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is called porphyritic by analogy with the widely found representatives of this group, the porphyries. When effusive rocks have cooled in a massive layer, their structure was similar to that of deep-seated rocks. When the layer was relatively thin, upper layers of the effusive lava became porous because of an intensive release of gases from the magma as pressure diminished.

Fragmental rocks have been formed as a result of rapid cooling of lava fragments ejected during volcanic eruptions. Referred to these rocks are pumice and volcanic ash. Some fragmental rocks (volcanic ash) have cemented to give volcanic tuffs.

Sedimentary rocks have been formed by the precipitation of sub­stances from solvents, mainly water. Precipitation occurred perio­dically and has resulted in separate layers and strata. By the charac­ter of formation and the composition, sedimentary rocks fall into three groups: chemical, organic and mechanical.

Chemical sedimentary rocks (e.g., gypsum, anhydrite, tufas) have formed by the precipitation of mineral substances from water solu­tions, the deposits then consolidating and cementing themselves.

Organic rocks have been formed of remnants of seaweeds and living organisms consolidated and cemented together. Classed with the or­gan.)genous rocks are most of the limestones, chalk, diatomites.

Mechanical sediments (fragmental rocks) have resulted from the deposition or accumulation of loose products in the process of phy­sical and chemical disintegration of rocks. Part of these has been subsequently cemented by clay substances, ferrous compounds, car­bonates or other carbonaceous cements to give cemented sedimen­tary rocks, conglomerates, breccias.

Metamorphic rocks have formed as a result of more or less com­plete modification of eruptive or sedimentary rocks under the in­fluence of high temperature and pressure and sometimes of chemical action. Under these conditions minerals may have recrystallized without melting, the resultant rocks being generally more compact than the original sedimentary ones. Metamorphism has also affected the structure of the rocks. In most cases, metamorphic rocks are dis­tinguished by a shaly structure.

2. Rock-Forming Minerals

The properties of rocks as building materials are governed to a great degree by their mineralogical composition. Some minerals feature great strength, hardness, resistance to chemical attack (qu­artz), others have poor strength, readily soak in water (gypsum). Some minerals display a great tendency to cleavage and split readily along one or several directions (mica), thus decreasing the strength of the rock they make up, etc.

Distinctive characteristics of minerals are their chemical composi­tion and physical properties, e.g., density and hardness.

Among the great diversity of natural minerals only a few form the bulk of rocks, and therefore, these minerals are called rock-form­ing. Falling into this category are quartz, feldspar, mica, ferrous- magnesia minerals, carbonates and sulphates.

In chemical composition, quartz is represented by silicon dioxide SiO_a. This is a very widely distributed mineral in the earth's crust, found naturally in the form of separate rocks (quartz sand and glass, rock crystal) or as a constituent of polymineralic rocks. The density of quartz is 2650 kg/m3; hardness 7 (according to Mohs' scale of hard­ness); compressive strength, about 2000 MPa. Quartz resists attack by acids, with the exception of the hydrofluoric acid, and is weather­proof.

At temperatures between 18 and 20°C quartz fails to react with slaked lime Ca(OH)₈, but in a medium of saturated water vapour and at temperatures between 150 and 200°C it combines with slaked lime to give hydrosilicates. This property of quartz is used to pro­duce artificial stone materials from a mixture of quartz sand and lime, known as lime-sand (silicate) materials. As temperature rises, quartz undergoes physical changes. Thus, at 575°C quartz undergoes a transition from modification p to modification a, its volume increas­ing instantaneously by approximately 1.5%. At 870°C quartz turns into tridymite, the process involving a substantial increase in vo­lume, as seen from the fact that the density of tridymite is 2260 kg/m³, and that of p-quartz, 2650 kg/m³. At 1710°C quartz fuses and if cooled rapidly forms quartz glass.

By chemical composition feldspars are alumosilicates, i.e., compo­unds of silica with aluminium oxide and oxides of alkali metals Ka₂O, Na₂O or CaO. Feldspars readily split along cleavage planes and differ in colour. Their hardness is 6 (one unit less than that of quartz).

