PROPERTIES OF GOOD SAND

Good sand should possess the following properties:

1. It should be clean and coarse.
2. It should be free from any organic or vegetable matter; usually 3-4 per cent clay is permitted.  Figure 2.1 Graph showing the percentage increase in volume to the percentage by weight of moisture 
3. It should be chemically inert.
4. It should contain sharp, angular, coarse and durable grains.
5. It should not contain salts which attract moisture from the atmosphere.
6. It should be well graded, i.e., it should contain particles of various sizes in suitable proportions.
7. It should be strong and durable.
8. It should be clean and free from coatings of clay and silt.

2.4.1 Functions of sand in mortar

Sand is used in mortar and concrete for the following functions:

1. Strength: It helps in the adjustment of the strength of mortar or concrete by variation of its proportion with cement or lime. It also increases the resistance of mortar against crushing.
2. Bulk: It acts as an adulterant. Hence, the bulk or volume of mortar is increased which results in reduction of cost.
3. Setting: In the case of fat lime, carbon dioxide (CO₂) is absorbed through the voids of sand and setting of fat lime occurs effectively.
4. Shrinkage: It prevents excessive shrinkage of mortar in the course of drying and, hence, the cracking of mortar during setting is avoided.
5. Surface area: It subdivides the paste of the binding material into a thin film and, thus, more surface area is offered for its spreading and adhering.

2.4.2 Substitutes for sand

Sand has now become a scarce and costly material and extraction of river sand is now said to affect the ecological balance. The use of substitutes for sand has gained great importance.

Surkhi, or finely grained burnt clay, is one of the materials used for this purpose. It can be used in all mortars, except that for plastering.

Processed quarry dust or stone screening is also being used as substitutes. It is abundantly available and if it is properly screened it imparts more strength to the mortar. This is now being industrially manufactured under various trade names.