Yahya Ghasemi Print III pdf

2.2. 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. see Section 2.3. 
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 re-
crystallized which makes them less durable and less stable in some cases, for example the 
formation of alkali-reactive silicates (Alexander and Mindess, 2005). 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 2.3. shows rocks formed by each of the explained processes. 

---

10 
$JJUHJDWHVLQFRQFUHWH
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. 
(Alexander and Mindess, 2005). 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
(Lagerblad et al, 2013). 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. (Alexander and Mindess, 2005; Neville, 1995;
Gram et. al, 2017)