Materials Used for Mortar

Sand

Sand is often referred to as a fine aggregate. It is normally dug from a pit or a quarry and should be clean and free from impurities.

Sea sand contains salt, which will adversely affect the mortar and should not be used unless it has been washed and supplied by a reputable firm.

Good mortar sand should be well graded so that it contains a mixture of fine, medium and coarse particles.

Poorly graded sands, with single-size aggregates, contain a greater volume of air, and will require a greater amount of binding agent to fill in the spaces between each grain and make the mortar workable. Mortar made from poorly graded sand will be weaker and is more likely to shrink, leaving cracks into which rain can penetrate.

The following guidelines should be followed to ensure that the mortar will be of a suitable quality to produce good brickwork.

• Use only specified sand – different sand may require different mix proportions.

• Obtain sand from one source of supply – different sands will result in a different mortar colour.

• Store and protect sand from rain and from contamination by other building materials.

• Dirty sand produces weaker, less durable and discoloured mortar.

   Difference between a poorly graded sand and a well graded sand

Cement

Cement is the most commonly used binder in bricklaying mortar. It is used to bind together aggregates such as gravels and sand to make concrete, and to bind sand and lime to create mortar.

The most common form of cement is Ordinary Portland Cement (OPC), which is used to a great extent in concrete and mortar. The name is thought to be derived from the colour of the cement resembling that of the stone found in Portland in Dorset. Portland cements are also known as hydraulic cements because, when mixed with water, they set to form a solid mass.

Water

It is essential when mixing cement or lime mortars that the water is clean.

The term ‘drinkable’ water is often used to explain the need for clean and not contaminated water.

Very often, a water barrel is used on a building site for storing water for mixing. This can become contaminated due to operatives regularly cleaning tools in the water. There is a risk that a loss of mortar strength may occur in this case.

Plasticizers

Plasticizers are used to improve the workability of a mortar mix. These are usually proprietary liquids which have the property of inducing a repelling force between cement particles and which therefore act to make the mortar less dense and easier to handle on the trowel.

The dispersed particles require less water to lubricate them and so mixes tend not to become waterlogged. This helps to avoid seepage from the joints when laying bricks with a low water absorption rate.

Never use domestic or commercial detergent as a plasticizer as they may contain harmful chemicals. Unlike proprietary plasticizers, they generate uncontrolled quantities of large air bubbles within the mix thus producing weaker and less durable mortar.

Retarders

Retarding plasticizers have added chemicals which increase the length of the initial setting time. Some retarders can increase the setting time from 3 to 7 hours without loss of strength at 2, 7 and 28 days.

Retarders are often used in hot weather when the sun tends to accelerate the setting.

Modern pre-mixed mortars contain high performance retarders that allow the bricklayer to use the mortar for up to, typically, 36 hours after mixing.

Accelerators

Accelerators are used to speed up the hydration of cement, producing higher strength at the early stages. It is particularly useful as a cold weather precaution.

Calcium chloride is a chemical which is commonly used as an accelerating agent. It is purchased in solid or flake form and needs dissolving in water before use. It is added at up to 2% of the weight of cement in the mix.

This will typically give a 25% increase in strength at 7 days.

One disadvantage of the use of calcium chloride is that it is corrosive to steel and should therefore not be used in steel reinforced brickwork etc.

Chloride-free admixtures should be used where there is a possibility of the mortar coming in-contact with any steel reinforcement.

Pigments or colouring agents

These generally consist of pigments added in specified quantities to OPC, usually at the rate of up to 1kg per 25kg bag of cement.

Larger quantities may reduce the strength and workability of the mixed mortar.

Various colours are available, typically red, black, brown, and yellow and marigold and the agents can be used in mortars for bricklaying, pointing or rendering.

It is virtually impossible to add accurate proportions on site and maintain a consistency of colour.

Hydrated limes

Hydrated lime can be used to improve the workability of a mortar mix. Being water retentive, the lime improves the bond with the brickwork and the strength of the brickwork as well as improving its rain resistance.

Mortar containing lime is often referred to as sand/lime mortar and must be treated with care at all times.