Abstract: A cementitious, convex structural block for forming parabolic walls is disclosed. The block utilizes a key and the keyway to facilitate placement and to add strength to the wall. The parabolic shape of the wall increases its compressive strength and when used underground as a seal or stopping in mining applications channels force from blast waves and dammed water into the mine shaft ribs. When constructed from a geopolymer, the block is lighter and has a smaller carbon footprint than cement.