Concrete basics for building: Mortar and Plaster

Mortar binds bricks and blocks together to give strength and stability to a wall. Sand-cement plaster is used as a decorative or protective coating to concrete, masonry walls and concrete ceilings.

In both cases, what we have said about concrete applies equally to mortar and plaster, but there are some special factors you should take into account.

Freshly mixed mortar must be soft and plastic so that it spreads easily and makes good contact. A brick should be easily pushed down to level by hand pressure. Too strong a mortar may crack and is more expensive.

Freshly mixed plaster must be workable and cohesive, i.e. it must be plastic and have good water retention.

  • A workable plaster will squeeze out from under the trowel and it will be possible to push the trowel to within a few millimetres of the underlying surface.

• An unworkable mix will ‘lock up’ once the trowel has moved a few millimetres and prevent further downward movement of the trowel.

1. Materials

1.1 Cement

Use cement complying with SABS EN 197-1/SANS 50197-1 strength class 32,5N or, where available, masonry cement complying with SABS EN 413-1/SANS 50413-1 strength class 22,5X.

1.2 Sand

The quality of the sand is especially important for mortars and plasters and is the main cause of cracking in plasters.

The sand should consist of hard particles ranging in size from dust up to about 2 mm. The sand should be clean (grass, leaves, roots, etc. are harmful) and should not contain excessive amounts of clay.

If possible, ask the supplier for the sand grading, carried out in accordance with SABS Standard Method 829/SANS 201.

Use the following grading of a good quality plaster sand as a benchmark.

Sieve size, mm Percentage passing sieve by mass
2,36 100
1,18 70 to 100
0,60 45 to 100
0,30 25 to 65
0,15 10 to 40
0,075 5 to 15

Pit sands are often well-graded and generally are best for plaster, but watch out for excessive quantities of clay which may cause cracking in wet/dry cycles.

The actual source of a sand is not a reliable guide to its quality – sand quality can be determined only by making trial mixes. If possible, ask the supplier for a sample of sand before ordering, and make up a small batch using 5 kg of cement and 25 kg of the sand.

Measure how much mixing water is needed to give a good consistency:

• If 5 ? is enough, the sand is excellent.
• If 6 ? is enough, the sand is average.
• If you need more than 6 ?, find an alternative (better) source of sand.

1.3 Building lime

If sand lacks fine material or is single sized, use a maximum of 40 ? of building lime complying with SABS 523/SANS 523 per 50 kg of cement to improve workability. Lime also helps the fresh mortar to retain water when it is placed against dry cement bricks or blocks and helps to prevent cracking of the hardened mortar.

2. Mix proportions and ordering materials

Table 1: Mix proportions for a one-bag batch of mortar or plaster, using ordinary cement

  Cement, 50 kg bags Sand Yield,
litres
Litres Wheelbarrows
Mortar Class II for normal load-bearing applications, and
Plaster for ceilings, internal and external walls
1 200 3 160
Plaster for reservoirs, swimming pools, etc. 1 130 2 115



Table 2: Mix proportions for a one-bag batch of mortar or plaster, using masonry cement

  Masonry cement,
50 kg bags
Sand Yield,
litres
Litres Wheelbarrows
Mortar Class II for normal load-bearing applications, and
Plaster for ceilings, internal and external walls
1 170 140
Plaster for reservoirs, swimming pools, etc. 1 100 95


To help you estimate how much cement and sand to order:

Plaster: A one-bag batch of plaster for an internal wall using the mix proportions shown in Table 1 will cover approximately 8,5 m2 with 15 mm thick plaster, assuming 20% wastage.

Mortar: The following table and its notes will help you calculate material quantities you will need.

Table 3: Quantities of masonry units and mortar for single leaf walls.

Masonry unit size, mm Masonry units per m2 Mortar m3
per
Length Width Height   1000 units 100 m2 walling
190 90 90 50 0,27 1,35
222 106 73 52 0,36 1,74
290 90 140 22,3 0,41 0,90
290 140 90 33,4 0,56 1,87
390 140 190 12,5 0,84 1,05
390 90 190 25 0,95 2,38
390 190 190 12,5 1,14  

 

The dimensions of the units given in Table 3 are for those that are commonly available at builders’ merchants.

NOTES:

The table is based on exact sizes of solid masonry units, with 10 mm thick bedding and vertical joints, and no wastage. For 15 mm thick joints, multiply the quantity of mortar required by 1,5; for 20 mm thick joints, multiply by 2.

Allow for wastage which could typically range from 15-20% to as much as 50-70% if site control is poor.

3. Handling

In addition to handling procedures described for concrete, a few special considerations relating to mortar and plaster should be noted:

• If mortar or plaster has to be left in the sun before being used, cover with plastic sheeting or a wet sack.
• Discard mortar or plaster that has stiffened so much that you can’t restore workability without retempering, i.e. adding more water. If you are working on your own or with one assistant, rather mix a number of small batches as they are required than a large one-bag batch.
• Use a thin layer of mortar between bricks or blocks; thick layers are wasteful and may lead to cracking.

4. Surface preparation for plastering

Plaster thickness should not exceed 15 mm per layer and should be as uniform as possible, so construct the walls to be plastered as accurately as possible, i.e. do not attempt to ‘plaster walls straight’ in one layer.

The surface should be rough, absorbent to a limited extent, strong and clean, i.e. free of any film such as dust, oil or paint.

• Roughness improves adhesion by providing a positive ‘key’ for plaster to grip.
• If the surface to be plastered is very smooth, e.g. ceiling or existing plaster, provide a key by chipping the surface and applying a thin ‘spatterdash’ coat. Mix one part cement with two parts sand with water to form a thick slurry. Throw the wet mix onto the wall to form nodules about 3 mm high, and leave for about two days. Scratch with a nail – if the spatterdash coat is hard and adheres firmly, apply plaster.
• Brickwork is usually sufficiently rough without further treatment. If the surface is dusty, clean by brushing, water jetting or vacuuming. If the units are very absorbent (most burnt clay and some cement bricks are), pre-dampen to control suction.

5. Applying plaster

Never work in direct sun. Plastering should be protected from the sun and drying winds.

The plaster should be used up within two hours of being mixed and never be retempered by mixing in additional water.

Ensure that plaster is not continuous across the line of a dampproof course.

Cut plaster through to the base material where different materials meet, e.g. masonry and concrete.

Before the wall is plastered, apply narrow strips of wood or plaster along the perimeter of the wall or at suitable intervals to act as guides for the striker board.

General guidelines for applying plaster are as follows:

• Using a rectangular plasterer’s trowel, push plaster onto the wall or ceiling using heavy pressure to compact the plaster and ensure full contact with the underneath surface or layer. The plaster should be slightly proud of the intended surface.
• Once the plaster has started to stiffen, strike off the excess to a plane (or curved) surface using a light striker board. Discard removed material.
• If applying plaster in more than one coat or layer, score the undercoat(s) with roughly parallel lines about 20 mm apart and 5 mm deep to provide a key for the next coat and to distribute cracking so that it is less noticeable.
• For the final coat, use a wood float to remove ridges made by the striker board. At the same time fill in any depressions and float flush with the surrounding plaster. Various decorative finishes can be obtained by brushing, flicking plaster on to the surface and lightly floating, etc.

Source: Cement & Concrete Institute. Photographs: Larsen Building Products and Planning Engineer.