THE HISTORY OF THE ROCK USAGE IN THE BUILDING INDUSTRY
Right from the most distant ages of the history, men have been using several materials for any kind of construction: the 'remains' that reached us are an evidence and enable us to notice that the use of these material in different Countries, has always been conditioned by the lithologic nature of the place.
In all continents there are remains of 'stone' monuments, giving the evidence of the most ancient civilizations. It is sufficient to remember the monuments of the ancient Celtic population, they had both ritual and funeral monuments, consisting of large stone slabs rammed in the ground, on which they put other large slabs creating in this way the shape of a table (the dolmen), or mere rough stones, vertically rammed in the ground and they have a ritual function (the menhir).
The use of the 'stone' for monuments had a great development with the Egyptians (the Pyramids); with the Greeks and the Romans who used more developed excavation and working methods.
The rock classification is based on a generic criterion, which considers the way and the phenomena through which they originated; on this basis, the rocks are divided into three big classes:
SEDIMENTARY ROCKS (or layered or exogenetic rocks)
METAMORPHIC ROCKS (or schistose-crystalline rocks)
It is important to remember that the set of processes that lead to the formation of stony or terrigenous material is called 'rock cycle' therefore any kind of rock belonging to a class can change into a rock belonging to another class subsequently to physico-chemical or mechanical changes.
The eruptive rocks, also called igneous or endogenetic rocks, have a deep origin and they form through the solidification due to the cooling of the liquid magma that is to say of molten masses with a very complex composition, which is mainly silica. They are called igneous because of the incandescence status of magma which is almost lower than 900° C.
are divided into intrusive rocks (rocks that consolidated slowly and that have never reached the earth surface: e.g. granites, syenites, diorites, and gabbros) and extrusive rocks (rocks that consolidated quickly because of the magma that was poured outside both on the earth and marine surface; e.g. porphyries, trachytes, liparites, and basalts).
Also called layered or exogenetic rocks, originate from the sedimentation or laying down of particles coming from the physico-chemical modification (limestones, dolomites, organogenic limestones, etc.) and mechanical break-up of pre-existing rocks (breccias, puddingstones, sandstones, marls).
(or schistose-crystalline rocks) originate from a 'metamorphosis', that is to say the change, that can also be deep, of pre-existing rocks, both eruptive and sedimentary (gneiss, phyllade, mica schist, slate, etc.); worthy of mention are those marbles called, by geologists and petrologists, saccharoidal, nonschistose crystalline limestones.
Granite is an composed acid eruptive rock which usually has a light colour and whose essential components are quartz, black mica, and biotite; the colour changes from white (Bianco di Alzo granite, granite of Montorfano), to grey-white (granite of Elba, granite of Giglio, granite of S. Fedelino), to pink (granite of Baveno, granite of Maddalena, granite of Tula), up to red (Rosso Imperiale granite).
The structure, called granitic rock, is holocrystalline and granular; the crystallization degree, the uniformity of the grain, the physical and chemical characteristics of the essential minerals of the rock, give to this rock a very good compression strength (variable from 1000 to 2500 Kg/cm2), a noteworthy hardness and, consequently, a big wear resistance and a very high durability. It is less malleable than marble and limestones however, if polished, they take a more lasting brilliance. It resists very well to the mechanical stress, and wears out uniformly when it is trampled: granite is used for the floors in the entrance, passages, etc.
Granite, nowadays, is mostly used in the building industry for façades, interior claddings in living rooms, entrance halls, passages, thresholds, granites for stairs; for inner floors, for commercial use and for the funeral industry.
They are very acid, paleovolcanic, eruptive rocks. They have a structure called 'porphyritic', that is to say that it consists of quartz phenocrysts (big crystals), potassium feldspar, plagioclases immersed in a microcrystalline and/or vitreous paste. The colour, which is generally reddish or brown-reddish, is due to a ferric or ferrous pigment. These rocks have excellent technological, mechanical and durability characteristics towards the atmospheric agents and e they are classified, according to the cutting criterion, as very hard rocks. Cutting and polishing procedures on these rocks are extremely difficult, that is why they are the most expensive.
Porphyries are used, because of their high wear resistance and thanks to their surface irregularity, for the pavements on sand beds, in the form of 'cubes' and 'flagstones'. They are also used, after crushing, as crushed stone for concrete used in the pavements or ballasts, etc.
