The aggregates used in the construction are usually derived from the natural resources. These are prepared for the purpose of construction by reducing their size by means of crushing. There are other types of aggregates that are made indirectly (Not through direct natural means).
Hence based on these criteria, the aggregates are classified as:
- Natural Aggregates
- Artificial Aggregates
These are aggregates that are obtained from the natural deposits of sand and gravel or from the quarries by cutting the rocks. Among the mentioned types, the natural sand and the gravel are the cheapest. These are reduced to present size by means of natural agents; water, snow, wind ..etc. The sand that are river deposits are most commonly used type which gain good quality.
The second most commonly used aggregate are the quarried rock, that is reduced to a size by means of crushing.
Some of the examples of natural aggregates are granite, quartzite, basalt, sandstone etc. The natural aggregates are originated from the bed rocks. They can be classified into the following types:
- Natural Aggregates from Igneous rocks
- Natural Aggregates from Sedimentary rocks
- Natural Aggregates from Metamorphic rocks
Natural Aggregates From Igneous Rocks
The main features are :
- Highly satisfactory fro the concrete construction
- These rocks have a hard, tough and a denser form
- Their structure is massive with crystalline or massive structure
- The bulkiness of coarse aggregates are of igneous origin
- These have a light or dark colour
- These are chemically active, having chances to react with the alkalies in the cement.
- Examples: Granite, Basalt
Natural Aggregates From Sedimentary Rocks
The main features are:
- The formation of these aggregates are due to the sedimentary action
- Depending on the formation of the sedimentary rocks, these can have soft to hard texture
- These are porous based on the variation in their density
- These rocks can yield flaky aggregates
- These aggregates are considered cost effective
- The suitability of the sedimentary rock aggregates in the concrete manufacture depends on the degree of consolidation, the type of cementation, the thickness of the layers and the contamination.
- Examples: Limestone, Sandstone
Natural Aggregates From Metamorphic Rocks
The main features are:
- These rocks have a foliated structure
- The metamorphic and the igneous rocks have chances to be subjected to high temperature and pressure that will result in the metamorphism. This process will change the structure and the texture of the rocks.
- These posses good quality
- The structure of the same is crystalline
- The physical and the chemical property of the rocks is dependent upon the quality of the parent rock.
- Examples: Marble, Schist etc
Nowadays the natural aggregates are found scarce which increase the demand of artificial aggregates. The artificial aggregates are the man made aggregates. These are either by-products of any other product manufacture. Or it can be obtained by recycling a material that is found to be harmful to the environment in its free state.
The most commonly used artificial aggregate are clean broken brick and the sir cooled blast furnace slag. The broken brick of good quality are good for mass concrete but not for reinforced concrete. These must be free from lime and sulfate plaster. These are not suitable for water proof concrete.
Other byproducts are fly ash, silica fume..etc