Origin and Classification of Aggregates

Origin and Classification of Aggregates

Most of the properties of an aggregate derive from its parental rock chemical and mineral composition which affects strength, stiffness, density, pore structure and permeability. Rocks experience geothermal and weathering processes which can produce granular materials in the form of natural gravels and sands, these types of aggregates can be used in concrete production without any modification or additional processes. On the other hand, crushed aggregates, defined as granular materials produced by human related activities such as blasting, crushing and so on, have rougher surface and more angular shapes comparing to natural sands. In that sense, the aggregates can be classified into two main categories: natural and crushed.

In terms of origin, rocks are generally classified as igneous, sedimentary, or metamorphic based on how the rocks were formed over long periods of geological time.

All rocks originate as igneous rocks, which are formed through cooling and solidification of molten materials that are underlying the earth’s crust. If the solidification occurs slowly, the rocks will be labelled as intrusive rocks (coarse to medium grained) and in case the molten material forces its way to the surface and crystallize more rapidly the extrusive rocks will be formed (fine grained).

Sedimentary rocks are formed by chemical and mechanical breakdown of pre-existing rocks in a process of deposition and cementation of the materials. The layered structure of particles settlement can result in undesired shapes and flaky grains in the case of aggregates crushed from such rocks.

Lastly, metamorphic rocks are made of pre-existing igneous or sedimentary rocks when the original rock is subjected to high pressure and temperature usually at great depth. Hydrothermal metamorphism can result in the minerals in these rocks being re-formed and recrystallized which makes them less durable and less stable in some cases, for example the formation of alkali-reactive silicates. Metamorphic rocks such as Quartzite make important contributions to the production of concrete aggregates.

In General, aggregates from different origins and types can be used for making concrete while some are more suitable for a specific application (e.g. porous material for light weight or self-healing concrete). Moreover, some undesired minerals such as Mica can be found in metamorphic or igneous rocks as well as sedimentary rocks.

Figure 1. shows rocks formed by each of the explained processes.

Figure 1. Typical rocks used as aggregate (a) Granite-igneous (b) Sandstone-sedimentary (c) Quartzite-metamorphic.
Figure 1. Typical rocks used as aggregate (a) Granite-igneous (b) Sandstone-sedimentary (c) Quartzite-metamorphic.

Figure 2.3. Typical rocks used as aggregate (a) Granite-igneous (b) Sandstone-sedimentary (c) Quartzite-metamorphic.

As it was mentioned above, the aggregates can be classified as natural or crushed based on their source and production technique. Natural aggregates can be obtained from variety of sources e.g. pits, river banks, beaches or other quarries that require minimum extra effort and cost in terms of granular material processing. Utilizing natural aggregates in mix design leads to better performance of the concrete since they have smaller surface area (comparing to crushed aggregate of the same size) and also their relatively spherical shape facilitate the flow inside the aggregate structure. However, the negative environmental impact of using natural aggregate is of concern which includes and not limited to atmospheric pollution, water pollution, changes in water course, increase in settlement, changes in ecosystem, etc. Therefore, In Sweden and in some other parts of the world there are regulations about the amount of natural aggregates that can be taken from the earth’s crust.

An alternative to usage of natural aggregates is to crush the rocks to desired size fractions. The crushing technique has a significant effect on quality and properties of the product. This is especially important in case of crushed sand due to their influence on properties of concrete in fresh state as a result of tendency of crushed sands to have elongated flaky shapes. These issues can be reduced by use of appropriate crushing techniques, removal of chips and fines, choke and closed-circuit feeding, or other additional processes that improve the quality of the product by different methods most of which are based on gravity or centrifugal separation.