Building stones must be tested to assess their properties to use for various purposes. Some tests are simple which can be conducted in the field, and some can be tested only in a laboratory. Accordingly, they may be grouped as field tests and laboratory tests (IS: 1121, 1974 and IS: 1124, 1998).
2.11.1 Field Tests
Simple field tests are the absorption test, Smith’s test, the toughness test, the hardness test, the acid test and the crystallization test.
1. The Absorption Test
This is usually called the water absorption test. This is a simple test conducted on all stones. It consists of keeping a cubical specimen of the stone of about 50 g weight immersed in water for 24 hours and finding the volume of the absorbed water. This is represented as a percentage of absorption by weight. This percentage should not be more than 0.6%. This test is also called the electrical resistance test.
2. Smith’s Test
This test is conducted to find the presence of earthly matter or the deterioration of stones when immersed in water. A specimen of the stone is placed in a glass container with water and stirred vigorously. It is placed in water for at least 24 hours. If the water shows turbidity, then the stone is assumed to contain earthly substances. This test is to be conducted for all stones.
3. The Toughness Test
It is a rough test without any rigid specification or procedure. A stone is struck by a hammer to gauge the toughness of the stone. The force required to break the stone reflects the toughness of the stone.
4. The Field Hardness Test
This test is also called Mohs’ scale of hardness test. This is a simple test in which the hardness of the test is based on the hardness of the surface. The surface of the stone is scratched with the help of a pen knife, and the hardness is gauged based on the relative abrasiveness of minerals (the soft being talc and the hardest being diamond). Such a classification is given by Mohs and the scale is divided into the ten following scales:
- Talc
- Gypsum
- Calcite
- Fluorspar
- Apatite
- Orthoclase feldspar
- Quartz
- Topaz
- Corundum (saphire)
- Diamond
Thus, if a mark is made by the pen knife on the surface of the stone, the hardness may be equated to that of a calcite and hence the hardness, H = 3. A siliceous rock surface can not be scratched which may be equated to that of quartz and in this instance, H = 7.
5. The Acid Test
This test is performed to determine the presence of alkaline or lime content. This test consists of placing a cube of 50 to 100 gm weight in 1% hydrochloric acid for 1 week. If the corners of the cube show roundish and loose particles deposited on the surface, the stone has some alkaline content. If the stones exhibit florescence when subjected to the action of acids, then it reflects the presence of lime content. Stones that do not react to sulphuric acid are highly fire resistant.
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