Quality of Aggregates

Quality of Aggregates

The aggregate should be free from impurities like deleterious material, salt contaminations and alkali reaction. These are elaborated as under :

(a) Deleterious Material: Impurities and deleterious material like clay, silt, coal, lignite, mica, shale etc. interfere with the process of hydration and prevent bonding of aggregate with cement matrix.

(b) Salt contamination: This is the main problem mainly with sea dredged aggregate. These aggregates should not generally be used for PSC works. Even if these are permitted for PSC works, the limits should be as given in “Concrete Bridge Code”(CBC) :

As per CBC:

Limit of chloride – 0.04% by weight of FA or

 0.02% by weight of CA

Limit for sulphate – 0.4%by weight of Total Aggregate

(c) Alkali Reaction: Some aggregates contain active silica which may react with alkalies present in cement like sodium oxide, potassium oxide etc. Alkaline hydroxide derived from alkalies in the cement may attack these constituents and gels of “unlimited swelling” is formed which results in cracking. This problem can be overcome if:

1.Low alkali cement is used (cement with alkali content < 0.6%)

2.Ground Pozzolana is added to the cement so that Pozzolana react with alkalies before they attack aggregates.