By the character of cleavage, feldspars are divided into orthoclases and plagioclases. Orthoclases K₂O×Al₂O₃×6SiO₂ are straight-splitting minerals; plagioclases, oblique-splitting minerals. Classed with the latter are albite, or sodium feldspar Na₂O×Al₂O₃×6SiO₂, and anorthite, or lime feldspar CaO×Al₃O₃×2SiO₂.

Feldspars have a compressive strength of 120 to 170 MPa, and density ranging from 2500 (orthoclases) to 2760 kg/m³ (anortithe).

When exposed to atmospheric agents, feldspars disintegrate more rapidly than quartz, i.e., decay under the action of such agents as moisture and carbon dioxide. Weathering results in the formation of alumosilicates, in particular, of kaolinite, a constituent of clays, and sometimes of calcite (CaCO₃).

Micas are hydrous alumosilicates of complex and varied composi­tion. They are divided into two kinds, the biotite and the muscovite. Biotite contains magnesium and iron oxide impurities owing to which biotite mica is opaque and dark, sometimes black; muscovite is transparent since it carries none of these impurities. Micas split readily into thin elastic plates, which is evidence of their perfect cleavage. The density of muscovite lies between 2760 and 3100 kg/m³, and that of biotite, between 2800 and 3200 kg/m³, their hardness being 2 to 3.

Biotite is a constituent of many eruptive rocks. It weathers more rapidly than muscovite, which is found in eruptive and sedimentary rocks.

Ferrous-magnesian minerals. This group of minerals embraces pyroxenes (the most widely distributed kind being augite), amphi- boles (hornblende) and olivine. Ferrous-magnesian minerals have a complex chemical composition, comprising mainly magnesium and iron silicates. They are green, brown and sometimes black in colour. Their hardness is 5.5 to 7.5, and density, 3000 to 3600 kg/m³. Mine­rals of this group feature high impact strength and resistance to we­athering, with tne exception of olivine.

Weathering of olivine results in serpentine, one of the varieties of which (chrysotile or asbestos) has a fibrous structure and consists of extra-fine and very strong fibres.

The above-mentioned minerals are constituents of mainly eruptive rocks, whereas chief minerals of sedimentary rocks are calcite, magne- site, dolomite, gypsum and anhydrite.

Calcite CaCO₃ or calcspar is one of the main minerals of the earth's crust. Calcite forms large-, medium- and fine-grained rocks; its density is 2700 kg/m³ and hardness 3. Calcite is poorly soluble in water (0.03 g/1), but reacts vigorously with acids. Calcite deteriorates in water containing CO_a> since calcium bicarboniate Са(НСO₃)₂ is formed which is over 100 times more soluble in water than CaCO₃.

Magnesite MgCO₃ is met in nature far less frequently and is harder and less soluble than calcite.

Dolomite MgCO₃×CaCO₃ is a mineral which in chemical composi­tion represents a bicarbonate of magnesium and calcium. In physical properties dolomite is similar to magnesite.

Gypsum CaSO₄×2H₂O is a mineral of lamellar, fibrous or granular structure; it is soft (hardness 2) and has a density of 2300 kg/m³. Gypsum is white, but it may be tinted by impurities into grey, red­dish, yellowish or black colours. Gypsum fairly easily dissolves in water, its solubility being approximately 75 times greater than that of calcite.

Anhydrite CaSO₄ is an anhydrous variety of gypsum. The density of anhydrite is 2800-3000 kg/m³; hardness, 3-3.5; colour, from red­dish-white to grey. When exposed to prolonged contact with water, anhydrite may turn to gypsum with a slight increase in volume.

Kaolinite is a hydrous aluminium silicate. Its individual platesand scales are colourless, whereas solid masses may be white, yello­wish, brownish and green-blueish; its hardness is 2.5.

The minor minerals found in rocks are pyrite FeS₂, apatite (calcium phosphate), etc.

