A good mortar should produce a satisfactory joint between bricks.  For external work the mortar must develop sufficient strength and bond to the units, to be durable and produce weather-resistant joints.  The mortar must not be unnecessarily strong as this increases the risk of shrinkage cracking.  Non-load-bearing internal work should be as weak in strength as practicable.  Generally, strength is not the most important factor, except for load-bearing  brickwork, where the strength of the mortar is important.  A good mortar should be easy to use and harden fast enough to avoid delay in construction.

The properties of mortars vary with the nature and proportions of the constituents – cement, sand, lime admixtures, quantity of water and method of mixing.  Cement:  sand mortars are very strong but harsh;  the addition of lime or plasticiser will reduce their strength but will make them more workable.  The table below sets out the various types of mortar, how they are made and where it is recommended they be used.

Mortar mixes
(Proportions by loose dry volume)

Mortar designation	Cement	Lime	Sand
I	1	0.25	3
II	1	1	6
III	1	2	9

 

Type of construction	Mortar designation
External walls, outer leaf of cavity walls above the DPC.	II
External walls below the DPC	I or II
Backing to external solid walls.	II or III
Internal walls and inner leaf of cavity walls.	ii or III
Coping and Sills	I
Parapets and domestic chimneys – rendered brickwork.	II
Parapets and domestic chimneys – facebrick work.	I
Retaining walls.	I
External freestanding walls	I or II
Underground work such as manholes	I

Note that all recommended mortar mixes contain hydrated lime.  This lime not only improves the workability of the mortar but also contributes significantly to the improvement in rain penetration resistance.

The selection of sand for mortar is a subject which deserves more attention than it is normally given.  Poorly graded or mono-sized mortar sand often leads to high permeability of the finished mortar-joint.  Mortar sands containing excessive clay (more than 10%) will display excessive shrinkage during curing with the possibility of setting up substantial cracks through the mortar joints.

Wherever practical, sand which complies with the grading requirements of S.A.B.S. 1090 should be used for mortars for brickwork.  The use of such sand will minimise the chances of water penetration through the joints, given good workmanship standards, and will more than repay any small additional first cost through savings in future remedial work in curing building leaks.