A mold, process of using the mold to form a block, and the resultant block are disclosed. The block may of course be connected to other blocks to form stuctures such as walls. The blocks and structures may have curved features for simulating a log structure. The mold and process utilize cemititious material for forming a the block, which has a pair of block members, one of which has a curved outer face. The block members are separated by a central insulation space and connected by rods extending from each block member across the central insulation space. The mold has a body including a pair of block forming cavities. Each cavity has a material ingress-egress opening adapted to receive a block forming composition and to output a block. The block forming cavities are separated a predetermined distance by a space forming central cavity. The mold also has a block ejector movably connected to the body. The ejector has a plate member associated with each cavity, and a handle. The blocks disclosed include pair of block members, one of which has a curved outer face. The block members are separated by a central insulation space and connected by at least one block connector extending from each block member across the central insulation space.