The cement becomes useless if its absorbed moisture content exceeds:
🔬 Understanding Premature Hydration
Cement is a hygroscopic material, meaning it actively absorbs moisture from the surrounding air. When cement powder absorbs moisture, it begins a slow, partial chemical reaction called hydration. This premature hydration causes the cement particles to form lumps and significantly reduces their potential to react fully when mixed into concrete. If too much moisture is absorbed, the cement loses a substantial amount of its binding ability and strength, rendering it unfit for structural use.
📝 Detailed Analysis of Moisture Limits
(d) 0.05
This is the correct answer. The generally accepted limit for moisture content in stored cement is 5% by weight. If the absorbed moisture exceeds this value, the cement is considered "useless" or unsuitable for important structural work due to the significant loss of strength. The value 0.05 is the decimal equivalent of 5%.
(a) 0.01 (1%), (b) 0.02 (2%), (c) 0.03 (3%)
These are incorrect. While any amount of moisture absorption is undesirable and will cause some loss of strength, these lower levels are generally not considered to make the cement completely useless. For example, an absorption of 1-2% might lead to a 10-20% loss in strength, which might be acceptable for non-critical work. However, the 5% mark is the critical threshold where the loss of quality is too great.
📊 Effect of Moisture on Cement
| Absorbed Moisture (%) | Effect on Cement |
|---|---|
| 1% - 2% | Some loss of strength, but may still be usable. |
| 2% - 4% | Significant loss of strength, noticeable lumps. |
| > 5% | Considered useless for structural purposes. Hard lumps form. |
💡 Study Tips
- Remember the 5% Rule: 5% is the critical failure point for moisture in cement. This is a common and important number to memorize.
- Moisture = Lumps & Lost Strength: The two main consequences of moisture absorption are the formation of lumps (caking) and a reduction in the final strength of the concrete.
- Connect to Storage Rules: The reason for strict storage rules (raised floors, gaps from walls) is to keep the moisture content well below this 5% limit.