Any sand used in the concrete mix shall be within certain sand specifications in terms of gradient and free of salts, sulphates, clay materials and impurities

thanks Rana Burhan Alshahwany for your answer

In addition to sand free from impurities it must be properly graded. Sand is graded in to four zones as per IS and BS standards whereas as BS has recently recommended three grading zones and even recommended finer sand for use in concrete. In deciding zones 600 micron sieve is most critical as percentage passing on this sieve have no overlaps.