Clay, silt, salts, mica and organic matter are a source of impurities in sand. All sands are generally found to contain some percentage of silt and clay. Mica is easily discernible from its shining surface. A certain percentage of impurities are inevitable in sand; a maximum of 6 % of silt and 2 to 3 % of mica is usually allowed. Sand should also be free from particles of shell. Coal residues are particularly harmful as they may have a corrosive effect on reinforcement.
Test for Presence of Silt or Clay in Sand
EFFECT OF SILT OR CLAY IN SAND
A small percentage of silt or clay (not exceeding 1 to 2%) is considered to improve the plasticity of a mortar to some extent, but an excess causes reduction in strength. In very coarse sand, it may sometimes be considered desirable to introduce a small percentage of silt in order to improve its harshness.
Presence of clay in sand forms a sort of film on the particles of sand and prevents or reduces the adhesion of cement to the sand particles; retards the setting of cement and increases drying shrinkage. Clay having a greater surface area than sand increases the amount of water required for the mix, and thus reduces the ultimate strength of the concrete or mortar.
The following two test methods can be done on site to check presence of silt or clay in sand.
METHOD – 1
A rough field test may be carried out by rubbing a sample of the sand between damp hands and noting the discoloration caused. Clean materials will leave the hands slightly stained and such sand is good for ordinary purposes. If the hands stay dirty after the sand has been thrown away, it indicates the presence of too much silt or clay.
METHOD – 2
Half fill a graduated glass cylinder (of 100 ml capacity) with sand and pour in clean water until the glass cylinder is three-quarter full. Shake up vigorously and leave it to settle for about an hour. Clean sand will settle immediately and presence of the clay will show the water muddy. Any clay or silt will settle slowly on the top of the sand layer. If salt is added in water, one teaspoonful to a pint, it will quicken the process and silt will settle in a layer on top of the sand layer. Thickness of silt layer should not exceed one-seventh, or 6 % of that of sand below. If thickness of the silt layer is more, sand needs washing. This is called decantation test. This test is not applicable to crushed stone sands.
Test for Presence of Organic Impurities in Sand
A simple test for determining the presence of injurious organic matter in sands is made by shaking some of the sand in a plain glass bottle with an equal volume of a 3 % solution of caustic soda (100 gm of caustic soda in 4 liters of water), and allowing the mixture to stand for 24 hours. The liquid above the sand should then not be darker than light straw (pale yellow) colour. If the colour 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 additional advantage of removing any salts in sand. Organic impurities in sand may be either due to decayed vegetation, humus, coal particles, or organic and industrial wastes depending upon the source of the sand. It is generally considered that organic impurities retard the setting of cement and thus have deleterious effect on the strength of concrete or mortar.