Sea water:
Try it first: pick an option, then read the explanation.
Correct answer: D. All options are correct
Detailed Explanation: Properties of Sea Water and Its Effect on Concrete
Sea water contains dissolved salts that affect concrete at both the fresh and hardened stages. Understanding these effects explains why IS 456 restricts the use of sea water as mixing water.
Why D (All options are correct) is correct:Statement A — Sea water contains approximately 3.5% total dissolved salts by weight, predominantly sodium chloride (NaCl, ~2.7%), along with magnesium sulphate, calcium sulphate, and carbonates.
Statement B — Chloride ions (Cl−) from sea water penetrate hardened concrete and destroy the passive oxide film on steel reinforcement, triggering active iron corrosion: Fe → Fe2+ + 2e−. The resulting rust expands 2–3×, cracking the concrete cover.
Statement C — Sodium chloride, carbonates, and bicarbonates in sea water accelerate the early hydration of C3A and C3S, shortening initial setting time.
Sea Water Effects on Concrete
| Effect | Cause | Impact |
|---|---|---|
| ~3.5% salinity | NaCl, MgSO4, Na2SO4 | Multiple chemical attacks |
| Corrodes reinforcement | Chloride ion (Cl−) attack | Structural failure |
| Accelerates setting | Carbonates + chlorides | Reduced workability time |
Key Concepts for Students
- IS 456:2000 Clause 5.4: sea water is not permitted as mixing water for prestressed or reinforced concrete.
- Sea water may be used for plain (unreinforced) mass concrete in some cases if tested and approved.
- Sea sand (not sea water) is also restricted: chloride limits per IS 456 are 0.06% in fine aggregate for RCC.