 

8. Chief Igneous Rocks

Among the igneous rocks, distinction is made between the mas­sive and the fragmental varieties, the latter resulting from the disin­tegration of massive rocks.

The chief representatives of massive deep-seated rocks are granites, syenites, diorites and gabbro. Slow cooling of magma at a great depthunder a considerable pressure pro­vides favourable conditions for complete crystallization of magma, with the effect that all the deep- seated rocks show evidence of high compactness and pronounced crys­talline structure (Fig. 1).

Fig.1. Granular crystalline structure of granite

 

Granite is the most widely found deep-seated rock, consisting ma­inly of quartz, feldspar and mica. Sometimes mica is replaced by dark-coloured ferrous-magnesian mi­nerals. The colour of granite depends on the main constituent (feldspar) and the presence of dark Fig 1. Granular crystalline structure minerals. It is also found in grey, nerals show so strong a cleavage that fractures mostly occur not along the planes of cleavage, but across the grains. The bulk density of granite averages 2600 kg/m³; its compressive strength is 100 to 300 MPa; tensile strength, 1/40 to 1/60-th of the compressive strength.

High mechanical strength, weathering and frost resistance prede­termine high building qualities of granite and building materials made of it. Granite is used for facing slabs, staircases, floors, curb stones, crushed stone, etc. Granite is also used in the construction of hydraulic engineering installations and for monuments.

Syenite consists mostly of feldspar (orthoclase) and some dark- coloured mineral. Syenite's structure is similar to that of granite. Its density is 2700-2900 kg/m³; bulk density, 2400 to 2900 kg/m³; compressive strength, 150 to 200 MPa. Syenites, which are softer and more resilient than granite and show greater amenability to polishing, are used for the same purposes as granites. There are (ransitional varieties between granites and syenites, the so-called granosyenites.

In mineralogical composition, diorites are represented by plagio- clases, hornblende, and less frequently, by biotite and augite. Diorite is green darkish to blackish in colour. Its bulk density is 2700-2900 kg/m³; compressive strength, 180-200 MPa. Diorites are hard to work, and have good polishing qualities, high resistance to abra­sion and to weathering. Diorites are used for road construction and as facing slabs.

Gabbro is a crystalline rock consisting mainly of plagioclase and dark-coloured minerals (pyroxenes, such as augite), and less frequ­ently of biotite and hornblende. Gabbro's colour ranges from grey and green to black. Belonging to the gabbro group is also labrado- rite, a rock composed essentially of the mineral labradorite (a variety of feldspar) of grey, green-greyish or dark colour with a blue lustre on cleavage planes. The bulk density of gabbro is close to its density and equals 2900-3160 kg/m3 (which is evidence of high density); compressive strength is 100 to 280 MPa, but sometimes it may be as high as 350 MPa. Gabbro is resistant to weathering, is hard to work, but' its polished surfaces are very durable. It is used for hydraulic engineering and other kinds of installations in the form of rubble, facing slabs, etc. Labradorite of beautiful colouring is used as a facing material.

Effusive rocks have formed during the cooling of magma as it flowed to the surface of the earth. The structure of effusive rocks may be semicrystalline, granular or glassy. Effusive rocks are of the same chemical and mineralogical composition as the deep-seated rocks and have approximately the same physical and mechanical properties, but are distinguished by a fine-crystalline (or even a glassy) structure.

Quartz porphyry (an analog of granite) has a glassy structure with embedded large grains of crystalline quartz. In the course of weath­ering the grains may fall out of the body of the rock. Its density ranges from 2400-2600 kg/m³, its bulk density is close to its density, while compressive strength lies between 130 and 180 MPa. It is used as crushed or piece stone.

Along with the quartz porphyry, there exists a quartz-free porphyry (an analog of syenites) in which no quartz is present.

Trachyte is a rock similar in chemical and mineralogical composi­tions to porphyry, but its origin dates from a more recent geological period. Trachyte features high porosity and a relatively low compres­sive strength ranging from 60 to 70 MPa.

