Effect of mica particles on the strength (especially for plaster work)
Mica being flaky & laminated affects strength & durability of mortar /concrete in the same way as other such particles do. The effect on workability & strength is more for leaner mixes.
In Indian river sands, mica content could be as high as 12%. The problem is further aggravated if the sand is of finer variety (Ganges sand could have fineness modulus as low as 0.6) .
Muscovite variety of mica is more harmful than the mica biotite variety. Jamuna sand is mainly biotite with small percentage of muscovite particles; whereas Ganga sand is mainly muscovite with small percentage of biotite particles;
Indian Standards do not prescribe any test for measuring the quantity of mica. You can, however, measure the mica content by Floatation/Density separation method or the Wind blowing method; former being more accurate.”