In which type of sand, salt is found?
Correct Answer: C. sea sand
📚 Detailed Explanation: Salt Content in Sea Sand
The origin of sand determines its chemical composition. Sea sand is saturated with seawater, which contains approximately 3.5% dissolved salts by weight, predominantly sodium chloride (NaCl) along with magnesium sulphate and other chlorides. These salts become embedded in and on the surface of the sand particles.
Why C (sea sand) is correct: River sand and pit sand do not naturally contain significant quantities of salt. River sand is continuously washed by fresh flowing water, and pit sand (sub-surface sand from inland quarries or riverbeds) is typically free from salt. Sea sand, by contrast, is soaked in saltwater and retains chloride ions even after wave action. Option D is wrong because sea sand genuinely contains salt. The chloride ions in sea sand are problematic for reinforced concrete as they accelerate steel corrosion.
Types of Sand and Their Properties
| Sand Type | Source | Salt Content | Suitability |
|---|---|---|---|
| River sand | Riverbeds | Nil (fresh water washes) | Best — clean, well-rounded |
| Pit sand | Inland pits/quarries | Nil to trace | Good after silt removal |
| Sea sand | Seashore / seabed | High chlorides (salt) | Not suitable for RCC without washing |
| Desert sand | Arid regions | May have mineral salts | Generally unsuitable |
Key Concepts for Students
- Chlorides from sea sand attack the passive oxide film on reinforcement steel, triggering active corrosion.
- IS 456:2000 limits chloride content to 0.3 kg/m³ of concrete for mild exposure, and less for severe/extreme exposure.
- Even after washing, sea sand may retain enough chloride to exceed IS 456 limits — testing is mandatory before use.