material thereby causing small cracks, which, due to the continuous impact of the water jet, "erodes" the material, which is why we can refer to this as "micro-erosion." There are two systems that are based on this principle. The one which uses only water is suitable for cutting all kinds of soft materials such as wood, food, plastic and so on. The one which uses an abrasive product apart from water is suitable for hard materials such as steel, titanium, alloys, ornamental stone and so on. Some of the main advantages of this method over more conventional ones are: cool cutting (there is no heat that can affect the material); the fact that it is multidirectional (it can cut in any direction), and that the cutting perforates most materials (without previous cutting). Finally, cracks do not appear and a lot of material can be saved due to reduced cutting width.
The main disadvantage of this method lies in the fact that, in the case of some materials with large thicknesses and high hardness, an excessively long cutting time is required, which involves higher costs. Besides, in the case of very thick materials, the “ideal” vertical cutting shape tends to become distorted. This is sometimes exacerbated by incorrect cutting speed.
Surface FinishThe surface finish of the stone determines its roughness or texture and also its tonality. The smoother the surface finish, the more the tonality of the stone darkens. This is specially the case with polished rocks. The most common surface finishes are:
- Polishing: this treatment is applied with fine grain grinds and subsequent polishing with alumina powder, iron oxalate or other similar products which continuously appear on the market.
- Grinding: this surface finishing is achieved using a carborundum grinding of grainsize 60.
- Honed: this surface finish is achieved with a carborundum grinding of grain size greater than 120.
- “Abujardado”: this finish is obtained through the continuous beating of a hammer, or buss hammer, with square pyramidal elements on its face side, which provide a rough finishing.
- Flaming: this kind of finish is obtained with a thermal lance at a temperature of 1,200°C, which causes the release of small particles. This provides a very rough surface finish.
- “Apiconado”: this surface finishing is obtained "chipping" the piece by hand with a suitable chipping hammer.
- Slicing: this is rough finishing which can be achieved by parting the piece with a press with two blades which breaks the material and creates a rough surface. Regarding colour, natural stone is probably the building material with the widest colour range, which, together with its natural or man-made textures, provides endless design possibilities for designers. This variability is related to rock formation mechanisms. Indeed, the large number of completely random variables involved in rock formation processes, which are controlled solely by the physical and chemical laws of nature, provide us with a large range of varieties which, from the point of view of the market, may seem chaotic, but is in reality full of harmony and sensitivity. However, such diversity does not occur at every quarry and may only apply to a single one, even at the same operation level. Occasionally, excessive variety may lead buyers to reject the product. Nevertheless, it should be noted that changes intonality, texture and overall appearance are to be expected in natural stone. This may give natural stone its competitive advantage over other monotonously uniform manufactured materials.
- Treatments for improvement
Boards are usually subjected to different improvement treatments in order to increase the durability of the product. The most common improvement treatments are the following:
- Board unseen side reinforcement with nylon mesh and resins.
- Filling of holes and hollows of the face side with special fillers.
- Priming with fluid resins in order to seal open pores of the stone.
- MACHINERY AND TECHNOLOGY
The extraction and transformation processes which have already been described require specific equipment machinery which, as in any other industrial activity, has evolved greatly in recent times. In the early industrial exploitations of the late 18th century, the extraction processes of natural stone were rather undeveloped, but certainly not lacking in effective technological foundations
The use of metallic or wood wedges, inserted following the direction of the weak plane of the rocks, which is called "hair", allowed for more or less regular blocks to be extracted by applying human strength only. The principle applied here was the fact that the resistance of the rock to tensional stress is ten times lower than to compressional stress. The introduction of helical steel wire, used in combination with sand load as an abrasive, led to the development of reduced thickness planks using well-squared blocks. Strappings made of hardened steel are able to cut the rock with the help of a steel abrasive product called "grit". Alternatively, they may be provided with small diamond plates along their axes, with the same function. Diamond wire machines with numerical control devices are also common in the cutting of medium-sized blocks. Currently, robotics is undergoing a major boost in the natural stone world, and machinery is being manufactured that allows for pieces to be cut using previously designed software or even software designed from the 2D or 3D scanning of the shapes that are to be reproduced.
