By definition, concrete block is a manmade concrete masonry unit, or CMU. Usually rectangular in shape, concrete blocks normally feature hollow cores and are stacked upon one another. As the blocks are stacked, they are held together by mortar, and as the wall is being erected the hollow cores are filled with grout in order to strengthen the wall as it takes height. Concrete block is manufactured in a number of architectural styles such as split face, fluted, ground face, etc. Concerning waterproofing product selection with reference to concrete block, one of two materials should always be specified…. either Combocrete KL-23 or Combocrete Stain- Blok. The essential difference between these two products is warranty duration. While Regular features a 5-year warranty, the Blok-Lok warranty may be extended to 10-years.