The specific surface expressed in square cm/ gm of a good Portland cement should not be less than.....................
🔬 Understanding Specific Surface Area
Specific Surface Area is a way of expressing the fineness of cement. It represents the total surface area of all the particles in one gram of cement. It is a more accurate measure of fineness than the simple sieve test because it accounts for the size distribution of all particles.
A higher specific surface area means the cement is finer, which leads to a faster rate of hydration and quicker early strength gain. This property is measured using the Blaine Air Permeability Apparatus.
📝 Detailed Analysis of the Options
(a) 1750 & (b) 2000
These values are too low. A specific surface area below 2250 cm²/g would indicate that the cement is too coarse and would not meet the standard requirements for Ordinary Portland Cement. The hydration and strength gain would be too slow.
(c) 2250
This is the correct answer. According to IS 4031 (Part 2), the specific surface area of Ordinary Portland Cement (all grades - 33, 43, and 53) shall not be less than 2250 cm²/g. This value ensures that the cement is fine enough to hydrate properly and achieve its specified strength in the required time.
(d) 2500
While a specific surface of 2500 cm²/g is perfectly acceptable (and better than the minimum), it is not the specified minimum limit. The question asks for the value it should not be "less than".
📊 Minimum Specific Surface for Different Cements
| Type of Cement | Minimum Specific Surface (cm²/g) |
|---|---|
| Ordinary Portland Cement (OPC) | 2250 |
| Rapid Hardening Cement (RHC) | 3250 |
| Portland Pozzolana Cement (PPC) | 3000 |
| Low Heat Cement | 3200 |
💡 Study Tips
- 2250 is the Magic Number for OPC: This is a fundamental value to memorize for Ordinary Portland Cement.
- Finer Cements, Higher Numbers: Notice that other types of cement that are required to be finer (like RHC, PPC, Low Heat) have a higher minimum specific surface area.
- Specific Surface = Fineness: Directly link the term "specific surface" to the property of "fineness".
- Method = Air Permeability: Remember that this value is determined using the Blaine Air Permeability test.