This Vandersanden sponsored CPD explains how salt, lime and gypsum efflorescence can stain brick facades and the options for prevention and correction
Brickwork can experience several types of discolouration, which tend to vary in colour depending on the brick type and stain. However, this CPD focuses on the three types of efflorescence that are usually expressed as a white or greyish discolouration. Recent studies have shown that this discolouration is often caused by mortar.
Mortars have evolved over time to offer extra plasticity, adaptive curing times and cost reduction. However, the potentially detrimental effects of some of their constituents on brickwork are not usually discovered until some time after construction, when the bricks start to change colour.
Discolouration in clay brickwork has been the subject of research over many decades. The most recent doctoral study by KU Leuven University in the Netherlands attributes these stains largely to the joint mortars used and divides the stains into three types: salt efflorescence and lime efflorescence – which both come under the category of primary efflorescence – and gypsum efflorescence, which is categorised as secondary efflorescence.
How does staining happen?
Discolouration usually starts when the facade, or an individual brick, gets wet from precipitation. Some brick types, for example those with a more open pore structure such as stock bricks, are similar to sponges in that they can absorb water into their pores through a suction effect.
In addition to water, bricks absorb minerals such as lime and salts. As the brick starts to dry out, water evaporates through the brick face and these minerals then deposit themselves on the face of the brick. They are usually visible as a white discolouration.
The colour of a facade can also be affected by dirt deposits – either from nature or human activity – that become attached to the bricks, particularly in wet conditions.
The most common form of discolouration on new brickwork is salt efflorescence, a form of primary efflorescence. This can occur for a number of reasons, starting with the salt content found in the clay used to make the brick. The primary salts involved here are all water soluble, which means they turn into a solution as soon as water reaches them, whether it is from the mortar or from the brick.
The most common salts are sulphates of sodium, and occasionally potassium or magnesium. It is also thought that nitrates or phosphates are possible sources of efflorescence, but research suggests this is seldom the case.
What research does show is that calcium oxide from the mortar will react with water to build up calcium hydroxide, which is extremely water-soluble. If a newly built wall is not covered and water runs over and through it, this can result in efflorescence staining.
Primary efflorescence is more visible on bricks and usually appears on facades towards the end of winter and the beginning of spring, as the masonry structure starts to dry out and the salts are expressed.
Primary efflorescence – salt efflorescence
To help prevent salt efflorescence, it is important to follow the manufacturer’s advice, using the correct mortar for the type of construction. Bricks and mortar need to be protected from excess water during construction, using wall covers where possible to add some additional protection.
After construction the masonry needs to be kept as dry as possible, ideally for up to 10 days. In cavity wall construction, enable some effective ventilation of the cavity to promote drying. As this is a transitionary phase, the salt efflorescence usually weathers away naturally after a couple of rain showers and dry periods.
Treatments for salt efflorescence include using a dry, soft bristle brush, along with a dustpan, to brush and collect the salts into the pan. Allowing them to fall to the ground could result in them being reabsorbed into the ground and being blown by wind onto the brickwork. Workers can also use a clean wet sponge to draw out some of the excess minerals from the body of the brick.
Primary efflorescence – lime efflorescence
The second type of primary efflorescence is lime efflorescence – or lime run-off, as it is also described. It usually appears on the mortar joints and can then travel down the face of the brick.
How does this happen? To establish adhesion between the bricks and mortar, excess lime is created that reacts with the carbon dioxide in the atmosphere. In wet conditions the brick pores can overflow, and the carbonation process required to achieve adhesion between bricks and mortar is less effective. As a result, excess lime is created and expressed out onto the brick facade.
The severity of stain is affected by the carbonation process. Under good conditions, carbon dioxide from the air converts the excess lime in the mortar into a water-insoluble lime and cures. However, under adverse conditions, calcium oxide from the mortar will react with water to build up calcium hydroxide; this is extremely water soluble and can then spread throughout the masonry.
The calcium hydroxide then reacts with carbon dioxide from the air to form the water-insoluble calcium carbonate (lime); this is what we see expressed on the facade.
At an early stage, lime efflorescence can easily be removed using water or brick cleaner. However, if it is left in place it will need to be professionally removed with special cleaning products.
While rain and excessive moisture are the main issues, it should be noted how important the carbonation process is to ensure the adhesion between bricks and mortar.
