Natural stone materials

PART II. CHARACTERISTIC OF BASIC TYPES OF CONSTRUCTION MATERIALS

5. Natural stone materials

Natural stone materials are the materials which are obtained both directly during output, and at the subsequent mechanical treatment of rocks.

Natural stone materials due to high mechanical strength, durability, decoratively are widely used since the ancient times as walling and facing materials in construction for paving, hydraulic and other buildings, for strengthening and cladding of slopes, embankments, etc. At present they are about 50% of all of the materials mass applied in the construction. Especially requirement in such construction materials as crushed stone, gravel and sand is large.

Natural stone materials are divided into two groups - regular and irregular shape. Sawn, sledged, rubbed, polished products belong to the first group, and mainly quarry, crushed and graded materials belong to the second one.

Basic ways of application in construction of different products from natural stone are shown in Table 5.1.

Table 5.1

Application different kinds of stone in construction

Setting	Materials and products
Basements	Rubble, sawn and sledged stone
Walls	Wall (sawn) stone, large wall blocks, cut stone
External cladding	Facing slabs and stone, profile elements
Internal cladding	Facings slabs, profile elements
External stairs and grounds, parapet walls and fencings	Steps, slabs for grounds, pillars and walls, facing slabs
Internal stairs and grounds, floors	Steps, slabs for grounds, stair and floors
Highways coatings	Edge stone, pavestone, sledged stone and crushed stone
Hydraulic facilities	Angular, sledged and cut stone, boulders and crushed stone
Concrete aggregates	Angular, sledged and cut stone, boulders and crushed stone

Raw materials for the production of natural stone materials are rocks - mineral aggregations of certain composition and structure, which are the products of geological processes in the earth's crust.

The problem of the careful use of natural raw material, utilization of by-products of its processing such as screenings, stone powder, oversize and others is gained the greater value.

5.1. Rocks

Types, composition and structure of rocks. About thousand types of rocks are known for today. By formation conditions (by genesis) they are divided into three classes:

- Magmatic, or igneous rocks, formed as a result of magma cooling in the bowels of the earth or on its surface, e.g. silicate melts;

- Sedimentary rocks, formed on the Earth surface are as a result of accumulation and transformation of destruction products of rocks formed before, remains of vegetable and animal organisms and products of their vital activity;

- Metamorphic rocks, formed on high deepness as a result of sedimentary and igneous rocks change under the action of high temperature and large pressure, caused by the influence of gaseous substances, escaped from magma, and hot solutions.

Slowly cooled hypogene (intrusive) rocks (granites, syenites, diorites, gabbro, etc.) and formed in overhead horizons of earth’s crust, (effusive) rocks (basalts, andesites, porphyrites, pumice stones, etc.) are outpoured rocks.

Chemical composition of igneous rocks can be expressed by content of oxides of silicon, aluminum, iron, magnesium, calcium, sodium, potassium, hydrogen. The major components of igneous rocks, called minerals, are quartz and silicates. It is calculated, that average minerals content in igneous rocks is following %: feldspars - 60, quartz - 12, amphiboles and pyroxenes - 17, mica - 4, other silicates - 6.

Sedimentary rocks by the way of their formation are divided into three groups:

Fragmental - the sediments mechanical by origin (boulders, pebble, sand, clay);

Chemical – the sediments chemical by origin (sulfates, carbonates, halogen compound);

Organogenic - the sediments biochemical by origin (carbonate, siliceous, carbonic rocks, etc.).

Fragmental rocks are appeared as a result of destruction (weathering) of igneous rocks influenced by temperature, water, glaciers and other external agents, chemical ones are appeared at sedimentation mineral substances from water solutions and organogenic ones are the products of sedimentation of weeds wastes and animal organisms at the bottoms of reservoirs.

C hemical and mineralogical composition of sedimentary rocks is more diverse, than igneous rocks. Along with minerals of initial mother rocks they can include also a series of other ones, formed in consequence of sedimentation (carbonates, mineral salts, etc.).

Among metamorphic rocks two groups are selected: modified changed igneous (gneiss) and modified sedimentary (for example: quartzite, clay shale). In the process of transformation, or metamorphism of rocks chemical and mineralogical composition undergoes certain changes depending on the actual values of temperature and pressure. Thus, at temperature up to 500°С and at pressure 40-90 MPa the various stratified silicates containing hydroxyl groups are appeared. If a temperature is higher, and pressure is lower, the crystals of amphibole, different micas and other similar minerals are appeared.

R ocks differ by the mode of occurrence, structural and textural features (Fig. 5.1-5.3).

The peculiarities of their internal structure, predefined by the crystallinity degree, sizes and form of crystals are meant by a structure of rock, and the complex of signs determined by relative location of rock components in the space they occupy is meant by a texture.

By crystalllinity degree there are differed шт holocrystalline, semi crystalline and glassy structures. Holocrystalline structures are representative for the hypogene rocks, formed as a result of slow magma cooling, and also for the most of the metamorphic rocks. Semi crystalline and glassy structures are representative for outpours rocks.

