Which of the following statements regarding the fineness of cement is INCORRECT?
🔬 Understanding Fineness of Cement
The fineness of cement is a measure of the size of its particles. It is a critical property because a larger surface area (finer particles) allows for a faster and more complete reaction with water (hydration). This directly impacts strength gain, heat generation, and other properties like shrinkage. Fineness is typically expressed as the specific surface area, which is the total surface area of particles per unit mass.
📝 Detailed Analysis of the Options
The question asks for the INCORRECT statement.
B. The unit of the fineness of cement is cm³/kg.
This is the INCORRECT statement. The unit cm³/kg represents volume per unit mass, which is a measure of specific volume, not specific surface area. The fineness of cement, when measured as specific surface area, has units of area per unit mass. The standard units are cm²/g or m²/kg. For example, the minimum fineness for OPC 43 grade is 225 m²/kg. Therefore, this statement about the unit is fundamentally wrong.
A. The fineness of cement can be tested by determination of the specific surface of cement.
This is a correct statement. Modern methods for determining fineness do not measure particle size directly but instead measure the specific surface area. The Blaine's air permeability test, for instance, calculates the fineness based on the total surface area of the particles in a given mass of cement. This is considered a more accurate representation of the cement's reactivity than just particle size.
C. increase in the fineness of cement increase the drying shrinkage of concrete.
This is a correct statement. Finer cement particles have a much larger total surface area. To coat all these particles with a film of water and achieve a certain level of workability, a higher water-cement ratio is required. This excess water, which is not used in the chemical hydration process, eventually evaporates from the concrete, causing it to shrink. More excess water leads to greater drying shrinkage. Therefore, increasing fineness leads to increased drying shrinkage.
D. the specific surface of cement can be determined by using an air permeability apparatus.
This is a correct statement. This is precisely what the Blaine air permeability apparatus is designed to do. It measures the resistance to airflow through a compacted bed of cement, which is then used to calculate the specific surface area. This is the most widely used method for determining cement fineness today.
📊 Summary: Effects of Increasing Cement Fineness
| Property | Effect of Increased Fineness |
|---|---|
| Rate of Hydration | Increases (Faster) |
| Rate of Strength Gain | Increases (Faster) |
| Heat of Hydration | Increases |
| Drying Shrinkage | Increases |
| Workability (for same w/c ratio) | Decreases |
💡 Study Tips
- Fineness = Surface Area / Mass: The fundamental definition of fineness is area per mass. This helps you immediately spot incorrect units like volume per mass (cm³/kg). The correct units are cm²/g or m²/kg.
- Finer means Thirstier: Think of finer cement as being "thirstier" for water to achieve workability. More water in the mix means more water has to leave, causing more shrinkage.
- Link the Test to the Property: Remember Blaine Air Permeability Test → Specific Surface Area → Fineness.