Importance of Natural Stone as a Building Material

Classification of Stones

Stone is classified as geophysical, physical and chemical in civil engineering works.

Geological Classification

Stone is classified as geophysical, physical and chemical in civil engineering works.

Igneous Rocks - are formed from cooling and solidifying of molten materials. Igneous rocks are strong and durable like granite and basalt. Granite stone have crystalline surface because of the slow cooling of lava under thick cover at the top of the surface of the earth. When the cooling of lava on top of the surface of the earth results to non – crystalline and glassy texture, basalt are formed.

Sedimentary Rocks - are formed from deposits of eroded and pre – existing rock that settle in layers on sea beds. These deposits merge together by pressure and heat. Chemical agents contributes to the cementing of these deposits. Thus, rocks formed are more uniformed, fine grained and compact in nature. Sandstone and limestone are examples of these type of stone.

Metamorphic Rocks - are formed when igneous and sedimentary rocks go through metamorphic changes because of pressure and internal heat. Granite becomes gneiss and basalt and trap change to schist and laterite, limestone to marble, sandstone to quartzite and mudstone becomes slate because of metamorphic action.

Physical Classification

Rocks can be classified based on their structure.

Stratified rocks - are rocks that have layered in structure and possess planes and stratification or cleavage. They are easily split near these planes. Examples are sandstones, limestones and slate.

Unstratified Rocks - are not layered or stratified in structure. They cannot be split into thin slabs and possess crystalline and compact grains like granite and marble.

Foliated Rocks - are rocks that have a tendency to split along a definite direction only and needs to be parallel to each other just like in stratified rocks. This is very common among metamorphic rocks.

Chemical Classification

Engineers prefer to classify rocks based on their chemical composition as siliceous, argillaceous and calcareous.

Siliceous Rocks - Engineers prefer to classify rocks based on their chemical composition as siliceous, argillaceous and calcareous.

Argillaceous Rocks - the main component of these rocks is argil that is clay. These stones are hard and durable but are brittle in nature and unable to withstand shock. Examples are slate and laterite..

Calcareous Rocks - the main component of these rocks is calcium carbonate (CaCO3). Examples are limestone that is of sedimentary origin and marble of metamorphic origin.

Uses of Stones

Stones are used in different engineering constructions:

• For the construction of foundations, walls, columns and arches in stone masonry.
• For flooring, walls, benches, vanities, fireplaces, etc..
• Stone slabs for damp proof courses, lintels and as roofing materials.
• Stones with good appearance for facade works of buildings like polished marble and granite.
• For paving roads, footpaths and open spaces around the buildings.
• For construction of piers and abutments of bridges and dams.
• Crushed stones for providing base course for roads and they form a finishing coat when mixed with tar. They are also used as inert material in concrete, for making artificial stones and building blocks and as railway ballast.

Requirements of Good Building Stones

An ideal or good building stone must be inert, does not contract and expand, and should not contain water or discharge moisture. Even so, we also need to take into consideration the stone type and climatic conditions which vary from place-to-place and where it will be utilized.
In order to know that a stone is good for building there are requirements that should be considered.

Crushing Strength: Although most stone are having a good strength it should still be able to resist the load coming onto it. It is also necessary to check on the strength of the stone when it comes to large structures.

Durability: A stone's durability can be see if it is capable of resisting adverse effects of natural forces like wind, rain and heat.

Hardness: This is seen when a stone is used in floors and pavements. A good stone should be able to resist abrasive factors like movement of men and materials placed over them.

Toughness: Stones for building should be really tough to withstand stress like vibrations. Vibrations may be caused by machineries or loads that are moving over them. Stone aggregates used in road construction should be tough.

Specific Gravity: A good building stone should have a specific gravity between 2.4 and 2.8. For the construction of dams, retaining walls, docks and harbours heavier variety of stones should be used.

Porosity and Absorption: A good building stone should not be porous. Porous stones allows rainwater to enter into it that causes the stone to get weak and crumble. When water freezes inside the pores like in higher altitudes, it disintegrates the stone. This phenomenon is called freeze-thaw.

Finishing: Finishing is when you give a stone its required shape | surface finish and to reduce the cost of finishing, a stone should be easy to work with to an extend. Care should also be taken when processing stone so it may not affect the required strength and durability.

Seasoning: A good building stone must be free from quarry sap. Laterite stones must not be used for 6 to 12 months after being quarried. They are allowed to get rid of quarry sap through the action of nature. Seasoning is the process of removing quarry sap.

Cost-effective: One of the important requirements in selecting a building material is the cost. Cost can be lowered if the building site is near the quarry. This can bring down the cost on transportation and may also bring down the cost of the stone. Although not all good building stone can meet all the requirements because one may contradict the other. Strength and durability for example may contradict the processing requirement. What is important is that engineers should look into the properties needed for the intended work when selecting the stone.

To make sure of the properties required in stones builders and contractors can conduct different tests like crushing strength test, water absorption test, abrasion test, impact test and acid test.

Common Building Stones

The following stones are commonly used in constructions:

Basalt and Trap - are igneous rocks and used as road metals and aggregates for concrete. They are also used as pavement and for rubble masonry work for bridge piers, river walls and dams.

Granite - is an igneous rock and is used for monumental and institutional buildings. Polished granite is used as bench tops, cladding for columns and wall. They are also used as coarse aggregates in concrete.

Sandstone - is a sedimentary rock and is therefore stratified. It is desirable to use it with silica cement for heavy duty structures. It is also used for masonry work for dams, bridge piers and river walls.

Slate - is a metamorphic rock and is used for roofing tiles, slabs and pavements.

Laterite - is a metamorphic rock and is used for rough stone masonry works, pavement construction, etc.

Marble - is a metamorphic rock and is used for facades and ornamental works. It is also used for columns, flooring, steps and as table top..

Gneiss - is a metamorphic rock and is used in minor constructions. They can be used for flooring, pavement and not for major purposes because of its weakness. The hard variety can be used for buildings.

Quartzite - is a metamorphic rock which is hard, brittle, crystalline and durable. It is used as aggregates for road aggregate and concrete aggregate. It is also used to construct retaining walls, rubble masonry, stone pitching, etc.