Diabase is an analog of gabbro. It consists of plagioclase and augite and carries a certain amount of quartz and hornblende. Its bulk den­sity is close to its density 2800-3000 kg/m3, its compressive strength lies between 200 and 300 MPa, its colour is dark-grey. Diabase is ame­nable to polishing. It is used in the form of crushed or piece stone, slabs, paving blocks, facing material. Various items may be cast from molten diabase at temperatures between 1200 and 1350°C. Cast diabase is resistant to acids and alkalis, has good dielectric proper­ties and a compressive strength somewhere around 500 MPa.

In chemical and mineralogical compositions basalt is an analog of gabbro. Its colour is dark, it has a cryptocrystalline structure with some amount of volcanic glass, and is composed of plagioclase and augite. The density and bulk density of basalt are close to each other - 2700-3300 kg/m³ - and the compressive strength ranges from 100 to 150 MPa. High hardness and strength of basalts make them suitable for road pavings and also for the manufacture of cast stone mouldings.

Porphyrite and andesite are analogs of diorite. Porphyrite is a more ancient rock than andesite; they are grey, green greyish and yellow greenish in colour. Their density is 2200 to 2800 kg/m³, and com­pressive strength, 60 to 240 MPa. Porphyrites are used as facing material, crushed stone and paving blocks, and andesite, being an acid-resistant material, as aggregate in acid-resistant concretes and also for special facing jobs.

Fragmental rocks fall into loose (pumice, volcanic ash, etc.) and cemented (volcanic tuffs) varieties.

Pumice was formed in the process of rapid cooling of magma, its mass expanding under the pressure of intensively evolving gases. Subsequent rapid cooling of swollen lumps of magma gave rise to a glassy porous rock. The colour of pumice is grey, black and sometimes white; it consists of silica SiO₂ (up to 70%) and alumina A1₂O₃ (up to 15%). Pumice occurs in fragments 5 to 50 mm across, ejected during eruptions of volcanoes. Bulk density of lump pumice is 400 to 1400 kg/m³; porosity, 80%; compressive strength, 0.4 to 2.0 MPa; hardness, 6. Pumice is used as aggregate for lightweight concretes, as heat insulating material and as an active mineral admixture to lime and cements.

Volcanic tuffs. During volcanic eruptions, ashes and sands were mixed with molten lava to form tuff lava. Cemented tuff lava is called volcanic tuff. Tuffs have a glassy structure due to rapid cooling. A typical representative of volcanic tuffs is the artik tuff (so named after a deposit near Artik, Armenia). The bulk density of lump tuff is 1250 to 1350 kg/m³; porosity, 40 to 70%; compressive strength, 8 to 19 MPa and sometimes higher; coefficient of heat conductivity, 0.21-0.33 W/m×°C. Its colour is rose-violet. Tuff is used as aggregate for lightweight concretes and mortars, for large wall slabs and as an active admixture to air-setting lime or cement. Excellent decorative qualities and frost resistance predetermine its wide use as a facing material for facades of buildings.

4. Sedimentary Rocks

As mentioned, sedimentary rocks resulted from the precipitation of salts in drying water basins (chemical deposits), the accumulation of plant or animal remains (organogenous rocks) and the deteriora­tion of massive magmatic or sedimentary rocks (fragmental rocks).

Chemical deposits comprise gypsum, anhydrite, magnesite, dolo­mite and lime tufas.

Gypsum is a rock consisting of a mineral of the same name. It is used for the manufacture of an air-setting binder (construction gyp­sum) and also as a facing material (artificial marble) for interiors of buildings.

Anhydrite is composed of a mineral of the same name-the anhy­drite CaSO₄. It is used for facing and as a source material for the ma­nufacture of anhydrite cement.

Magnesite is so named after its chief constituent mineral-mag­nesite MgCO₃. Sometimes it carries impurities, such as calcium and iron carbonates. The hardness of nlagnesite is 3.5-4.0, its colour being white or yellow to brown. Magnesite is also a raw material for the manufacture of air-setting binding materials (caustic magnesite) and refractories.

