Concrete blocks are a must for numerous construction projects--particularly outdoor projects, such as fences and walls. You can purchase concrete blocks from a number of different manufacturers, but if you need tailor-made concrete blocks for specific projects--of if you'd just like to save a little money--you can make your own. Do-it-yourself types can make decent money by molding concrete blocks for sale. Once you get the hang of it, the process isn't difficult.
Set up concrete molds in the exact shape you want your blocks to be. You can purchase them from a number of different outlets or make your own by using bricks or stones as models.
Use a concrete mixer to mix cement with clean sand. The ratio should be 3 parts sand to 1 part cement for normal types of concrete. For stronger concrete, make the mixture 2 parts sand to 1 part cement.
Add gravel or crushed limestone to the mixer. The amount of gravel should roughly equal the combined sand-and-concrete mixture: 3 parts gravel to 2 parts sand, plus 1 part cement. You may need to experiment a bit: With too much gravel, you won't be able to get a smooth consistency to your concrete.
Slowly add water into the mixture, stirring it until it reaches the right consistency. You can check as you stir by scooping a sample of the mixture into a slump cone, measuring it it off and seeing how much the mixture settles. When it settles about 3 to 4 inches into the cone, the mixture contains the right combination of ingredients.
Continue mixing the concrete for several more minutes until it is consistent throughout.
Spray the inside of your mold with a water-based concrete-releasing agent. This will let you remove the concrete block from the mold once it has hardened.
Pour the concrete carefully into the mold, then level it off the top using a trowel or a wooden board. Tap the mixture to help it settle, and to get rid of any air pockets in the mold.
Let the concrete harden for at least 24 hours before attempting to remove it from the mold. Then press down on the completed block while simultaneously pulling up on the mold. The block should spring free, and the mold can then be reused to make more blocks.