Silt Content of Sand

Silt content is the percentage of very fine clay/silt-sized material present in fine aggregate. Excess fines coat the sand grains, increase water demand, and prevent proper cement-paste bonding — directly lowering concrete/mortar strength and increasing shrinkage and permeability. It is checked on site by the quick volumetric sedimentation test (sand + water + salt shaken in a measuring jar, silt layer measured after settling) and in the lab by wet-sieving on the 75-micron sieve per IS 2386 Part 1/2.

IS 383 limits deleterious fines (silt + clay) in sand to about ≤3% for uncrushed natural sand (higher allowances for crushed-stone sand fines which are non-plastic). A common site failure is unwashed river/pit sand exceeding the limit; the remedy is washing the sand or rejecting the source. The field jar test is a standard stores/QC acceptance check on every sand delivery.