Depending on the grains sizes the structures are divided onto coarse-grained (sizes of grains over 5 mm), medium-grained (2-5 mm) and fine-grained ones (less than 2 mm). If minerals grains which compose a rock are close by sizes, the structure is evenly grained, in opposite case — unevenly grained. The typical varieties of unevenly grained structures are porphyritic structures, which are characterized by a presence of glassy or fine-grained great bulk in the rock, where large crystals - inclusions are dissipated.

Basic types of textures are uniform, or massive, ordinary texture in igneous rocks, nonuniform (taxite) and gneissoid (foliated structure).

Granites are the most important in the natural stone materials manufacturing among the igneous rocks, carbonate rocks among sedimentary ones (limestones and dolomite), metamorphic ones - gneisses, quartzites and marble.

Granites are acidic (SiO₂>65%) hypogene rocks which are easy to distinguish (Fig.5.1), typical minerals: quartz (20-40%), feldspar (40-70%) and mica - muscovite or biotite (5-20%). Among other minerals amphiboles and pyroxenes occur. Grainy-crystalline structure is typical for granite. The color of granite is determined the colors of feldspars entering into composition, and more often grey, bluish-grey, dark red.

Syenites, diorites, gabbro are close to the granites. Unlike granites they practically do not contain quartz and consist mainly of feldspars and dark colored minerals - hornblende, augite, etc. These rocks are mostly grey or dark-green in colour.

The rocks of granite group belong to the strongest and most resistant rocks, they are well polished. They are applied in construction for cladding of constructions and structures, in particular structures subjected to impact and wearing loads (floors, starlings, etc.). The rocks of granite group are processed also on crushed stone, used as concrete aggregate.

Basalts are the most common outpoured rocks. Feldspar and large amount of dark-colored minerals are included in their composition. The structure of basalts is glassy or cryptocrystalline. High hardness and brittleness complicates their processing. The color of basalts is darkly grey or black. Basalts as well as granites are widely used as a building stone. They are applied in the industry of stone casting, as electrical insulation and acid-resistant materials. In those industries, that basalt, other outpoured rock diabase is applied.

Andesites and porphyrites are the widespread outpoured igneous rocks. Basalts and diabases belong to the high basic rocks (40-50% SiО₂); andesites and porphyrites to the low basic rocks (52-65% SiО₂). Porphyrites and andesites are applied for pavestone making, acid-proof elements, etc.

Carbonate rocks - are mainly limestones and dolomite. Limestones are formed mainly of calcite CaCO₃, dolomite CaMg(CO₃)₂, clay and other minerals are as admixtures in them. Structure and properties of limestones are predefined by terms of their formation. As a result of CaCO₃ precipitation calcareous tuff is formed from the sources of carbonate waters; it is a soft easily-sawn porous rock. A type of calcareous tuff is a travertine which is the result of CaCO₃ precipitation from thermal springs. Travertine has dense, fine-grained structure and is applied as cladding stone.

Some types of limestones are organogenic by origin. They are formed as a result of compression and cementation of skeletal residuals of elementary animals (shells, shellfishes, etc.). Shelly limestone and chalk are the limestones organogenic by origin.

Dense limestones are applied for obtaining cladding details, crushed stone for heavy-weight concretes, and porous ones for wall stone and blocks. Limestones widely are applied as raw materials for obtaining lime, Portland cement and other artificial construction materials.

Along with limestones there are also used conglomerate rocks - sandstones which consist of quartz sand grains, consolidated by clayey, siliceous, calcareous and other substances. Siliceous and calcareous sandstones are the most strongest and resistant.

Marbles, which are applied mostly as cladding materials, are formed as a result of limestones and dolomites recrystallization.

Typical metamorphic rocks - gneisses by mineral composition correspond to igneous rocks of granite type. For them, as well as for other metamorphic rocks, crystalline-grainy structure and foliated (gneissic) texture are typical. Gneisses are applied mainly as crushed stone for highways and for ballasting of railway bed.

Quartzites are formed as a result of metamorphization of quartz sandstones. Depending on the admixtures they can be white, yellowish and reddish colors. Quartzites are used as cladding, acid-proof material, for refractory materials production.

Rocks properties. Application area of rocks is determined by their physical-mechanical properties, predefined by peculiarities of formation, chemical and mineralogical composition, structure and texture. The values of basic properties of a series of construction rocks are shown in Table 5.2.

