Mortar is an ancient mix that has been used for centuries to hold stone, bricks and blocks together. It is a combination of cement, lime, sand and water, which together go through a chemical change forming the rock-hard mortar used in construction. Mixing these ingredients starts the chemical reaction immediately, so make only as much as you can use within one hour. The typical ratio for mortar is 1 part water, 1 part cement, 1 part lime and 6 parts sand.
Pour half the sand into the mixing container, add all the lime and cement, and then the rest of the sand. Mix thoroughly with a hoe, pulling and pushing it across the container.
Add the water gradually, working it into the dry materials with the hoe. Continue adding water as needed until the mixture resembles a soft pudding that holds its shape when you lift some with a trowel.
Apply the mortar immediately to the concrete blocks, as it will be hardened within an hour.
Getting a bag of mortar compound at your local hardware store will save a lot of time and effort. Simply pour the compound into a bucket, add the correct amount of water per package directions, and stir.
Use rubber gloves to protect your skin, because the mixture is very caustic.