Cement
Made from limestone or chalk and clay, cement provides the adhesive component in concrete and mortar. It binds the other components – aggregate and sand – together and dries to a very hard finish. The most common form of cement is Portland Grey but there is a lighter variation called Portland White. Cement is sold in 25kg (55lb) or 50kg (110lb) bags.
For repair jobs fast-setting cements are available which set hard in about 30 minutes. Another variation is sulphate resistant cement. This is used when the ground the mixed concrete is coming into contact with has a high clay content or is high in sulphates. Cement, whatever the type, must be stored somewhere dry as it will set rock hard if it gets damp or wet and will be unusable.
Fire cement
An asbestos-free, fire cement for repairs to fire backs. Supplied as a ready to use paste that sets as hard as cement producing smoke tight joints that can withstand extremely high temperatures.
Aggregate
Aggregates are primarily used in a mix when producing concrete and as base materials under foundations and roads.
Hardcore
This is the coarsest aggregate made up of large stones and gravel, and is used as a base for concreting. Hardcore is compacted before the concrete is laid on top.
Coarse aggregate or gravel
Coarse aggregate is widely known as gravel, and consists of small stones that are generally 10mm (2/5 in) to 20mm (4/5in) in diameter. It is generally used in mixing concrete or to improve drainage. Gravel is also available in a variety of attractive colours and is resilient to vehicles being driven over it, therefore is a very popular material for driveways.
Ballast
Ballast is a mixture of sand and gravel and is used in the mixing of concrete.
Sand
If an aggregate has particles less than 5mm (1/5in) in diameter it is classified as sand. There are three main types of sand used in building and DIY and all have specific uses. The three types of sand are: Sharp sand, builder’s and plasterer’s sand, and kiln-dried silver sand.
Sharp sand
This coarse sand has large particles and is used in mixing concrete. Sharp sand can also be used to mix a tough, extremely hard mortar.
Builder’s and plasterer’s sand
Builder’s sand (or bricklayer’s sand) is finer and softer and mixed with cement and water to produce the mortar for laying bricks and blocks. Plasterer’s sand is a finer grade than sharp sand and mixed with cement and water to produce render.
Kiln-dried silver sand
Very fine sand with a pale colour, kiln-dried silver sand is used in grouting the joints between exterior paving or patio slabs.
OTHER MATERIALS USED IN MIXING CONCRETE AND MORTAR
Lime
Although cement already has a lime content, adding more lime to the mix produces mortar that is easier to work with and increases its resistance to cracking after is has set. Although very effective for this purpose, lime has generally been replaced with plasticizer
Plasticizer
Plasticizer is available in powder form but is generally supplied as a liquid. It eliminates the use of lime when mixing mortar, while making the mix easier to work. Mixing instructions will come with the plasticizer (usually on the label), which will include the correct quantity to add.
Cement pigment
Powdered pigments that can be added to the mix to permanently change the colour of the mortar, rendering concrete or pointing.
Waterproofer
Used when the mortar is to be applied to an area that is prone to damp such as exterior walls. Comes in liquid form and works by sealing the mortars pores making it impervious to moisture. Mixing instructions will come with the waterproofer (usually on the label), which will include the correct quantity to add. Waterproofer may increase the time it takes for the render to dry and consequently make it workable for longer.
Frostproofer
If working in colder weather is unavoidable, frostproofer can be added to the mix. The frostproofer speeds up the time it takes the mortar to set if there is a threat of a frost attack. Mixing instructions will come with the frostproofer (usually on the label), which will include the correct quantity to add.
Concrete and mortar mixes
The following mixes give the quantities of cement, lime, sand and coarse aggregate for different types of concrete and mortar mixes used in DIY. The proportion indicates the ratio by volume, usually a shovel-load. However, if greater accuracy is required in the mixing to maintain the strength or colour throughout then it is best to use a bucket to measure out the quantities. In some of the mixes below we have included lime, but you can replace this with a plasticizer if you prefer.
General-purpose mortar for laying bricks, blocks and pointing:
- 1 cement
- 1 lime
- 5 builder’s sand
- Mix with water to produce a consistency that allows the mortar to stick to a trowel when turned upside down
For laying paving stones and patio slabs:
- 1 cement
- 4 sharp sand
- Mix with water to produce a consistency that is stiffer than general-purpose mortar (above)
Concrete for foundations for an extension:
- 1 cement
- 2.5 sharp sand
- 3.5 coarse aggregate
- Mix with water to produce a consistency that pours easily
Concrete foundations for garden walls:
- 1 cement
- 5 ballast
- Mix with water to produce a consistency that pours easily
General purpose concrete for a base for slabs or a garden shed:
- 1 cement
- 2 sharp sand
- 3 coarse aggregate
- Mix with water to produce a consistency that should pour easily
Alternatively, another general-purpose concrete mix is:
- 1 cement
- 4 ballast
- Mix with water to produce a consistency that should pour easily
Render mix for an undercoat for plaster or an exterior coating:
- 1 cement
- 1 lime
- 6 plasterer’s sand
- Mix with water to produce a consistency that allows the mortar to stick to a trowel when turned upside down