- APPLICATIONS AND USES OF THE NATURAL STONE
There are many applications of natural stone in the world of construction, covering a wide range of commercial products, which include paving, façades veenering, masonry, roofing and individual cut stones. The design possibilities of ornamental stones are almost endless, especially if we take into account the sheer variety of rocks existing in nature, the combinations that can be obtained with the size and shape of the rock pieces, their colour and appearance as well as the different surface finishes that can be achieved.
PRODUCTS AND MATERIALS
The use of natural stone for the construction of pavements represents the single most important application of this material. The paving of roads resulted from the need to adapt existing communication routes to new transport systems, which led to the construction of the first cobbled road surfaces in the Roman era. Cobbled roads still maintain their validity today from a conceptual perspective.
This kind of application underwent a period of crisis at the start of the second half of the 20th century, which led to the destruction of many cobbled roads in different cities, maybe due to budgeting restrictions or to a lack of expertise regarding adequate laying techniques. Fortunately, stone paving is currently being recuperated, so much so that, these days, the restoration of the old part of a town cannot be envisaged without the use of stone.
However, apart from cobbled streets, other types of outdoor pavement and indoor flooring are also made of stone. In this case it is larger pieces of stone, or tiles, which are used.
Veneering for cladding and facades
These construction work units have replaced traditional masonry as the visible external elements in buildings. From the technological point of view, façade veneering, unlike traditional masonry, does not have a structural function but it can act as an insulating material. Although veneering can be fixed to the building with chemical adhesives or cement mortars, nowadays, the laying is more and more frequently carried out by means of anchorages, thereby creating what is known as ventilated facades.
This construction system, which is considered by many as one of the greatest architectural achievements of the 20th century, requires metallic anchorages made of stainless steel acting as connecting elements between the stone and the wall. An air gap between them can thus be created, which facilitates ventilation, thereby significantly increasing the design life of the rock. It is commonly said that natural stone, like man, needs to breathe, which is why this kind of solution is perfect.
Although the main technological function of claddings for façade claddings is for insulation, day by day innovations in this field appear, creating shapes from different claddings as those of the image at the top, in granite and glass, or on the left, a skyscraper with limestone cladding
Masonry is the most traditional use of stone in architecture. From the regulatory standpoint, masonry can be defined as those stone pieces with a structural function and thicknesses greater than 80mm. If the pieces are irregularly shaped, they are generally called rough ashlars, and if it they are square blocks, ashlars.
The joining of the pieces is carried out with lime mortars or mixed lime and cement mortars in the load-bearing walls, arches, retaining walls, and so on, which constitute the different structural elements of this type of architecture. The use of stone masonry has been largely replaced by other materials such as bricks or concrete. However, there are still exceptional examples thanks to which stone masonry has taken a new impulse as a symbol of wealth, durability and comfort, especially as compared to other materials. A clear example of this is rural Galicia in NW Spain, where the use of stone masonry for a family home is almost a cultural imperative.
Natural stones, like any other construction material, are not unaffected by the development of new products, which mainly result from technological advances both in cutting processes and adhesives. In this regard, it is worth mentioning the tiles or panels involving a stone slice of about 4-5mm in thickness to which a cheaper material has been adhered with suitable adhesives, thereby providing the material with enough rigidity. The materials used on the stone may be ceramic or made of fibre cement, or they may be lightweight honeycombed aluminium structures or a nylon mesh with a thick enough layer of resin. The main advantage that these products share is that they all entail a more efficient use of mineral resources since they require a smaller volume of stony material per square metre of covered area. Some products, especially with honeycombed structures, allow us to manufacture large but very light panels, which significantly improves laying time, even though the final cost may still not be the lowest. Other rather novel products which are increasingly entering the market are aged stone units. These are pieces that have been subjected to physical erosion treatments with abrasives, such as steel shot projection, or chemical erosion, which provides them with a rough, altered look similar to that of those materials that have been degraded by use and or by the passing of time.
