Cinder blocks are often used for small structures that don't require significant weight-bearing responsibilities (although you can fill the hollow areas of the cinder blocks to increase their ability to support a load). When comparing concrete blocks to cinder blocks, the cinder blocks are lighter and have a far greater porosity and permeability for water. If your cinder blocks are constantly wet because of this permeability, you may notice mold growing on them. Cinder blocks have pores that retain moisture and give mold a chance to begin to grow. If you find mold on cinder blocks, you should take immediate steps to eliminate it.