Sturdy concrete blocks, sometimes called cinder blocks, supply a durable material for a wall. The blocks withstand weathering without any signs of wear when erected properly. Concrete cinder blocks come in standard sizes, which simplifies the process if you later add height to the wall. Mortaring the new blocks in place using the correct methods ensures a sturdy addition that blends with the rest of the existing wall.
Mix the concrete mortar powder with water in a 5-gallon bucket, following package directions. Only mix the amount of mortar you expect to use within one hour so it doesn't dry out.
Apply a 1-inch-deep layer of mortar, spreading it 8 inches wide, over the top of the existing block wall, using a mortaring trowel. Draw the side of the trowel through the center of the mortar bed to create a furrow. Only cover three to four blocks at a time.
Set the corner block in place. Corner blocks have a finished edge on the outer side of the corner. Use a half or whole corner block as needed to ensure the joints on the new tier are staggered above the joints of the block tier directly below. If the previous top tier begins with a whole block, begin the new tier with a half block, or vice versa.
Apply a 1-inch-thick layer of mortar to the edge of the second block. Set it in place, abutting the mortared edge against the edge of the first block. Continue to add mortar to the edges of each new block and the top of the wall as you set each block in place.
Draw the edge of a masonry jointer along the mortared seams before they harden. The jointer smooths the seams between the blocks. Apply enough pressure to the jointer to create seams that mirror those present on the existing wall.
Mix quick-setting concrete in a clean bucket, following package instructions. Fill in the holes in the top of the blocks with the concrete mix. Smooth the top level with the back of the trowel.