Mortar is used a bond in between concrete blocks and bricks, among other things. Mortar is a calculated mix of portland cement, hydrated lime, sand and water. Using different measurements of each ingredients will create a different kind of mortar every time. If you're using brick or cement blocks for a project that still has the mortar around it, it needs to be removed. The safest way to remove to mortar is to stay away from harsh chemicals (muriatic acid, or hydrochloric acid, is extremely dangerous and should be used by professionals) and use some tools.
Hold your chisel up, pressing it into the mortar. Start in the upper left-hand corner, working your way to the right.
Tap on the end of the chisel lightly with the hammer. Continue tapping on the chisel until the majority of the mortar (on that part of the block) is gone. Move your chisel to the right and continue hammer the mortar off.
Repeat Steps 1 and 2 until almost all of the mortar around the concrete block is gone.
Brush the top part of the block with the steel brush. This will shave off any left over mortar around the block. Use heavy pressure as you brush the block until there is no mortar left.
Repeat Step 4 with the other three sides of the block.