Which of the following statements is INCORRECT regarding the fineness of cement?
- AIncrease in the fineness of cement increases the drying shrinkage of concrete.
- BThe unit of the fineness of cement is cm³/kg.Correct
- CThe fineness of cement can be tested by determination of the specific surface of cement.
- DThe specific surface of cement can be determined by using an air permeability apparatus.
Solution
Concept
Fineness of cement is expressed as specific surface, i.e. surface area per unit mass (cm²/g or m²/kg).
Formula
Specific surface = surface area / mass (cm²/g or m²/kg)
Step-by-step solution
1. Fineness is measured as specific surface, which is area per unit mass. 2. So its unit is cm²/g or m²/kg, not cm³/kg. 3. Statement B gives the unit cm³/kg, which is volume per mass. So B is wrong. 4. Statements A, C and D are correct.
Explanation
Fineness of cement is the total surface area of particles in a unit mass of cement. So its unit is area per mass, like cm²/g or m²/kg. The unit cm³/kg is volume per mass, so statement B is incorrect.
Why the other options are wrong
A: Correct. Finer cement needs more water and gives more drying shrinkage. C: Correct. Fineness is found by measuring specific surface. D: Correct. Blaine's air permeability apparatus gives the specific surface.
Shortcut
Fineness = surface area per mass, so the unit must contain an area (cm² or m²), not a volume (cm³).
Common mistake
Mixing up specific surface (cm²/g) with specific volume (cm³/g).
Correct answer: B) The unit of the fineness of cement is cm³/kg.