The proper size of cube mould for testing compressive strength of cement is:
📏 Standardization in Material Testing
To ensure that the quality of cement is consistent and meets required standards, its properties must be tested in a uniform way. The compressive strength test is arguably the most important of these tests. To get reliable and comparable results, every aspect of the test is standardized by codes like IS 4031.
This includes the mix proportion of the mortar, the type of sand, the water content, curing conditions, and the size of the specimen being tested. This question focuses on the specific size of the cube mould used for testing the strength of the *cement* itself.
📝 Detailed Analysis of the Options
(d) 15 cm
This is a very common point of confusion. A 15 cm (or 150 mm) cube is the standard size used for testing the compressive strength of concrete, not cement. Concrete cubes are larger because they contain coarse aggregates.
(c) 10 cm
A 10 cm (or 100 mm) cube is also sometimes used for testing concrete, particularly if the maximum aggregate size is less than 20 mm. However, it is not the standard size for the cement mortar test.
(b) 10.05 cm
This is an arbitrary size and not a standard dimension for any common cement or concrete test.
(a) 7.05 cm
This is the correct answer. The standard size of the cube mould for testing the compressive strength of cement mortar is 7.06 cm, which is often rounded to 7.05 cm in questions. This specific dimension (which is 70.6 mm) is chosen because it results in a face area of approximately 5000 mm² (or 50 cm²). This makes the calculation of stress (Force / Area) straightforward during the test. The test uses a 1:3 cement-sand mortar to create these cubes.
📊 Standard Cube Sizes in Testing
| Material Being Tested | Standard Cube Size | Key Reason |
|---|---|---|
| Cement (in Mortar) | 7.06 cm (70.6 mm) | Provides ~50 cm² face area |
| Concrete | 15 cm (150 mm) | Accommodates coarse aggregates |
💡 Study Tips
- Cement vs. Concrete: This is the most important distinction. For CEMENT strength, think small (7.06 cm). For CONCRETE strength, think big (15 cm).
- The "50 cm²" Trick: Remember that the odd-looking 7.06 cm dimension is chosen for a reason: it gives a neat face area of about 50 cm², which simplifies calculations.
- Visualize the Materials: A cement mortar cube only has fine sand, so it can be smaller. A concrete cube needs to be larger to contain the large pieces of stone (coarse aggregate).