GB2 8" x 8" 16" Cinderblock are hollow rectangle-shaped structures usually made of concrete and coal cinders. Cinder blocks are lighter than concrete blocks. Cinder blocks are similar in shape and construction to concrete blocks, except much of the aggregate filling material is ash. Generally used as the material for load-bearing walls or in the foundation of a house, but their sturdy, heavy composition and stackable design also make them usable in many DIY projects around the house.