Impurities in Sand
• Impurities are always undesirable in sands. They reduce the strength of concrete and mortar. Since it is impossible to have sand without impurities. Therefore maximum 6% impurities are permissible.
• Clay, silt, salts, mica and organic matter are sources of weakness in any sand.
• Impurities in sand may also give the dull appearance.
• Clay and silt will prevent the bonding between aggregate particles and cement by making a thin layer around the aggregate particles which reduce the strength.
Sand taken from the seashores or from some rivers or canals may contain salts. The simplest solution is to wash the sand in fresh water but special care is required where large quantities of salt are present. An allowable limit for salts present in sand and coarse aggregate of the concrete mix is about 0.05% by weight. If salt is not removed, it will absorb moisture from the air and cause efflorescence. A slight corrosion of reinforcement may also result. Salts will also retard the setting and hardening.
Generally the bulk density is the direct measure of strength. Greater bulk density has positive effect and vice versa. Sands of greater bulk density need less amount of cementing materials and it leads to economical side. Since Lawrancepur sand has greater bulk density so it should be preferred for concrete mixes.
Specific gravity of a material gives the relative crushing strength, specific gravity can also be helpful to explore the relative porosity of the material. It may also be used to calculate the quantity of materials. It is used in mix design calculations.
Water absorption property of material would have negative effect on the strength of concrete. Concrete mix made with materials with greater water absorption capacity will need relatively greater amount of water to complete hydration reaction. Lawrancepur sand has less water absorption capacity than other sands.
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