robo sand vs natural sand

The sand particle should be coarser and have minimum silt contents. The silt contents should not be more than 4%. 

The quality of the sand has more impact on the strength of the structure in spite of the quantity of the cement used.
1) If there is more silt in coarse sand, the strength of cement mortar is reduced from 1 % to 30%.
2) If you take best quality coarse sand, it will give maximum strength.
WASH the sand before use so that silt contents should be reduced, the problem of cracks is mainly due to silt contents.
    See the Article below

http://www.gharexpert.com/articles/Fine-Aggregate-1475/Specification-Fine-Aggregate-Coarse-Sand_0.aspx

http://www.gharexpert.com/home/HOW_TO_PREVENT_CRACKS_IN_BUILDING.aspx

http://www.gharexpert.com/articles/Cracks-718/Which-Various-Reasons-Cracks-in-Buildings-_0.aspx