- CLASSIFICATION OF STONE
Natural stone may be classified according to the genetic classification of crustal rocks:
Plutonic rocks They are formed by slow cooling and consequent solidification of magmas inside the terrestrial crust. Theyare also called crystalline rocks because the slow solidification process allows complete mineral matter crystallization. Among plutonic rocks, the most commonly used ones are acid rocks, above all granite,which is the basic constituent of continental plates, and syenite, also called pink quartz due to its highalkali feldspars (orthose) content.
Volcanic Rocks The outcropping magma in volcanic complex has reached the surface and it undergoes rapid cooling, which hinders minerals crystallization. Basalt is probably the best-known rock among volcanic rocks. Its composition is the same as that of the Earth’s crust, although it is rare in continental zones.
They originate as a result of the external geodynamic activity on the surface of the Earth's crust. On the surface, rocks become weathered, disintegrate or decompose. Later on, different types of rocks generate in different environments due to diverse crystallization, sedimentation and burial processes. These rocks can be further classified into two main groups depending on the genetic process involved in their genesis, namely,
Detrital Rocks, which are generated by accumulation of more or less cohesive fragments of otherrocks. In turn, they may be classified according to the size of the fragments. Sandstones are the mostappreciated as natural stone due to their cohesion.
Chemical rocks, which are formed by mineral Carbonates are the most frequently occurring and alsothe most used subgroup, i.e. limestone, mainly composed by microscopic calcite crystals (calciumcarbonate), although other minerals may also occur, but far less frequently: silica, clays, iron and manganese oxides, organic matter… and other carbonates. Iron oxides confer interesting yellow, orange,red and black coloration. Meanwhile, organic matter confers black or grey colours. Fossils are very common and can be clearly observed in polished sections. If dolomite (magnesium carbonate) content is high, the term dolomite rock is used. Depending on the content of either carbonate (calcium or magnesium carbonate), rocks will receive different names, such as dolomite rock, limestone or calcareous
In industry, both sedimentary rocks, i.e. limestone and dolomite rock, may be called "marbles" due to their appearance once polished, although, as discussed below, marble is not a sedimentary but a metamorphic rock. The presence of fossils in rocks determines their sedimentary origin. Tufas are limestones in whose genesis the photosynthetic activity of microorganisms and vegetables is involved. Carbonate material accumulates around them, thus preserving a high porosity which leads to a decrease in weight. Tufas have been widely used in popular architecture (for arches and vaults) as they can be easily worked, maintaining good consistency and compactness. Evaporites are also a type of chemical rock, so called because they form in water masses that are subjected to intense or complete evaporation. Only rarely have these rocks had any interest as natural stone, but there exists an evaporite rock in the Ebro Valley that, due to its post-sedimentary evolution, displays characteristics that have turned it into a valued material. This rock is alabaster, to which a chapter will be dedicated. At a global scale, the most important extracting sites for this rock are located in Aragon.
All types of rocks may be subjected to intense pressure and/or high temperatures, as a result of the activity of the Earth's crust. This gives rise to major transformations in texture, structure and atomic organization in minerals. Rocks that have undergone such transformations are called metamorphic rocks. This group includes a variety of lithologies, among which marble, quartzite, slate or serpentine stand out.
Marble is the metamorphic rock that is most important in mining. It is a carbonate rock that has undergone metamorphism due to the proximity of hot magma, thus presenting different mineralogical composition and texture with respect to the original rock. It is, together with alabaster, the most representative natural stone because it was used by the greatest masters of the Renaissance as a raw material for their sculptures.
Slate is used mainly for roofing, as an alternative to tiles, due to the development of schistosity planes that have resulted from pressure metamorphism. Slabs can thus be obtained, their usual thickness being 2-8mm.
Quartzites are mainly composed of quartz, which is a stable mineral in surface conditions. From the point of view of alterability, this confers the rock interesting characteristics. Its use as natural stone is scarce given its high hardness.
Serpentinite is another metamorphic rock with great ornamental interest. Its colour is green and may display various tonalities, from light to dark. It is formed by regional metamorphism of ultramafic magmatic rocks (peridotites). It is frequently used in emblematic spaces such as palaces and magnificent hotels, both in floor coverings and wall linings or skirting boards. It has also been called "green marble".
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