Clay, silt, salts, mica and organic matter are the sources of impurities in sand. All sand usually contains some percentage of silt and clay. Mica can be easily recognized by its shiny surface. a certain percentage of impurities in the sand is inevitable; Usually a maximum of 6% silt and 2 to 3% mica are allowed. The sand should also be free of shell particles. Coal residues are particularly harmful because they can have a corrosive effect on reinforcement.
Testing for the presence of silt or clay in the sand
silt effect in sand
A small percentage of silt or clay (no more than 1 to 2%) is believed to improve the plasticity of the mortar to some extent, but the addition causes a reduction in strength. In very coarse sand, it may sometimes be considered desirable to add a small percentage of silt to improve its hardness.
the presence of clay in the sand forms a kind of film on the sand particles and prevents or reduces the adhesion of cement to the sand particles; Slows down the setting of cement and increases drying shrinkage. Soil with a greater surface area than sand increases the amount of water required for mixing, and thus reduces the final strength of the concrete or mortar.
The following two test methods can be performed on site to check the presence of silt or clay in the sand.
Method – 1
A rough field test can be done by rubbing a sample of sand between damp hands and noting the discoloration caused by the discoloration. Clean material will leave hands slightly stained and such sand is good for general purposes. If the hands remain dirty even after the sand has been dumped, it indicates the presence of too much silt or soil.
Method – 2
Fill a graduated glass cylinder (of 100 ml capacity) halfway with sand and pour in clean water until the glass cylinder is three-quarters full. Shake vigorously and leave it to settle for about an hour. Clear sand will freeze immediately and the presence of clay will make the water appear muddy. Any clay or silt will gradually settle on top of the sand layer. If a teaspoon of salt in water is added to the pint, it will speed up the process and the silt will settle in a layer on top of the sand layer. The thickness of the silt layer should not exceed one-seventh or less than 6% of the bottom sand. If the thickness of the silt layer is high, sand needs to be washed off. it is called decay test, This test does not apply to crushed stone sand.
Testing for the presence of organic impurities in sand
A simple test for determining the presence of harmful organic matter in sand is made by mixing sand in a plain glass bottle with an equal volume of a 3% solution of caustic soda (100 grams of caustic soda per 4 liters of water). , and let the mixture stand for 24 hours. The liquid on top of the sand should then be no darker than a pale straw (pale yellow) colour. If the color is a marked yellow or brown, the presence of an excessive amount of organic matter is indicated.
Such impurities can be removed by washing the sand. Washing has the added benefit of removing any salts in the sand. Organic impurities in sand can be caused by either rotting vegetation, humus, coal particles, or organic and industrial waste depending on the source of the sand. It is generally believed that organic impurities slow down the setting of cement and thus have a detrimental effect on the strength of concrete or mortar.