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The essential requirements of building stones used for structural work are given below:
Strength
Ordinarily, the strength of a stone is not of primary importance, as very often the loads to which it is subjected in a structure are much below its permissible crushing strength
Durability
Stones should possess a natural durability to withstand the destructive effects of various agents continuously operating on them. In fact, the life of a structure depends upon the durability of the materials with which it is built. The durability of a stone depends upon the relation between its chemical composition and that of the atmospheric surroundings. The texture of the stone also influences its durability. Crystalline homogeneous and close-grained varieties of stones with a dense structure should be selected for good works. The surface of a freshly broken stone should be uniform in texture, colour and hardness. Porous stones or those containing patches of soft or objectionable materials are liable to disintegrate quickly.
Cost
An important consideration in the selection of building stone is its cost. Other things being equal, the cost of a stone depends upon the ease with which it can be quarried out, the proximity of the quarry to the place of use, and the transportation facilities available. The subsequent cost of dressing a stone, before it is placed directly in the structure, should also be low.
Appearance
In the case of the stone used for face work, where appearance is a primary factor, its colour and ability to receive polish is an important factor.
Hardness
The stone used in floors, pavements and aprons of bridges, should be able to resist the abrasive forces caused due to wear and friction. Hardness of stones can be tested by the Mohr's scale of hardness in the laboratory and in the field by scraping the surface with a sharp knife. A hard stone will not show any scratches.
Fig: Building stones
Toughness
Building stones should also be tough enough to withstand stresses developed due to vibrations of machinery and moving loads over them. The stones used in the construction of roads should be hard as well as tough.
Specific Gravity and Weight
The stones used for the construction of dams, weirs, barrages, docks and harbours should be of a heavier variety. In case of dams and roof coverings, lighter varieties of stones are preferred. The specific gravity of good building stones should be between 2.4 and 2.8.
Porosity and Absorption
A good stone should not be porous, rain water, enter the pores which is generally acidic, Chemical Composition of Stone Limestone and weak sandstone are relatively less durable than a good sandstone, granite or gneiss. Air and water, containing carbon dioxide, seriously affect limestone. Iron pyrites also tend to disintegrate stones; excess iron oxide or carbonate in stones develops rust in the presence of moisture; presence of clay affects the efficiency of the cementing materials; soda and potash also have a disintegrating effect; all varieties of mica are soft and are readily decomposed by exposure to atmosphere. Stones with silicates as the cementing material will weather better than those with calcareous or ferruginous binding material.
Texture
The texture of a stone indicates the arrangement of its constituent minerals. Good building stone should be homogeneous in structure. Stones with homogeneous and crystalline texture are hard and compact and superior to non-crystalline and open-texture varieties. Generally, igneous and metamorphic rocks such as granite, trap and gneiss are hard and compact. Sandstone, limestone and some of the metamorphic rocks are porous. Amorphous rocks like glass, flint, etc. have a fused texture and are hard and compact.