Dolomite consists mainly of the mineral dolomite CaCO₃×MgCO₃ with admixtures of argillaceous, ferrous, siliceous and other sub­stances. Its colour is grey or yellowish to brownish, and the structure, granular. In properties, dolomites are close to dense limestones, and sometimes they feature mechanical properties higher than those of limestones. Dolomite is the source material for crushed stone, facing slabs, refractories and binding materials.

Lime tufas have formed in the process of separation of CaCO₃ from calcium bicarbonate dissolved in water. Very porous lime tufas are used as raw materials for the production of lime, whereas dense tufas with characteristic fine and uniformly distributed pores are employed as piece stones for walls and aggregate for lightweight concretes.

Organogenous rocks include carbonate and siliceous varieties. Limestone, shell limestone, marble, diatomites and tripoli are used in building work.

Limestone has formed from animal and plant remains in water basins (or as a product of chemical deposits). Loose accumulations of shells and their fragments were cemented together by calcium car­bonate. Limestone consists mainly of _t the calcium mineral CaCO, with admixtures of clay, dolomite, quartz, etc. The bulk density of limestone ranges from 1700 to 2600 kg/m³, and its compressive strength, from 10 to 100 MPa. The colour is white or yellowish to brownish. Limestones are used for the manufacture of crushed stone, facing slabs and architectural items, and also for the production of lime and portland cement.

Shell limestone is a porous rock composed of shells and their frag­ments cemented together by lime. Its bulk density is 900-2000 kg/m³, and the compressive strength, 0.4 to 15.0 MPa and over. It is used for the manufacture of wall stones and blocks (slabs), and also as aggregate for lightweight concretes.

Chalk is an earthen rock composed of almost pure calcium carbo­nate. Admixtures of argillaceous substances and quartz grains are possible. Chalk is a highly dispersed material. Its colour is white and it is used as a white pigment and in the manufacture of putty, lime, Portland cement and glass.

Diatomite is a poorly cemented very porous siliceous rock formed of shells of diatom algae and partly of skeletons of living organisms. Its bulk density lies between 400 and 1000 kg/m3, and porosity is from 60 to 70%.

Tripoli is lightweight, clay-like rock carrying amorphous silica in the form of fine opal balls. Its bulk density equals 500 to 1200 kg/m³, porosity, 60 to 70% and heat conductivity, from 0.17 to 0.23 W/m×°C.

Diatomites and tripoli are used for the manufacture of heat insu­lation materials, lightweight brick, and also as an active mineral admixture in hydraulic-setting materials.

Mechanical deposits. Physical weathering by water, and tempera­ture fluctuations resulted in the disintegration of rock to lumps, small pieces and fine particles. Products of disintegration were trans­ported by winds and particularly by water streams over vast distances and then settled, thus giving origin to clays, sands, crushed stone and gravels from massive rocks.

Chemical weathering consists in an interaction of rock constituents with various substances contained in the atmosphere. Thus, under the action of water and carbon dioxide of the air, feldspar (orthoclase) formed a mineral called kaolinite

K₂O • Al₂O₃×6SiO₂+2H₂O+CO₃=K₂CO₃+4SiO₂+Al₂O₃ • 2SiO₂ • 2H₂O

Physical and chemical weathering is often accompanied by bio­chemical weathering which is due to the activity of animals and plants.

Weathering of rocks produces fine particles, grains and large frag­ments; some of these are bonded into rocks by clay, calcite or silica. By the size of grains and degree of cementation, mechanical deposits of sedimentary rocks are divided into the following types.

Sand is a loose mixture of grains of various rocks ranging in size from 0.14 to 5.0 mm. By their origin, sands fall into rock, river, sea, dune, etc., varieties.

Gravel is rounded off stones measuring from 5 to 70 mm. It is used as an aggregate for concrete.