Table 5.2

Basic properties of rocks

Rocks	Average density, kg/m3	Compressive strength, MPa	Module of elasticity, 104 MPa	Frost-resistance, cycles
Igneous:
granites	2500-2700	100-260	5-10	100-300
gabbros	2800-3000	100-350	9-11	100-300
porphyrites	2500-2700	60-150	6-8	59-200
basalts	2200-3100	110-500	8-8.3	50-200
Methamorphic:
gneisses	2000-2500	10-200	6-7	25-200
quartzites	2500-2700	100-250	7-9	100-300
Sedimentary:
carbonates	1700-2700	5-200	0.2-9	3-300
sandstones	2000-2500	10-250	1.4-5	15-300

Compressive strength is the most important property of natural stone. This parameter is highest for rocks which have a homogeneous crystalline structure. If glass prevails in rocks, their strength decreases, they are more affected by temperature changes. Fine-grained rocks, composed of grains of the irregular, ragged form, have higher strength. Porosity makes significant influence on compressive strength of rocks as it diminishes contact area between. If, for instance, porosity of limestones diminishes from 40 to 2%, their compressive strength grows from 5 to 180 MPa. Flexural strength of rocks is in 10-20 times lower, than compressive strength.

In rocks, especially sedimentary rocks, weak forms can be found, contained as separate layers. Homogeneity of rock properties is the most important indicator of their quality.

While selecting the type of the stone its average density is in great importance; for the rocks of certain mineral composition and structure it can describe their strength and durability. As far as chemical basicity of rocks increases (ratio of basic oxides content to acidic ones) the density of rocks grows.

Frost resistance of the most rocks can be defined approximately by water absorption. Water absorption of dense igneous rocks not subjected to the weathering does not exceed 0.7%, water absorption of sedimentary rocks is 10% and more.

Weathering resistance of minerals containing in the rock has substantial value for durability of natural stone. Quartz is resistant to weathering; orthoclase and microcline are low-resistant; plagioclases, amphiboles, olivine, calcite, dolomite, gypsum and others are nonresistant. Chemically active minerals like sulfides, sulfates and others can negatively influence on the stone preservation. For example, sulfides oxidization on the polished surface of cladding stone causes foxing and results in destruction, oxidization of sulfides in crushed stone - volume increasing and reducing in concrete strength.

In the case of application of natural stone as walling material thermal and sound conductivity, air permeability are very important properties along to porosity, frost-resistance, weathering resistance. All of these properties are correlated and predefined mainly by porosity of rock. Effective wall materials are elements from such high-porous rocks, as tuff, shell limestone.

For cladding natural stone their decoratively and processability are of important value.

The color of the rock is determined by the color of minerals, that it is consisted of. Igneous rocks have the most stable coloring, sedimentary and metamorphic rocks are less proof.

Workability of rocks (abilities to smoothing, polishing) reduces as far as their strength growths, at a coarse-crystalline structure, also at the presence of clots.

Soils as natural materials. Rocks which are the objects of structural engineering activity are called soils. Soils are natural basis, environment and construction material for various constructions. Soils are divided onto:

• Rocky - consolidated rocks, which are practically uncompressed and have compressive strength in the water-saturated state more than 5 MPa (granites, basalts, some types of sandstones, etc.);

• Semirocky or loose rocks with compressive strength in dry or water saturated state less than 5 MPa (gypsum conglomerates, etc.);

• Earth – coarse fragmental (uncemented rocks which contain more than 50% of particles more than 2 mm by mass ), sandy (friable in the dry state rocks which contain less than 50% of particles larger than 2 mm by mass) and clayey (for which certain plasticity is typical).

Soils are divided also by softening coefficient, weathering degree, by solubility in water. Humidity has large influence on strength properties and especially on the soils cohesion. Dry clayey soils at considerable moistening become fluid. At the soil compression the most its density at minimal work is reached at ring optimal humidity.

In construction process of earthworks soil consolidates due to approaching of particles between each other under load action. Compressibility of soils is characterized by reduction of porosity coefficient at increasing in compressive stress. Determination of dependence between the porosity coefficient and compressive stresses is defined by compression equipment.

Strength of soils is characterized their ability to resist to sliding stress. Cohesion c (MPa) and angle of internal friction  (degree), applied for the calculations of load-carrying capacity and resistance of soils are rated to a number of parameters of soil strength. Density and humidity of soil, loading rate influence significantly on the values с and  .

Important parameter of soil at its irrigation estimation is filterability. Depending on the type of soil, volume of its pores and structure of pore space soils have different filtration coefficient (Table 5.3).

Table 5.3

Filtration characteristics of soils

Degree of water permeability	Filtration coefficient, m/days	Type of soil
Practically waterproof	0.01	Clay
Low waterproof	From 0.01 to 0.1	Loam
Waterproof	From 0.1 to 1	Loamy sand
High water permeable	From 1 to 10	Fine sand
Very high water permeable	Over 10	Coarse sand, gravel, pebble stone

For earthworks construction all the types of soils are accepted, with the exception of pulverescent sands, soils which contain the water soluble inclusions, and also soils which contain the decayed organic substances, etc. Clays are used for construction of waterproof elements of buildings, where they are at permanent humidity. Hydraulic earth-fill dams are built of gravel sandy, loamy sand soils. At the construction of earth-to-rockfill and rock-fill dams rocks are used with softening coefficient equal 0.9 (for igneous and metamorphic rocks) and 0.8 (for sedimentary). Stones should have sufficient strength and frost resistance.