ABOUT STONES

 The engineering structure are composed of material. These materials are known as the engineering materials or building materials or material of construction. it is necessary for an engineer to become conversant with the properties of such materials.

The services conditions of building demand a wide range of materials and various properties such as water resistance, strength, durability, temperature resistance, appearance, permeability, etc. are to be properly studied before making final selection of any building material for a particular use. It is significant to note that a universal building material for application in all conditions is yet to be a found out.

The grouping of building materials is done by considering specific properties of the materials such as stones, ceramics, cement concrete, timber, metals, etc. eaach group is then suitably sub-divided. The application of each building material in the engineering field is predetermined by its chief or main properties. When alternatives are available, the final choice is made from the considerations of engineering and economics. Thus the materials of construction, for the purposes of convenience, can be broadly classified in the following three groups:

(1) cement materials such lime, cement, mortar, etc.;

(2) protective materials such as paints, varnishes, plaster, etc.; and

(3) solid materials such as stones, bricks, iron, timber, etc.

Classification of rocks:

The building stones are obtained from the rocks which are classified in the following three ways:

(1) Geological classification

(2) Physical classification

(3) Chemical classification

(1) Geological Classification:

According to this classification, the rocks are of the following three types:

(i) igneous rocks

(ii) sedimentary rocks

(iii) metamorphic rocks

(i) Igneous rocks: The inside portion of the earth's surface has high temperature so as to cause fusion by heat at even ordinary pressures. The molten or pasty rocky material is known as the magma and this magma occasionally tries to come out to the earth's surface through cracks or weak portions. The rocks which are formed by the cooling of magma are known as the igneous rocks.

The igneous rocks recognized in the following three classes:

Plutonic rocks(Intrusive rocks)	Hypabyssal rocks	Volcanic rocks(Extrusive rocks)
Such rocks are formed due to cooling of magma at a considerable depth from the earth’s surface. The cooling is slow and the rocks possess coarsely grained crystalline structure.       The igneous rocks commonly used in building industry are of the plutonic type.       The granite is the leading example of this type of rock.	Such rocks are formed due to cooling of magma at the relatively shallow depth from the earth’s surface.        The cooling is quick and hence these rocks possess finely grained crystalline structure.         The dolerite is an example of this type of rock.	Such types of rocks are formed due to pouring of magma at the earth’s surface.       The cooling is very rapid as compared to the previous two cases. Hence these are extremely fine grained in structure.       The basalt is an example of this types of rock.

(ii) Sedimentary Rocks: These rocks are formed by the deposition of product of weathering on the pre-existing rocks. All the products of weathering are ultimately carried away from their place of origin by the agents of transport. Such agent are frost, rain, wind, flowing water, etc. following four types of deposits occur;

Residual deposit	Sedimentary deposits	Chemical deposits	Organic deposits
Some portion of the product of weathering remain at the site of origin. Such deposit is known as a residual deposits.	The insoluble product of weathering are carried away in suspension and when such products are deposited, they give rise to the sedimentary deposits.	Some material that is carried away in solution may be deposited by some physio-chemical processes such as evaporation, precipitation, etc. It gives rise to the chemical deposits.	Some portion of the product of weathering gets deposited through the agency of organisms. Such deposited as known as the organic deposits.

The example of the sedimentary rocks are gravel, sandstone, limestone, gypsum, lignite, etc.

(iii) Metamorphic rocks: These rocks are formed by the change in character of the pre-existing rocks. The igneous as well as sedimentary rocks are changed in character when they are subject to great heat and pressure. The process of change is known as the metamorphism.

(2) Physical Classification:

This classification is based on general structure of rocks. According to this classification, the rocks are of the following three types:

(i) stratified rocks;

(ii) unstratified rocks; and

(iii) foliated rocks.

(i) Stratified rocks: These rocks possess planes of stratification or cleavage and such rocks can easily be split up along these planes. The sedimentary rocks are distinctly stratified rocks.

(ii) Unstratified rocks: These rocks possess structure may be crystalline granular or compact granular. The igneous rocks of volcanic agency and sedimentary rocks affected by movements of the earth are of this types of rocks.

(iii) Foliated rocks: These types of rocks have a tendency to be split up in a definite direction only. The foliated structure is very common in case of metamorphic rocks.

(3) Chemical Classification:

This classification is known as the scientific or engineering classification and according to this classification, the rocks are of the following three types;

(i) silicious rocks;

(ii) argillaceous rocks; and

(iii) calcareous rocks.

(i) Silicious rocks: In these rocks, the silica predominates. The rocks are hard and durable. They are not easily affected by the weathering agencies. The silica however in combination with weaker minerals may disintegrate easily. It is therefore necessary that these rocks should contain maximum amount of free silica for making them hard and durable. The granites, quartzites, etc. are example of silicious rocks.

(ii) Argillaceous rocks: in these rocks, the argil or clay predominates. Such rocks may be dense and compact or they may be soft. These stones are hard and durable but brittle. The slates, laterite, etc. are example of the silicious rocks.

(iii) Calcareous rocks: In these rocks, the calcium carbonate predominates. The durability of these rocks will depends of these rocks will depend upon the constituents present in the surrounding atmosphere. The limestones, marble, etc. are example of calcareous rocks.