Cinder blocks are often used in building lightweight structures and walls, and may contain hollow areas, which you can fill. By filling them with different materials, you change the purpose of your cinder blocks to make them practical all around your home and garden. It's worth noting that although the names are often used interchangeably, there is a difference between concrete blocks and cinder blocks, specifically in the materials used to make them, which also influences strength. So whether you're looking to use actual cinder or concrete blocks, knowing what to fill them with can come in handy. More concrete is a common way to fill cinder blocks.