Question

Write a short note on decay of concrete .

Answer 1

1) As concrete contains free lime (CaO) ,it is susceptible to chemical attack.
2) In acidic water (pH<7) ,lime dissolves thus making concrete weak .Higher is the acidity ,more is the deterioration of concrete.
3) Lime is more soluble in soft water than in hard water .Hence deterioration of concrete is more in soft water than in hard water .
4) Lime of concrete is removed by sulphates and chlorides ,present in water .If concrete is soaked in mineral oil for sometime ,its resistance to abrasion decreases .If old sugar bags are used to carry sand or cover fresh concrete during curing ,the setting time of concrete is delayed and strength is greatly affected during first four weeks .

5) The most serious type of damage to concrete takes place in the presence of sulphates .The sulphates combine with tricalcium aluminate to form sulpho-aluminates ,which occupies more volume .This causes expansion ,thereby the life of concrete is greatly reduced .Such a danger can be avoided by eliminating tricalcium aluminate from the cement composition and manufacturing cement containing
tetracalcium aluminoferrite ,instead of aluminate .