Unregistered material purchase - bricks, sand & jally tax to be paid in rcm

I am doing contract business i want to pay tax for unregistered purchase ( Bricks, Sand, Jally ) without GST monthly i am purchasing upto 10,00,000/- i want to pay tax in GSTR 3B in which column i should pay the tax the tax in RCM. If i pay the tax in rcm i can avail the tax payment of rcm in same month or next month