Chapter 2
Sand
Sand is an important building material. It abundantly occurs in nature and is formed by the decomposition of rocks. Sand particles consist of small grains of silica (SiO2). It forms a major ingredient in concrete, lime mortar, cement mortar, etc.
2.1 NATURAL SOURCES OF SAND
Sand is formed by the weathering of rocks. Based on the natural sources from which sand is obtained, it is classified as follows:
- Pit sand
- River sand
- Sea sand
2.1.1 Pit sand
This sand is obtained by forming pits in soils. It is excavated from a depth of about 1-2 m from the ground level. This sand is found as deposits in soil and it consists of sharp angular grains, which are free from salts. It serves as an excellent material for mortar or concrete work. Pit sand must be made free from clay and other organic materials before it can be used in mortar. Also, the coating of oxide of iron over the sand grains should be removed.
2.1.2 River sand
This sand is widely used for all purposes. It is obtained from the banks or beds of rivers and it consists of fine rounded grains. The presence of fine rounded grains is due to mutual attrition under the action of water current. The river sand is available in clean conditions. The river sand is almost white in colour.
2.1.3 Sea sand
Sea sand is obtained from the sea shores. It consists of fine rounded grains like the river sand. Sea sand is light brown in colour. Since the sea sand contains salts, it attracts moisture from the atmosphere. Such absorption causes dampness, efflorescence and disintegration of work. Sea sand increases the setting time of cement. Hence, it is the general rule to avoid use of sea sand for engineering purposes even though it is available in plenty. However, after removing the salts by washing, it can be used as a local material.
2.2 CLASSIFICATION OF SAND
Based on the grain size distribution, sand is classified as fine, coarse and gravelly.
- Fine sand: The sand passing through a sieve with clear openings of 1.5875 mm is known as fine sand. Fine sand is mainly used for plastering.
- Coarse sand: The sand passing through a sieve with clear openings of 3.175 mm is known as coarse sand. It is generally used for masonry work.
- Gravelly sand: The sand passing through a sieve with clear openings of 7.62 mm is known as gravelly sand. It is generally used for concrete work.
2.3 BULKING OF SAND
The increase in the volume of sand due to the presence of moisture is known as bulking of sand. This is due to the fact that moisture forms a film of water around the sand particles and this results in an increase in the volume of sand. The extent of bulking depends on the grading of sand. The finer the material the more will be the increase in volume for the given moisture content. Bulking of sand can be expressed in a graphical way as shown inFigure 2.1.
For a moisture content of 5–8 per cent, the increase in volume may be about 20–40 per cent depending upon the gradation of sand. When the moisture content is further increased, the sand particles pack near each other and the amount of bulking is decreased. Hence, dry sand and the sand completely flooded with water have practically the same volume.
The volumetric proportioning of sand depends upon the extent of bulking. It is more with fine sands than with coarse sands. If proper allowances are not made for bulking of sand, the cost of concrete and mortar increases and it results in mixes with inadequate sand. This makes the mix harsh and difficult for working and placing.
2.4 PROPERTIES OF GOOD SAND
Good sand should possess the following properties:
- It should be clean and coarse.
- 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 - It should be chemically inert.
- It should contain sharp, angular, coarse and durable grains.
- It should not contain salts which attract moisture from the atmosphere.
- It should be well graded, i.e., it should contain particles of various sizes in suitable proportions.
- It should be strong and durable.
- 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:
- 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.
- Bulk: It acts as an adulterant. Hence, the bulk or volume of mortar is increased which results in reduction of cost.
- Setting: In the case of fat lime, carbon dioxide (CO2) is absorbed through the voids of sand and setting of fat lime occurs effectively.
- Shrinkage: It prevents excessive shrinkage of mortar in the course of drying and, hence, the cracking of mortar during setting is avoided.
- 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.
REVIEW QUESTIONS
- What is the importance of sand in building construction?
- What are the sources of sand?
- How is sand classified?
- What is bulking of sand and how can it be determined?
- What are the properties of good sand?
- Describe the functions of sand in mortar.
- What are the substitutes of sand used due to the scarcity of sand?
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