Among the most famous we remind the porphyry of Gleno (from the Alpes near Bergamo), the porphyry of Trentino (Bolzano and Trento) with remarkable characteristics and durability.
Gneiss are schistose-crystalline rocks coming from the dynamic metamorphism of both endogenetic and exogenetic rocks. They are commercially known as serizzo granites, ghiandone granites, and beola. They have very good technological characteristics, their compression strength, usually on the schistosity surface, is included between 900 and 1800 Kg/cm2; its durability is equal to the one of the eruptive rock.
They are used in the building industry, in the form of carved or cleft slabs, for building façades and outer skirting, for door and window jambs, etc.; in the form of cut stone for building perimeter walls, buttress of motorways and railways.
Marbles, are called by geologists and petrologists, metamorphic crystalline limestones, saccharoidals coming from the regional or dynamic or thermal metamorphism; they are made up of calcite to which are often associated authigenic accessory minerals (impurities). Erroneously, in the general language, the term 'marble' is used to indicate any kind of rock used in the building industry.
Here below we cite, broadly speaking, the most significant marbles: white marbles (Statuary), they are without pigmentation or have it in minimal quantity. They have a saccharoidal structure with a very thin, compact, and translucent grain that can be easily carved; breccia marbles, they are polychromatic and originate from the metamorphism of tectonized calcareous breccia; from a qualitative point of view the best ones are those with a white background, then those with a white-yellowish background.
The arabesque marbles are characterized by a kind of natural ornament or decoration of the surface.
The bardiglio, a typical apuano marble, is a saccharoidal, with an ash grey-bluish background due to organic pigmentation traces which presence makes it an excellent decorative material suitable only for indoor use.
The Cipollino is a saccharoidal marble having a white background with rectilinear, curvilinear or corrugated green veins or colour areas.
The colour of the rock is one of the important features that exalts its quality. The black-grey colour is due to the presence of organic-carbonaceous pigments that can change over time because of the oxidation.
The red colour is due to the presence of the 'haematite' mineral (iron oxide with trivalent iron), while the green colour is due to iron oxide (ferrous oxide with bivalent iron); the yellow colour is due to the presence of 'limonite' (iron oxide with hydrated iron oxide). In some limestones there can be some pink-violet shades or areas, this is possible because of the presence of manganese in the form of silicate and carbonate (the rocks of this type, from a commercial point of view, are called 'peach blossoms ').
They are rock originated by the activity of animal organisms (hardly ever vegetable organisms) and by the accumulation of their sloughs at the bottom of the sea or in areas that were occupied during the very ancient geologic epochs; the calcium carbonate represents the predominant mineral component that, together with the animal sloughs, forms strong pure and impure limestone layers (with a calcium carbonate value equal to 50%).
The pietra di Finale (Savona), limestone with visible fossils, has a variable colour going changing from white to reddish, is used as covering material which is not very fine. The pietra di Vicenza (Lessini mountains and Berici cols) is a limestone having a variable colour from white/grey to pale yellow; it shows good durability characteristics and it is used outdoor. It had the peculiarity of 'hardening' over time, therefore it is used in the manufacture of ornamental accessories for the outdoor use.
The yellow and red colours from Verona (giallo Verona and rosso Verona) are characterized by ammonite fossils, sometimes they can be very big. The background colour can be whitish, red, and yellow. They all contain a certain quantity of clay, therefore their use shall be limited to the outdoors. If exposed to the atmospheric agents they tend to crumble by forming cracks and to fade.
From a commercial point of view there are different varieties and among them there is the Broccato (red, very decorative and therefore precious, mainly used indoor); the Aurisina (organogenic limestone very compact, with a grey-earth colour with darker stains, used both outdoor and indoor), the Repen (cliff limestone, with the same characteristics as Aurisina, it can be used without limitation), the Botticino (from Brescia, it is a compact crystalline organogenic limestone, it is slightly metamorphosed, with good technological characteristics, its background colour is white-cream with a shade that sometimes is darker; it is used in the civil and religious building industry both outdoor and indoor), the Pietra di Trani (quite compact limestone, its colour is more or less light and plain on which the fossils stand out, it is used both outdoor and indoor), the Nero and Nero Assoluto (compact limestones that, even if they have reasonable technological characteristics, have a short durability because of the organic pigmentation that can degrade due to oxidation, producing a more or less intense brightening of the surface) it is used above all in the religious building industry and for architectural works.
By Dr. Mario Curcio
A study of applied geology