Concrete blocks have been used in building construction for many years. Traditionally, they are laid with mortar between blocks, much like bricks are laid. But dry-stacking is an option, which is much easier for a do-it-yourselfer and can produce a wall as strong as a conventional concrete block wall. Dry-stacking has been done for more than a century, but new products, including interlocking blocks and improved surface bonding cements, make it even easier and better. You can use interlocking blocks, which have ridges and hollows at joints, or plain blocks.
Lay out your dry-stacked concrete block walls. You can use this technique to build a retaining or privacy wall or to build walls for a garage, shed, house or other building. The first step is to mark a foundation area for the wall, then excavate it to pour a concrete footing---the depth will vary with the height of your wall. Level the concrete, and let it cure for a week. Some builders recommend installing reinforcing bar at predetermined locations so it will set in the foundation; others wait to add rebar after the wall is built.
Set your first course of blocks on the foundation in mortar; do not put mortar between the block ends. This is the most important step in building a dry-stack concrete block wall. The blocks must be precisely level, straight and square; for a building, corner blocks must be accurately placed. Any deviation in the first row will affect the entire structure. Some builders recommend "striking" the foundation mortar, using a curved piece of metal to force additional mortar between the bottom of the blocks and the foundation.
Stack your additional blocks, overlapping joints in a "running bond" pattern. If you are using interlock blocks, align the tongues and grooves on the tops and bottoms and sides of the blocks carefully. Set strings with stakes as square and level references; once you have a few rows of block, you can set the string between corner blocks. Work from the corners to the centre as you lay blocks; otherwise, your wall may be wavy. Check your level, square and plumb on every row. For very tall walls, erect wooden bracing to prevent any tipping; this can simply be a horizontal 2 by 4 board held up by diagonal boards.
Finish your dry-stacked concrete block wall with a surface bonding cement, about 1/8-inch thick, on both sides of the wall. Apply this with a trowel or as recommended by the cement instructions. If you installed rebar in the foundation, you will have stacked blocks over it. Otherwise, add rebar about every 4 feet, through the hollows in the blocks down to the foundation. Then fill these openings with concrete.