This document discusses chloride and sulfate attack on concrete. Chloride attack is a major cause of reinforcement corrosion, accounting for 40% of structure failures. Chlorides enter concrete through water, cement, aggregates or admixtures and exist in both bound and free forms, with free chlorides causing corrosion. Sulfate attack results in the formation of gypsum and ettringite, increasing concrete volume and causing disintegration. Preventative measures for chloride attack include proper cover, epoxy coatings, and permeability reduction. Sulfate attack can be controlled through the use of sulfate-resistant cement, quality concrete work, and pozzolanas or high alumina cement.

1. Chloride attack on Concrete
and
Sulphate attack on Concrete
Er.Pratap Nanasaheb Shinde
Sanjivani COE Kopargaon.

2. Chloride attack-
•Chloride attack is important from durability point of view
•Statistics indicates almost 40% of structure failure due to corrosion of
reinforcement
•Chloride attack reduces strength of structure drastically.
•Oxide protective layer can also destroyed by presence of chloride in presence
of water and oxygen.
•It is primary cause of corrosion of reinforcement.
•Chloride enters in to concrete through cement,aggregate,water,and some
times from admixtures. it enters in concrete by diffusion from environment.
•Forms of chloride-
It present in concrete in two forms viz. i)bound ii) free
i)Chemically bound chloride incorporated in the product of hydration of
cement and physically bound chloride absorbed on the surface of gel pores.
ii) Free chloride is available which results in to corrosion of reinforcement.

3. Ways of Chloride attack-
i) Due to inside of concrete.
ii) Through ingress of chloride from out side to inside of concrete structure.
Reason of chloride present in concrete –
i. Use of sea water for mixing
ii. Use of calcium chloride as an additive to increases setting time.
iii. Aggregate contain chloride not washed before mixing
iv. Aggregate it self more chloride contain.
from external agency-
i) Use of sea water for mixing.
ii) Use of salt to melt the ice.
iii) Presence of chlorides in the substances placed for storage.
Preventative measure for chloride attack–
i. Proper cover to reinforcement.
ii. Coating of rebar by epoxy.
iii. Decreasing permeability.
Values of chloride ion for intitation of reinforcement corrosion –
i. Upto 0.2% by weight of cement-no risk. ii) 0.2 – 0.4 % by wt.of cement- low risk
iii) 0.4-1 % by wt,of cement- Moderate risk iv) more than 1% -High risk of corrosion.

4. Sulphate attack-
•External sulphate attack ( sulphate ion from external source)
•Internal sulphate attack ( Delay in Ettrignite formation)
•A characteristic whitish appearance is the indication of sulphate attack.
•Due to sulphate attack there is increase in volume of concrete /mortar
(chemical reaction between product of hydration and solution containing sulphate)
•Due to sulphate attack there is gradual disintegration of concrete.
•CH+NS+2H CSH2+NH (Gypsum formation)
•CH+MS+2H CSH2+MH (Gypsum formation and brusite formation)
•Gypsum cause softening and strength loss also reported to expansion.
•Further reaction of gypsum with aluminates (C3A)and it’s hydrate and mono
sulphate leads to formation of expansive ettringite.
•CH loss creates loss in PH hence reduce alkalinity.
•Maximum deterioration occurs with Magnesium sulphate.
C3S2H2+3MS+ (X+21) H 3CSH2+ 3MH + 3SH+(12+X-Y)H
•This process could occurs whenever the PH reduces [(as a result of loss of (CH)]
•Later stage conversion of CSH to non cementious MSH leads to complete
disintegration.

5. Methods of Controlling Sulphate attack-
a) Use of sulphate resisting cement and C3A content.
b) Quality of concrete- Well designed, placed and compacted with less W/C
ratio is used
c) Use of Air entering agent- up to 6% reduce segregation improve
workability also reduce bleeding and generate
greater impermeability.
d) Use of Pozzolana-enhance impermeability
e) Use of High pressure steam curing.
f) Use of High Alumina Cement.