Below are details of concrete and mortar for different applications, how to estimate quantities and what these quantities mean. The various mixes are quoted as volume ratios, however the individual components are normally sold by weight, so a little calculation work is required.

For normal ‘around the house’ types jobs, Ordinary (or Portland) cement is required. This is normally available in 50kg bags, although smaller 25kg bags are increasingly becoming available to take account of the UK manual handling regulations. Try not to buy more cement than you need as it cannot be stored for very long. Some special cements are available (i.e. quick drying) which may be required for special situation.

Lime:

• hydraulic – which sets by combining with water.

• Non-hydraulic – this hardens by drying out.

• Semi-hydraulic – which is a half-and-half, it mainly needs to dry out but has some hydraulic properties.

Each type is available as either hydrated lime and quicklime – hydrated is more convenient to use as it has been processed ready for use. Hydrated hydraulic lime must be used straight from the bag. Hydrated semi- or non-hydraulic are better to use after soaking in water for 24 hours – stir it into water and leave it overnight – the lime sinks to the bottom and excess water can then be poured off. Mix the lime with the sand before adding the cement.

Lime is sometimes used in cement and reduces the amount of water drawn into the bricks, thus preventing the cement from drying out too quick and shrinking. The main types of lime are:

Sand:

Soft sand (or Builders sand): a smooth sand, non-gritty, loamy and with cohesive properties, can be used for:

• Bricklaying mortar

• For bedding paving slabs

• For rendering walls

 

Sharp sand: this has a gritty feel and is similar to that used to condition soils and potting composts, can be used for:

 

• Concrete

• For rendering floors and walls

 

Two types of sand are available but are not interchangeable in all applications, whichever type is being used, always pass it though a sieve before use to remove any small stones etc.

Aggregate

(also referred to as ballast): is used for concrete and consists of various sized particles from fine grains of sand to small stones. Aggregate is normally graded by size of the largest stones; 10mm Aggregate will have been sieved though a 10mm mesh to remove all larger stones.

Mixes:(see bottom of page for render/stucco mixes)

Mortar

sand : cement

sand : lime : cement

general building (above ground)

5:1

5:1:1

general building (below ground)

3:1

6:1:1

Internal walls

8:1

9:2:1

 

Cement in concrete and mortar creates its strength by chemical reaction, it is not like wallpaper glue which ‘drys out’, it ‘goes off’. In fact, if the concrete or mortar should ‘dry out’ before the chemical reaction is substantially complete, it will fail. Likewise if it is subjected to a frost, it can also fail.

Concrete/mortar will take months to reach it’s full strength however after about 24 hours it should be fairly hard and after 4 or 5 days, the strength should be enough to stand upto normal punishment.

The various components:

Cement:

none

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