These aggregates are obtained from coal and are very light. Some types of coal produce clinker which expands on coming in contact with water and is injurious to concrete causing it to crack. The more the combustible matter remaining in the clinker, the more moisture movement is there. The other injurious matter is sulpher which may be found in these aggregates. Its percentage should be kept low. To minimize the failure of concrete prepared with this aggregate, the fine aggregate is almost omitted from the concrete. In general clinker should be used for such partition walls etc. which are not exposed to weather directly.
It should not be used in reinforced concrete where the steel is liable to get corroded. The crushing strength of this concrete is about 15 to 30 kg/cm2 and its density varies from 800 to 1000 kg/cm3.
It is also a suitable material to produce light weight concrete provided it is free from volcanic dust or clay etc. It gives concrete having density 800 to 1100 kg/cm3 and has a high thermal insulation. Its strength is about 15 to 40 kg/cm2.
Foamed slag is also used for preparing light weight concrete. It is obtained by rapidly quenching the blast furnace slag in water.
It should be free from combustible materials like coke and sulphur. It also shows expansion on getting moist. It produces concrete having a density of about 1300 to 1450 kg/cm2. It has a high crushing strength.
Expanded Vermiculite Slate
This aggregate is obtained by heating some natural products like vermiculite (a mica product) to suitable temperatures. By heating its volume increases several times and forms a light weight aggregate. The weight of this concrete may be as 600 kg/cm3.