Artificial stones are also called as cast stones or reconstructed stones. These are not commonly used in ordinary building construction because of the heavy cost.
Artificial stone can be moulded into most intricate forms easily and so it is most economical when it is substituted for the carvings and ornamentations in natural stone. For practical use, it is formed into a mortar or is used as concrete.
Manufacturing of Artificial Stone
The procedure of manufacturing artificial stone is as follows:
a. The natural stone is crushed into size less than 6 mm and the stone dust is removed.
b. A mixture of 1½ parts of stone of size 3 mm to 6 mm, 1½ parts of stones of size less than 3 mm and 1 part of cement by volume is prepared.
c. To impart colour to the stone, necessary colouring pigments are added to the dry mix. Its proportion should not exceed 15% of cement by weight.
d. Water is then added to the dry mix to obtain a mixture of workable consistency.
e. Then the wet mixture is then pressed into moulds ( mould may be of steel or wood, made with utmost care), cured with water and then seasoned in the air for a suitable length of time.
f. The polishing is done if required.
g. To produce the colour of light shade, white cement may be used in place of ordinary cement.
h. It is usual into the manufacture of cast stone that the facing or skin of cast stone is 25 to 38 mm in thickness and consists of the above-mentioned mix, while the remaining thickness of the stone slab consists of cheaper material (such as the lean mix of gravel and Cement or lean Cement concrete).
Varieties of Artificial Stones
The different varieties of artificial stones are:
a. Cement Concrete Blocks
~ These are made from a mixture of cement, fine aggregate, coarse aggregate and water. They may be cast-in-situ or cast-in-moulds (Note: cast in situ is used in the construction of piers and cast in the mould is used for steps, window sills, etc.)
~ Artificial paving slabs and stones composed of cement concrete and sometimes treated with sodium silicate solution also come under this class.
b. Ransome’s patent stone
~ It is created by mixing dry sand with silica of soda and a small portion of powdered stone or chalk.
~ The mixture while in a plastic state is poured into moulds and the casting on withdrawal are immersed in a solution of chloride of calcium, which cements the particles of sand.
~ It can be dressed and carved like natural stone.
~ The stone has a compressive strength of 3.2 KN/m² and weighs 19.2 KN/ m³.
~ Its cost is greater than that of national stone.
~ It is used in Nawab’s Palace at Murshidabad, and Bombay Post Office.
c. Artificial Marble:
~ It is made through fusing and moulding constituent mixture, which is composed of 80 elements of plaster in powder;20 parts of pulverized marble, 20 parts of sulphate of potash with a 5 % solution of glue mingled together with water.
~ It is used for outside work. It is generally utilized in France.
~ It is made by mixing marble chips with white cement and some pigments.
~ It is either laid in situ or pre-cast.
~ It is used for bathrooms, residential buildings, temples etc.
e. Mosaic Tiles:
~ The pre-cast concrete tiles with marble chips at the top surface are known as mosaic tiles. They are available in different shades and are widely used.
f. Reconstructed Stone
~ This stone is made from the debris of limestone quarries by crushing them into the grit, mixing it with lime made from Dolomite, heating in a closed retort up to 980°C to drive off CO2, slaking the powdery residue of CaO and MgO, mixing with water, and consolidating under great pressure into blocks.
~ It is then dried and CO2 is admitted till the carbonization of hydrate of lime blocks is complete.
g. Bituminous stone
~ Diorite and other granite stones are often impregnated with prepared or refined tar to form the bituminous stone.
~ Such stones are used where high durability is required. It is also used in dust resistant stone surfaces.
Advantages of Artificial Stones
Artificial stones claim the following advantages over the natural stones:
a. More durable than the natural stone.
b. Can be easily cast and seasoned at the site of work with great promptness and hence avoids the expenses of dressing and transportation.
c. Can be easily cast into any desired shape and can be easily and economically moulded to a required ornamental shape.
d. Artificial stone can be made in a single piece and subsequently the trouble of getting massive blocks of stones for lintels, beams, etc., may be avoided.
e. It is comparatively easy to carve artificial stone; it can be worked before becoming hard.
f. The artificial stones which are carefully cast, are free from defects that are likely to be present in natural stones.
g. In the artificial stones, cavities can be kept (to carry pipes, electric wires, etc.)
h. Their strength may be regulated by definitely proportionating the elements by using metallic reinforcements.
i. Equally exact in resisting deterioration and disintegration triggered by various atmospheric agencies, (e.g. rain, frost, etc.).
j. There is no need to take precaution concerning the natural bed of stones since the natural bed is absent in artificial stones.