by John Crowe
Clay Brick and Paver Institute
To achieve the practices specified below a high level of co-operation between architect, builder and bricklayer is necessary. It should be recognised that it may be difficult to insist upon the specification, especially about mixing procedures, but small matters may affect the quality of the work.
Where a standard 1:1:6 mix is specified and good batching practice followed, few problems arise on site. This is for mixes using both ordinary Portland and off-white cement (the use of white cement being rare because of its high cost and fickle drying characteristics).
To ensure that variations in mortar colour do not occur, the following practices must be followed:
The same materials must be used for the entire job.
˙ The same sand type.
˙ The same lime and cement brand and type.
The same batching and mixing procedure must be followed for the entire job.
˙ Sand must be properly batched - bucket or box - but also, if possible, the moisture content of the sand must be maintained. If sand dries out, it settles, and this can cause up to 30% excess sand when batched by volume, obviously affecting the shade.
˙ Cement must be always added dry from the bag.
˙ Lime can be added dry from the bag, or slaked on site, usually overnight, and added to the mix as lime putty.
Where lime is added directly from the bag dry, the mixing action alone does not give the lime sufficient time to produce a "fatty" mortar, like the immediate action achieved by an air-entraining agent. The practice of slaking sufficient lime overnight for the following day's mortar solves this problem and also reduces any problems associated with dust affecting the labourer.
Where the lime is slaked, settlement of the lime particles occurs overnight. The air is removed from the lime and accordingly less volume of wet lime is required for the mix. As a guide, a container of approximately 50% of the cement volume is required.
The water quantity per mix should also be measured. This may entail three containers of water being filled for the first mix, and recorded as added to the mixer. The addition of water during mixing is often the biggest cause of variations in mortar consistency, and ultimately mortar colour and strength. Where too much water has been added to the mix, sand only is often used to dry up the mortar. If cement or lime is also added, it is not always accurately measured.
The mixer should be revolving at the correct speed and ingredients should be mixed for a minimum of three minutes.
Half to two-thirds of the water should be placed in the mixer, half of the sand should then be added, followed by all the cement and lime. The rest of the sand should then be added to complete the batch. The remaining water can be added to produce a wet, workable mix.
The ingredients should be added in such a manner that the mix does not dry out, or migrate to the back of the mixer and stick to the back of the bowl. Every effort should be made to keep the materials moving over the tines and mixing together. This may require adjustment to the way water is added during the mix. If ingredients are added too fast the mixer may not be able to combine them into a mortar, and the dry materials will stick around the bowl. This can be a problem if not enough water is placed in the mixer before adding the dry ingredients.
To produce good mortar on-site from a tilting bowl mixer the following can be used as a guide:
˙ Set up the mixer so a barrow can fit under the bowl without touching.
˙ Make sure the mixer is level and plumb so the bowl is at the correct angle to ensure proper mixing.
˙ Always use a bucket or box to batch materials and accurately measure the water in the first two batches to avoid adding too much.
˙ Keep sand moist to maintain consistency when volume batching.
˙ Slake lime to obtain a "fatty" mortar.
˙ Add ingredients at a rate to maintain the mixing process and in the same order as the first mix.
˙ If too much water is placed in the mixer, add dry materials only in the mix ratio 1:1:6, etc. Never use sand only to dry up wet mortar.
˙ Allow a minimum of three minutes for proper mixing. Mortar turned out too quickly can be streaky, can contain clay balls, and partially mixed lime and cement. This is extremely critical where colour pigments have been added to the mortar.
˙ Put mortar into moistened barrow and distribute and use while fresh. Mortar can be retempered to maintain workability, but should be dumped if not used after one to one-and-a-half hours.
Coloured mortar requires precautions and care during mixing and laying. Several unconnected operations can affect the colour of the mortar in the finished wall. For good results with coloured mortar, the ingredients, the proportions, job conditions and workmanship must be consistent.
The above notes on mortar mixing apply equally to coloured mortars.
Synthetic or natural pigments are supplied dry and premixed as liquid.
Most dry colours are made from iron oxide pigments. Iron oxides are non-toxic, colourfast, chemically stable in mortar and resistant to ultra-violet radiation. They include yellow, reds, browns and blacks. Chromium oxide produces greens and Cobalt produces blue. Ultramarine blues, made from sulphur, sodium, carbonate and kaolin are less stable in mortar. Carbon black and lamp black (used to make blacks and browns) are less weather resistant than iron oxides. Beyond a certain point, called the saturation point, the colour intensity of mortar does not increase in proportion to the amount of pigment added. Synthetic iron oxides generally are saturated at about 5% of the weight of cement and natural oxides at about 10%. Over use of pigment may weaken mortar.
Mixing - Coloured Mortar
For lighter colours - creams, yellows or buffs - an off-white cement and lime will produce a better colour. Lime included in any coloured mortar certainly assists the pigment to produce a true colour.
To maintain colour consistency:
Keep moisture content of sand consistent. Dry sand is more dense in volume batching and excess sand puts more space between pigment particles lightening the colour. Select a light coloured low clay content sand.
Carefully record proportions of mix and sequence of mixing process. Coloured mortar should not be retempered as the extra water tends to change the colour. Only mix that which can be used immediately.
Bricks with high to medium initial absorption rate should be pre-wetted. High suction bricks not only affect compressive and bond strengths, but also absorb colour pigments from the mortar.
Finishing the joints can also affect the colour. All round iron jointing should be carried out when mortar has reached the same consistency (stiffness). Jointing mortar that is too soft lightens the colour, but if that has set too much, the colour.
Some bricklayers add a small amount of a darker colour to the mortar to improve and set the colour.
To yellow - add brown
To red and brown - add black
Lime improves all colours when added to the mix.
Brickcleaning can have a pronounced effect on coloured mortar. High pressure water jets and hydrochloric acid can remove the cement and colour from the surface of the joints. If any cleaning is required, soft methods such as stiff brush and diluted cleaners should be used. Where possible, work should be cleaned during construction, leaving the minimum completion.
Pre-mixed dry and wet mix coloured mortar is available and may be of higher quality and colour consistency than site-mixed mortar, which could offset higher costs.
With dry pigment extra care needs to be taken to produce a consistently coloured mortar. Start the mixing procedure by adding of the pigment to half to two-thirds of water and one-third to half of the sand. Let these ingredients mix for about one minute before adding any other ingredients, then follow the mixing procedure outlined for normal mortar.
Always use the same pigment - do not change type or supplier. A small amount of extra pigment may be required in the first batch, as some of the pigment will be left inside the bowl of the mixer, so the first batch is usually lighter in colour. The first batch can be used for unseen work or an extra 10% of pigment can be added to first mix of the day.
Liquid pigments can be added at any time during the mixing process. As the pigment is already 100% dispersed in the liquid, the colourant can be added after the sand, lime and cement are in the mixer. Final water to produce a workable mortar is added after the colourant. Because the amount of water may have to be adjusted to accommodate variation in sand moisture content, liquid pigments should not be added last.
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