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Which of the following type of cement is preferred for concrete making which is used in construction of sewage treatement plants located where, soil is infested with sulphates?
Portland slag cement
Extra rapid-hardening cement
Sulphate-resisting cement
Rapid-hardening cement
Sulphate-resisting cement
Sulphate-resisting cement (SRC) is specifically formulated to mitigate the chemical attack caused by sulphates present in soil or groundwater. It is preferred for structures like sewage treatment plants and foundations where the concrete is in direct contact with sulphated environments.
IS 12330: 1988
Sulphate-resisting cement (SRC) is specifically formulated to mitigate the chemical attack caused by sulphates present in soil or groundwater. It is preferred for structures like sewage treatment plants and foundations where the concrete is in direct contact with sulphated environments.
Limit┬аon┬аC3тАЛA<5%┬а(by┬аmass)
Limit┬аon┬аC4тАЛAF+2C3тАЛA<25%┬а(by┬аmass)
The primary cause of sulphate attack in normal concrete is the reaction between sulphates and C3тАЛA (Tricalcium Aluminate) in the presence of calcium hydroxide, leading to the formation of expansive ettringite. SRC minimizes this risk by limiting the C3тАЛA content to less than 5% and reducing the C4тАЛAF+2C3тАЛA content to less than 25%. This chemical modification prevents the formation of secondary ettringite which causes internal cracking and disintegration of the concrete matrix.
Sulphate attack leads to volume expansion and loss of strength in concrete.
Sewage contains high concentrations of sulphates resulting from organic decomposition.
The reduction of C3тАЛA is the key chemical modification in SRC.
SRC should not be used in structures exposed to sea water due to potential chloride-induced corrosion of reinforcement.
Prevents internal expansion due to ettringite formation.
Increases the service life of structures in chemically aggressive environments.
Maintains structural integrity in sulphated soils.
Slower rate of early strength development compared to Ordinary Portland Cement (OPC).
More expensive than OPC.
Higher risk of reinforcement corrosion in the presence of chlorides.
Sewage treatment plants
Foundations in marshy or sulphated soils
Culverts and pipelines exposed to groundwater
Option A (Portland Slag Cement) contains granulated slag which improves sulphate resistance but is not the specialized cement type explicitly intended for high sulphate environments.
Option B and D (Rapid hardening types) are formulated for high early strength and have higher C3тАЛS content, which is irrelevant or even detrimental to sulphate resistance.
C is correct тАФ Sulphate-resisting cement is designed to resist chemical deterioration by limiting the C3тАЛA compound which reacts with sulphates to form expansive minerals.
Always remember that C3тАЛA (Tricalcium Aluminate) is the most reactive component regarding chemical attacks; lowering it improves durability against sulphates but slightly slows down the initial setting properties.
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