Classification of Construction Aggregate Based on Geological Origin

The aggregates used in the construction are usually derived from 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:

1. Natural Aggregates
2. Artificial Aggregates

Natural Aggregates

These are aggregates that are obtained from the natural deposits of sand and gravel or from the quarries by cutting the rocks. The natural aggregates are originated from the bedrock. These are reduced to their present size by means of natural agents; water, snow, wind ..etc. The sand commonly used is river deposits. Among the mentioned types, natural sand and gravel are the cheapest. 

The second most commonly used aggregate is quarried rock, which is reduced to size by means of crushing. They provide coarse aggregates.

Some examples of natural aggregates are granite, quartzite, basalt, sandstone, etc.  They can be classified into the following types:

1. Natural Aggregates from Igneous rocks
2. Natural Aggregates from Sedimentary rocks
3. Natural Aggregates from Metamorphic rocks 

1. Natural Aggregates From Igneous Rocks

The main features are :

• Highly satisfactory for the concrete construction
• These rocks have a hard, tough, and denser form
• Their structure is massive with crystalline or massive structure
• The bulkiness of coarse aggregates is of igneous origin
• These have a light or dark color
• These are chemically active, having the chance to react with the alkalies in the cement.
• Examples: Granite, Basalt

2. Natural Aggregates From Sedimentary Rocks

The main features are:

• The formation of these aggregates is 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 concrete manufacture depends on the degree of consolidation, the type of cementation, the thickness of the layers, and the contamination.
• Examples: Limestone, Sandstone

3. 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 temperatures and pressure that will result in metamorphism. This process will change the structure and the texture of the rocks.
• These possess good quality
• The structure of the same is crystalline
• The physical and chemical property of the rocks is dependent upon the quality of the parent rock.
• Examples: Marble, Schist, etc

Artificial Aggregates

Nowadays natural aggregates are found scarce which increases the demand for construction aggregates. Artificial aggregates are man-made aggregates that are produced to decrease the dependence on natural resources. 

Artificial aggregates are either by-products of any other product manufacturer or they 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 is clean broken brick and air-cooled blast furnace slag. The broken brick of good quality is good for mass concrete but not for reinforced concrete. These must be free from lime and sulfate plaster. These are not suitable for waterproof concrete. Other byproducts are fly ash and silica fume. etc

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