Concrete practice (IS 456) — permissible pH of mixing water: as per IS 456:1978, the pH value of water shall be what minimum?

Introduction / Context:
Water quality affects cement hydration and the durability of reinforced concrete. Standards specify acceptability criteria (including pH) to avoid deleterious reactions and corrosion risk. This item tests memory of the basic pH requirement in IS 456:1978 (retained in later editions as a practical lower bound).

Given Data / Assumptions:

• Reference: IS 456 provisions for water used in mixing and curing concrete.
• We only need the minimum acceptable pH value.
• Alkalinity sufficient to avoid aggressive acidic conditions is desired.

Concept / Approach:

Acidic water (pH < 7) can attack cement paste and increase corrosion likelihood. IS 456 specifies that water for concrete should be free of harmful quantities of oils, acids, alkalis, salts, organic matter, and that its pH should not be less than 6 to ensure it is not aggressively acidic.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Other concrete codes also set similar constraints; typical potable water comfortably meets the requirement.

Why Other Options Are Wrong:

“Less than” or “equal to 6” allows borderline/acidic water; “equal to 7” is more restrictive than required; “not less than 7.5” is unnecessarily stringent.

Common Pitfalls (misconceptions, mistakes):

Assuming only neutral water is permitted; ignoring other chemical limits besides pH.

Final Answer:

not less than 6