Classification of aggregates

1. On the basic of geological origin

(a) Naturat aggregate

• Natural aggregate are obtained by crushing form quarries of igneous , sedimentary or metamorphic rocks 
• Most widely used aggregate are from igneous origin 
• Aggregates obtained from pites or dredged from river most ofter are not clean enough or well graded to suit the quality requirement . They therefore requird sieving and washing before they can be used in concrete 

(B) Artifical aggregates 

• Broken bricks , blast furnace slag and synthetic aggregates are artificial aggregates 
• Broken bricks called as brick bats are suitable for mass concreting, for example, in foundation bases. They are not used for reinforced concrete works. 
• Blast furnace slag aggregate is obtained from slow cooling of the slog followed by crushing . 
• These dense and strong aggregates as obtained are used for making precast concrete products . 
• The blast furnace slag aggregate has good fire resistant properties but are responsible for corrosion of reinforcement due to sulphur content of slag 
• Synthetic aggregates are obtained by thermally processed material such as expanded clay and shale used for making light weight contact .

2. On the Basis of size 

(a) coarse Aggregate 

• Aggregate ranging form 80 mm - 4.75 mm are identified as coarse aggregates. 
• These are obtained form natural disintegration or artificial crushing of rocks. 

(b) Fine Aggregate 

• Aggregate passing through 4.75 mm sieve are defined as fine 
• They may be natural sand - deposited by rivers , crushed stone sand - obtained by crushing stones and crushing gravel sand .
• Smallest size of fine aggregate ( sand ) is 0.06 mm 
• Depending upon the partical size , fine aggregate are described as fine , medium and coarse sands