Introduction to mortars
The quality of the mortar in a build is of great importance as it has a large bearing on both the look of brickwork and the strength of a building, and it can also increase or reduce the risk of cracking in areas of contraction/expansion.
An ideal mortar should be easy to work with, i.e. sticks to blocks. Use of plasticiser in the mix helps to improve the workability of the mix. It should also be strong enough for purpose and not cause efflorescence in brickwork. Generally, weak mortars are able to account for very small changes in dimension in joints. However, strong mortars will not; this could eventually result in a large crack in a location of high stress.
Components of mortar
The 3 components of mortar are:
- Aggregate
- Binder
- Water
Aggregate refers to sand: the sand must be appropriate for purpose, i.e. of suitable grain size. Binder refers to the combination of lime and cement. Plasticiser or colouring may also be added in place of lime depending on requirements.
The purpose of plasticiser improves the workability of a mix by entraining small air bubbles that act as lubricant. The air bubbles increase the volume of the binder by filling the voids left by the sand, thus improving the workability of the mix. There are many types of plasticiser available and they should always be used in accordance with the manufacturer’s recommendations.
Generally in mortars, cement provides the strength while lime/plasticiser provides the workability. Ideally, mortar should cling to blocks; this can be tested while mixing by testing if it clings to the trowel. It should also spread easily and not stiffen too rapidly.
Mortar Types
Cement and Sand
Quick development of strength in early stages
Adequate strength in fully hardened mortar
Use of Portland cement is the most convenient way of achieving the characteristics above. The strength of the mix should not be changed purely by varying the cement to sand ratio as lean mixes of sand and cement are unworkable.
Cement, Lime, and Sand
The use of cement and lime in appropriate proportions allows a mortar to be workable, adhesive, have good early strength, retain water and, also, not have too high a mature strength. The principle of this type of mix is that the lime replaces the same amount of sand so that the binder-paste fills all the voids in the sand. The use of both cement and lime allows the good qualities of both to be used. Either only non-hydraulic or semi-hydraulic lime should be used.
Air-entrained (plasticiser) mortar
Plasticisers are an alternative to lime in the mix. Essentially, the plasticiser added improves the workability of the mix by creating air bubbles that increase the volume of the binder-paste; this in turn fills the voids in the sand.
Ready-to-use retarded cement, lime and sand mortar/cement and sand mortar
Ensure manufacturer’s instructions are followed when using these types
of mortars.
Mortar mix
The designer should take all the different factors and requirements of the mortar into account, including the type of masonry unit, the type of construction, the position in the building, and the possibility of early exposure to frost.
Table C11 - Typical mortar mixes
Further information is available in I.S. EN 998-2:2010 Specification for masonry – Part 2: Masonry Mortars.
