Why quartz sand is preferred in cement mortar industries to bag it instead of river sand.?

electrical resistivity may be a cause,but why river sand cant take the place of ready mix dry mortar of cement sand mix when packed in bags for long time for plastering, brick -block laying of walls.cracks in walls develops due to good river sand or due to cement if standard ratio maintained.why quartz sand cement mortar manufacturer guaranteed for non cracking of walls.why cement manufactures are using quartz silica sand for their quality cement.