Because of crystallization on reaction with some chemicals, the stones may show a loss of weight and some defects. This test consists of immersing a cubical specimen of 40 mm side in a sodium sulphate solution for 2 hours and then drying it in an oven at 100°C. This procedure is repeated for five times. The loss of weight and the presence of cracking on the surface are noted. The presence of visible defects and loss in weight should be minimal indicating high durability and good resistance to weathering.
2.11.2 Laboratory Tests
Laboratory tests on stones comprise of the attrition test, the crushing test, the freezing and thawing test, the hardness test, the impact test and the microscopic test.
1. The Attrition Test
This is also called the abrasion test. This test is performed to determine the percentage of the wear of stones that are used for road construction. This test is carried out in Deval’s Attrition Testing Machine.
In this test, a certain quantity of stone is placed on Deval’s Attrition cylinder. The cylinder is rotated about its horizontal axis at the rate of 30 rpm for 5 hours. After the end of rotation, the contents of the cylinder are sieved, and the quantity of material retained on the stone is weighed. The percentage weight is the ratio of loss in weight to the initial weight which is expressed as a percentage.
2. The Crushing Test
This test is performed on the stones which are to be subjected to compressive stress. In this test a cubical specimen of 40 mm side is tested in a compression testing machine. The maximum load at which the stone specimen fails by crushing is noted. Then the crushing strength of the stone is the ratio of maximum load at failure and area of bearing face.
3. The Impact Test
The toughness of a stone is determined by the impact test using an impact testing machine. Here, a stone specimen of 25 mm diameter and 25 mm height is placed on the machine. A steel hammer of 2 kg weight is allowed to fall down axially on the cylinder from 1 cm height for the first blow, 2 cm height for the second blow, 3 cm height for the third blow and so on. The blow at which the specimen breaks is noted, which is the Toughness Index of the stone.
4. The Laboratory Hardness Test
This test is different from the field hardness test and the attrition test. This test is carried out in Dorry’s testing machine. A cylinder of 25 mm of the stone is rubbed against a steel disc spindled with coarse sand. The specimen is subjected to a pressure of 1.25 kg/mm2. After 1000 revolutions in the machine, the loss in weight is found, and the coefficient of hardness is determined as
Coefficient of hardness = 
5. The Microscopic Test
This is a geologist’s test. In this test, a thin section of a stone is taken and placed on a microscope to study the following properties:
- Mineral constituents
- Texture of stone
- Average grain size
- Nature of cementing material
- Presence of pores, fissures and veins
6. The Freezing and Thawing Test
This test is conducted for the use of stones in regions where the temperature goes below the freezing point. The test consists of keeping a specimen of stone in water for 24 hours and then placing it in a freezing mixture at –12°C for 24 hours. The specimen is taken out and thawed (warmed) to atmospheric temperature. The procedure is repeated at least seven times after which the specimen is examined for any damage.
2.12 ARTIFICIAL STONES
Artificial stones are known as cast stones or reconstructed stones. In many localities natural stones are not available at a reasonable cost. In such cases, these stones are useful. However, because of their high cost, they can not be treated as a substitute for natural stones.
2.12.1 Types of Artificial Stones
Different types of materials are combined, and a variety of stones are manufactured. Some of the types of artificial stones are discussed below.
1. Cement Concrete Blocks
These are made using cement, fine aggregate, coarse aggregate and water similar to that of concrete. They may be cast-in-situ or cast-in-moulds in the required shapes. Sometimes they are precast with steel and are therefore called reinforced cement concrete blocks.
2. Artificial Marble
It is made using sand with Portland cement or one of the gypsum or magnesite cements. It is either precast or cast-in-situ and is used for external work.
Another form of artificial marble is made by fusing and moulding constituent mixture, which is composed of 60 parts of plaster in powder, 20 parts of pulverised marble, 15 parts of sulphate of potash with 5% solution of glue mixed with water. Such marble stones are mostly used in France.
3. Terrazo
It is prepared using mixtures of marble chips with white cement and some pigment. It is also either precast or cast-in-site. It is generally used in bathrooms, residential buildings, etc.
4. Mosaic Tiles
These are precast tiles where the upper surface is made of marble chips. They are available in different sizes and colours.
5. Reconstructed Stone
This is made from the debris of limestone quarries by crushing them into grit. This is mixed with lime made from dolomite and heated in a closed retort up to 980°C to drive out CO2. This is slaked, mixed with water and consolidated into blocks under pressure. It is then dried, and CO2 is admitted until the carbonization of the hydrate of lime blocks is complete.
6. Bituminous Stone
Diorite and other granite stones are impregnated with prepared or refined tar to form bituminous stones. These are preferred where noise-, wear- and dust-resistant stone surfaces are needed.
Leave a Reply