Natural stone is perhaps the oldest, most abundant and most durable "readymade" building material, found predominantly in hilly areas. Various types and forms of natural stone can also be processed to produce other building materials.
The main stones used in building are divided into three geological categories:
1. Igneous rocks, generally crystalline, formed by the cooling of molten magma forced up through cracks in the earth's crust. It, therefore, cannot contain fossils or shells. Most common examples: granites and volcanic stones.
2. Sedimentary rocks, commonly found in layers, formed by the disintegration and decomposition of igneous rocks due to weathering (water, wind, ice), or by accumulations of organic origin. Most common examples: Sandstones and limestones.
3. Metamorphic rocks, which are structurally changed igneous or sedimentary rocks, caused by immense heat and pressure. Most common examples: Slates (derived from clay), quartzites (from sandstone) and marble (from limestone).
Extraction of rocks is possible with simple tools such as drills, wedges and hammers, but skill and experience is essential to ensure accurate cuts. Harder rocks, such as granite, require more sophisticated mechanized equipment. Natural stone can be used as quarried, ie irregularly shaped, or can be shaped with simple tools or machines, depending on the ultimate construction. The material can be used completely, without wastage.
• Rubble (undressed stone) for foundations, floors, walls, or even corbelled roof structures, in all cases with or without mortar.
• Ashlar (squared or shaped stone) for regular course masonry, window sills, lintels, steps and paving.
• Impermeable stone (eg granite) as damp proof courses; also as external cladding of walls, though less suited for low-cost constructions.
• Slate for roofing.
• Gravel and stone chippings as aggregate for concrete and terrazzo.
• Granules for surfacing bituminous felts.
• Powders for extending paint.
• Limestone for lime and cement production.
BUILDING STONE MATERIALS AND APPLICATIONS
(from United Natons: Stone in Nepal, 1977)
• Usually abundantly and easily accessible in hilly regions; extraction generally requiring low investment cost and energy input.
• Immense strength and durability of most varieties of stone; negligible maintenance requirements.
• Impermeability of most stone varieties, providing good rain protection.
• Climatically appropriate in highland and arid zones, due to high thermal capacity of stone.
• Deterioration may result from atmospheric pollution, eg when sulphur compounds dissolved in rainwater produce sulphuric acid, which reacts with carbonates in limestones, causing skin formation and blisters.
• Efflorescence and spelling caused by certain salts and sea spray.
• Damage due to thermal movement of some stones, especially when fixed rigidly to materials with differing thermal movement, eg concrete.
• Surface damage due to water, which slowly dissolves limestones; or by prolonged wetting and drying of certain sandstones; or by freezing of water trapped in cracks.
• Low resistance to earthquake forces, thus likelihood of destruction and endangering lives.
• Avoidance of using limestones and calcareous sandstones close to sources of atmospheric pollution eg where sulphur dioxide is emitted (from burning coal and oil).
• Avoidance of surface treatments that seal in salts; occasional sponging of affected stones helps to remove salts, especially in coastal areas.
• Construction of movement joints to accomodate differences between the thermal movements of adjoining materials.
• Construction details that will allow water to be removed by evaporation or drainage, to avoid frost damage or washing out of limestones.
• Careful building design, especially with corner reinforcements, ring beam, etc., in earthquake prone areas; especially avoidance of stone vaults or corbelled roofs.
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