Concrete blocks are an economical alternative to poured cement for building pillars and posts. Unlike poured cement, you can build a pillar out of concrete blocks without having to build and reinforce extensive forms. Concrete blocks are nominally 8 inches by 8 inches by 16 inches in size, but in fact are slightly smaller than that; the block plus a proper mortar joint will equal this size. For a pillar, buy blocks with flat ends. The main thing to remember is that pillars are only as strong as what they are sitting on, so they need to be built on a solid footing or foundation.
Check the foundation or footing that you are building your pillar on to ensure it is level. If you are not sure about its strength, or if you are building a new footing, make it at least 6 inches wider on each side than your pillar, and at least 6 inches thick. Putting rebar into the footing so that it extends up through the holes in the blocks will add strength, but will make building the pillar more difficult as you will have to lift each block over the rebar. It is adequate to simply put the rebar down through the blocks when you fill the centre holes with mortar.
Clean off any dust or debris, then mark the outside dimensions of the pillar with a chalk line or pencil. Get all your materials on site before you begin work. You will need enough blocks for the pillar you are building, enough mortar for the blocks, a water source and your tools. You will need one bag of mortar for every 40 blocks, plus several extra bags for filling the centre holes.
Build a 16 inch square pillar by laying two blocks next to each other. Build a 24 inch square pillar by using four blocks laid in an end-to-side pattern, leaving an 8 inch square hole at the centre.
Mix a bag of mortar in the wheelbarrow. Add water in small amounts until the mortar is a workable consistency but not too thin. It should be thick enough that when you stir it with a shovel, it stops moving within one second after you remove the shovel.
Put down a layer of mortar about 1 inch thick over the area where the blocks will go. Place the first block and tap it into place with the handle of your trowel. Be sure that its edges line up with the lines you made. Using the torpedo level, be sure that the block is level both side-to-side and front-to-back.
If you are building a 16-inch column, spread mortar on one side of the next block and press it firmly against the first block and down into the base mortar. Be sure that it is level in both directions, and with the first block.
Spread mortar on the tops of your first blocks, then lay the second course of blocks perpendicular to and on top of the first, so the joint between the two blocks crosses the joint between the first two blocks at a right angle. Check to ensure they are level on the top with the torpedo level, then check the sides with the 4-foot level.
Continue laying courses of blocks, checking to ensure courses are level horizontally using the torpedo level and plumb vertically using the 4 foot level at every step, until you have reached about 4 feet in height. Now you have holes in the centres of the blocks that reach all the way to the footing. Fill these holes with the same mortar used in the joints and tamp it down with a pole or the handle of a tool.
Continue building until you reach the desired height, stopping every 4 feet to fill the inside holes with mortar.
An alternative to checking with the level is to build a framework around where your pillar will go and secure strings showing where the corners will be. Making these strings plumb and level, you can then follow them as you build. For added strength, you can put 1/2 inch steel rebar into the holes when you fill them with mortar.
Clean your tools well when you are done. Be especially careful to clean the wheel of your wheelbarrow, as the lime in the cement will wreck the rubber if it is left on.