Using Cement to Make Concrete or Mortar-What is Cement Made Of and How is it Used in Construction?

Using Cement to Make Concrete or Mortar-What is Cement Made Of and How is it Used in Construction?

Many DIY and building jobs require the use of cement to make either concrete or mortar. Portland cement is a fine powder that is made up of mixed lime and clay. Ordinary Portland Cement (sometimes called OPC) is grey in color, although a more expensive white cement is also available as are various color additives. Aggregate, a mixture of sand and stones, is added to the Portland cement to make the concrete or mortar. You can mix the sand and stones yourself, or buy them ready mixed. When sand and cement are mixed with water a chemical reaction takes place, which causes hardening. Unfortunately, this means that due to the moisture content in the air the cement will start to harden and deteriorate if kept for a long time after opening the bag. Therefore, you should only buy what is required for the job at hand.

When mixing your own mortar the proportion of sand, cement, and water have to be varied according to the job. Therefore for small repairs, it can be more sensible to buy ready-mixed mortar, as the essential ingredients are all ready correctly proportioned and mixed together.

For a basic mix, the recipe is 3 parts of sand to 1 part of cement. If the mortar is being used in an exposed location lime is often used as an additive to the cement to reduce the amount of water drawn into the bricks. This prevents the cement from drying out too quickly and shrinking and creates a stronger bond. Please bear in mind that if you do add lime you also need to add more sand to balance out the recipe, an appropriate ratio would be 6 parts of sand to 2 parts lime and 1 part cement. You also need to be aware that the mortar will set more quickly and you will therefore need to work more quickly or mix a smaller batch.

The recipe should also be altered according to the weather and may require more or less sand or water be added to the cement. If very cold lime can be added and warm or hot water used to help it set more quickly.

Sands and Aggregates

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

The maximum size of the 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. For example 10mm gravel will have been passed through a 10mm mesh to remove all larger stones.

  • 20mm gravel is the largest acceptable-sized stone and is suitable for heavy work.
  • 10mm gravel suits paths and also finer concrete mixes.
  • Ballast is a mixture of sharp sand and stones and is used for concrete.
  • Sharp sand has a gritty feel and is similar to the sand used to condition soils and composts. Use sharp sand for:
    • Concrete
    • For rendering floors and walls
  • .Soft sand is a finer grade of sand used for
    • Bricklaying mortar
    • For bedding paving slabs
    • For rendering walls

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.

Making Concrete

Cement in concrete and mortar creates its strength by a chemical reaction, unlike wallpaper glue which 'drys out', it 'goes off'. In fact, if the concrete or mortar should 'dry out' before the chemical reaction is complete, it will fail. Likewise if subjected to a frost, it can also fail.

Concrete or mortar will take months to reach its full strength however after about 24 hours it should be fairly hard and after 4 or 5 days, it should be strong enough to cope with normal punishment.

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 to restore the desired consistency. If you are making larger amounts of concrete, or wish to reduce the manual work required, a cement mixer can be used.

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