Secondary efflorescence – gypsum efflorescence
The second efflorescence category, gypsum efflorescence,is different from the primary category in that it appears much later in a building’s life and can have considerable aesthetic implications.
Gypsum efflorescenceis usually a white-grey semi-transparent discolouration that appears months or several years after the construction has been completed. It normally occurs on the south-west elevation of a building, and because it is grey in colour, is more visible on the brickwork than on the mortar joints.
Gypsum is added to cement as an auxiliary substance to help with curing. The effect of carbon dioxide on mortar can lead to excess gypsum being released from the cement. The additives in the mortar then transport the gypsum into the brick pores, where it then crystallises.
Rain and wet conditions stimulate this process, particularly on uncompleted brick facades that are left exposed to rain followed very quickly by heat from the sun; this causes the gypsum to crystallise in the brick pores.
The consequences of this type of staining are more than aesthetic. Architects will be less inclined to showcase and promote their projects if such stains are prevalent.
Contractors are likely to receive questions and complaints from clients and architects about the causes, and to suffer the costs of remedial work to put it right.
Homeowners are less likely to be proud of their home and it could result in a lower house price, if there is visible staining when selling the property.
A prevalent issue
The problem has existed for around 20-30 years, with its commencement thought to coincide with the changes in mortars introduced to make them more pliable.
Research suggests that 80%-85% of facades built across Europe in the past 15 years suffer from these types of stains in one way or another. Bricks with low water absorption rates are less affected, but this accounts for only 10%-15% of all brick facades.
How can you prevent these stains from occurring and design in some protection for brickwork facades?
Ideally, a design should be arrived at that is easily achievable in practice, to avoid any modifications to the detail which could result in parts of a facade becoming saturated by rain. Detailing the brickwork to ensure the maximum shedding of water is important.
Consideration should also be given to the storage of bricks on site; this would ideally be under some form of cover, so as to prevent additional saturation from the rain and to stop minerals from the ground being absorbed into the bricks which may then be expressed later.
Protective coatings applied on site
Another solution is to apply a protective treatment after the structure has been completed.
This would usually be undertaken on site by a specialist contractor and should be carried out in a timely manner, with the facade having been cleaned thoroughly but given time to dry.
The detail of the application method and the coating itself being used should be discussed with the contractor or manufacturer as early as possible prior to construction. It is essential to confirm at this stage that any coating being applied will not adversely affect the fire performance of the brick.
Preparation work, coating performance and coverage, and also application timings and any environmental impact should all be discussed. The coating selected must of course be provided by a reputable manufacturer who complies with the relevant UK building products standards.
A second option is designing in preventive solutions such as pre-coated bricks and brick slips. Brick manufacturers are investing in specific coating facilities whereby bricks and brick slips are coated as part of production process.
When a brick is coated during production, the pores of the brick can be coated with material that is of a consistency which ensures the main faces of the brick become water repellant while allowing the brick to breathe, rather like a Gore-Tex raincoat. The coated brick can be handled for construction in the usual way once it has been delivered to site, and any unused bricks can be stored or recycled in the usual way.
The coating applied is often transparent, so the aesthetic of the visible face of the brick is unaffected. Coated bricks also stay drier, meaning facades will attract fewer dirt deposits, and the pores of the brick are kept open to perform as required, enhancing mortar adhesion.
Coated bricks have been designed to add additional protection and prevent unsightly stains appearing across facades during and after construction. Coated bricks are also designed to enhance environments and are ideal for any residential or commercial construction project where a brick facade will be highly visible and exposed to the elements or to pollution.
From a design perspective, when considering using brick colours that may be more susceptible to showing the effects of efflorescence – for example, deep reds, greys or blacks – then coated bricks are a better option than standard or uncoated bricks that are coated after construction, as their use saves time and money not only during the building phase but beyond, if restorative or remedial work is needed at a later stage.
Compliance and adherence to industry standards is essential when selecting bricks and any coatings being applied. All products must comply with the current standards as defined by the British Standards Institution.
The certification standards for clay bricks in the UK are:
- BS EN 771-1:2011+A1:2015 – specification for clay masonry units
- ECO standards ISO 14001.
Other recognised standards that must also be followed are:
- BS 8000-3:2020 – workmanship on building sites
- PD 6697:2019 – recommendations for the design of masonry structures
- PAS 70:2003 – guide to appearance and site measured dimensions and tolerance.
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