Sandstones are rocks composed of grains of quartz cemented by clay­like, siliceous or limestone substances. The strength of sandstones is governed by the kind of the binding substance, the size and the shape of cemented grains. The most resistant siliceous sandstones have a compressive strength of 200 MPa and over. Siliceous and partly lime sandstones are used as crushed stone for concrete, for facing bridge piers and building foundations, and for road surfaces because they are highly resistant to frost and abrasion.

Carbonate conglomerates and breccia. Rocks consisting of cemented stone fragments are called breccia, and those of grains of gravel are called conglomerates. Conglomerates and breccia are used as crushed stone for concretes, as well as piece stones and facing slabs.

1.               Metamorphic Rocks

The metamorphic rocks widely used in building practice are gneis­ses, clay shales, marble and quartzites.

In mineralogical composition gneisses are similar to granite and are shaly in structure. They are used mainly as facing slabs, rubble stone for foundations and walls of non-heated buildings, paving stones for sidewalks.

Clay shales are composed of solid shaly clays. Their colour is dark grey, sometimes black. Clay shales split readily, are weather-resis­tant and durable, which makes them particularly suitable for use as roofing materials.

Marble is a crystalline rock, formed of limestones or dolomites. Its crystals are bound together without the intermediary of a cement­ing agent. The strength of marble may be as high as 300 MPa. Its hardness is relatively low, 3.0 to 3.5. It can readily be sawn and polished and is applied for facing building interiors, as its poor che­mical resistance against sulphurous gases and atmospheric moisture makes it unsuitable for external application.

Quartzites are metamorphic varieties of siliceous sandstones with recrystallized quartz grains intergrown to such a degree that the cementing substance is indistinguishable. Quartzites are weather resistant, their strength is as high as 400 MPa. Applications of quart­zites include facing of buildings, bridge piers and manufacture of dinas (silica) refractory items.

2.               Mining and Working of Natural Stone Materials

Rocks suitable for the manufacture of stone materials are called useful minerals. Rocks accompanying the minerals and not used for the aforementioned purposes are referred to as waste rock. Operations involved in obtaining minerals are called mining. Voids formed in the process of mining are termed excavations, and the mined deposits, quarries.

Natural stone materials are mined chiefly in quarries by the use of excavators, hydromechanization techniques, stone-cutting ma­chines for sawing massive rock, blasting, etc. Up-to-date methods involve extensive mechanization of all production processes.

The choice in the method of mining the natural stone materials depends on the type of rock, depth and conditions of occurrence, hardness, etc. Loose rock materials, as sand, gravel, and clay, are mined in quarries with the use of various machines, of which most popular are single- and multi-bucket excavators, and also by applying the hydromechanization techni­que. Hydromechanization consists essentially of washing the soil away by jets of high-pressure wa­ter ejected from a monitor nozzle. This is followed by the separation of the commercial product (sand or gravel) from the water-soil mixture (pulp).

Sand and gravel are classified at the quarries into two or more size fractions. Crushed stone is. obtai­ned by crushing rocks mined by blasting or any other technique. Rock is crushed, screened and con­centrated at crushing-and-screening plants located near the quarries. A flowsheet of a plant for produ­cing crushed stone from hard rocks is given in Fig. 2.

Rock brought from the quarry is dumped into bins from which it is discharged by vibratory feeders and conveyed to a jaw crusher and then to an intermediate open sto­rage area. Crushed material is then reclaimed and transported by a conveyor to screens, small-size material going to a gyratory cone crusher, while large-size lumps are recrushed in a roll breaker. From the cone crusher, crushed stone is transported to screens for sizing.

 

 

Fig. 2. Flowsheet of multi-stage cru­shing

1 - jaw crusher; 2 - conveyor; 3 - screen; 4 - bin; 5 - dump buck; 6 - roll breaker

 

Massive igneous rocks are mined, as a rule, by blasting. Mechanical means (wedges, power shovels, etc.) are used to separate lumps of stratified, fissured and columnar structure rocks.

 

Fig.3. Machines for mining and treating rock