Chamber Bricks - RCM Payable or Not

I am running chamber bricks month sales upto 10 lakhs and paying GST @ 12% and i am  purchasing coal for bricks and i am taking that ITC in GSTR - 3B and for preparing bricks i am purchasing  sand and other items from unregistered purchase for preparing bricks i want to know whether i want to pay RCM for Unregistered purchase