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The use of cement

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The use of cement

Many outside repairs need concrete or mortar. The important constituent is Portland cement, which is bought in a fine powder. It is made up of lime and clay mixed, which sets hard when mixed with water. It deteriorates and hardens if kept for a long time after the bag is opened. It may prove more economical to buy only sufficient for a job. Ordinary cement comes in a grey colour, but white cement is available, but more expensive. There are also colour additives available for use with cement. These are not always successful as they often fade and can start to look patchy after a while.

A mixture of sand and stones - called aggregate, is added to the Portland cement. You can mix sand and stones, or buy them ready mixed. When sand and cement are mixed with water a chemical reaction takes place, which causes hardening. For small repairs, it might be more sensible to buy ready-mixed mortar, as the essential ingredients are already correctly proportioned and mixed together. When mixing your own mortar, the proportions have to be varied according to the job.

Sands and Aggregates

The size and shape of the material determine the type of sand or aggregate. Sand itself is classed as a fine aggregate.

The maximum size of stone can be specified when ordering aggregate. It is made up of gravel and crushed stones, which are retained, after passing through different sized sieves.

The aggregate in foundations is very coarse, whereas only finer graded builders sand would be used in a mortar mix for pointing or filling a crack. If you are using white screening blocks, use silver sand for a lighter coloured mortar mix.

All-in Aggregate

If you ask for all-in aggregate, sand and stones are already mixed.

For cracks and similar repairs use a proportion of one part cement to three or four parts all-in aggregate. For foundations use one part cement to four or five parts all-in aggregate.

If sand and stones are separate, the second mixture is conveniently remembered as 1:2:3 (cement:sand:stones).

Mixing concrete

Use clean water to mix concrete. The amount of water should be kept to a minimum, as too much gives a weaker result. Put the aggregate on a flat hard surface (such as a sheet of plywood) in a broad pile, and lay the cement on top. Using a spade or a shovel, mix the aggregate from the bottom with the cement until the whole pile is an even colour. Make a hollow in the top and pour in a little water, mixing from the edge to the centre. Add more water and continue to mix, turning in dry material until the whole mixture is wet. Try flattening the heap and cut into it with the spade. The ridges formed should stay there. If they flow back into a smoother surface, the mixture is too wet. It is important to use the right consistency because if the concrete is too dry it will not make a strong bond with masonry. If the concrete is too thick, use a little more water in order to restore the desired consistency.

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