Prevention is better than cure!
There are some great myths that circulate in the building industry, but one that continually pops up is that the sand is the root cause of efflorescence. In this blog post we will explore what efflorescence is in detail, debunk the myth and detail the best way to avoid its unsightly white marks ruining your job.
Does the sand cause efflorescence?
No! The rumour that “the sand is not washed properly” most likely stems from the common phrase; “the salts are leaching out”. It’s likely an assumption is drawn that the problem therefore comes from sea salt in the sand, yet we all know that sea salt does not turn to stone! The term ‘salt’ in fact relates to all kinds of compounds in chemistry, not just the type of salt we put on our fish and chips!
So what is efflorescence?
The word ‘leaching’ is arguably a more technically correct term, but efflorescence has become the popular term for those unsightly white marks that we are only too familiar with. The salts to blame are in fact an inherent part of cement, are easily dissolved and ‘leached out’ of the cement binder by water. The most common types of soluble salts involved are calcium, potassium and sodium and they become carbonates as the moisture/water dries, leaving the deposits behind to react with carbon dioxide in the air. Importantly, many cement based products, such as tile adhesive, also contain polymers that can remain emulsified for a long time, and can also migrate with water and add to the problem.
Can you stop efflorescence?
Every batch of cement fired in a kiln (think an industrial sized oven for making cement) will undergo a unique chemical reaction, and some batches of cement will cause the problem more than others (this explains why sometimes you can be lucky or unlucky). The key to preventing efflorescence is to control water. A small amount can form in the curing stages, but it’s usually negligible. It is the ingress of rain water afterwards that brings on the big problems, but with diligence, the right products and good design, it can be avoided.
Is efflorescence your responsibility?
Since January 2015, the NSW Home Building Act has defined two categories for defects: Major defects and minor defects. A builder is liable for major defects for 6 years, and minor defects for 2 years. As long as there isn’t an associated structural waterproofing failure, efflorescence is arguably a minor defect.
So what are my design options to avoid efflorescence?
External tiling is where we most commonly see efflorescence problems. Every job is different, and there are many ways to “skin a cat” depending on many factors, not least the many choices of materials that are available. Below is a list of considerations that will help you prevent efflorescence on your next external tiling installation:
- Know the two most important reference documents relating to external tiling. i) AS 3958.1 - Guide to the Installation of Ceramic Tiles. ii) NSW Guide to Standards and Tolerances 2017.
- Begin with correct falls in the substrate. A screed should be of uniform thickness, particularly when it is an un-bonded screed.
- Consider adjacent details that might feed water into the tile system. I.e. Inadvertently burying brick cavity weep holes or sub-sill drainage of window and door frames below the finished height of your tiles can cause problems.
- Construction joints, sheet joints and movement joints in a substrate should either be expressed through a screed and tiling, or isolated from the finished tiling using slip sheets or crack isolation systems (i.e. a bonded or un-bonded tile system). N.B: A screed installed on top of a membrane is considered an un-bonded screed. To bond a screed, it should be slurry-bonded on to pre-scabbled/ shot-blasted concrete.
- Use General Purpose Cement and Efflock in the screed and mix your screed using a screed mixer. Liquid polymer additives can also be used in the screed to provide additional strength. N.B: Polymer additives will delay the repellent effect of Efflock until those polymers begin to dry. Inclement weather should be avoided for the first few days.
- Provide shade over the work area to avoid premature moisture loss. A cement slurry may be needed to bond the screed together between each batch. Thoroughly compact the screed and avoid voids.
- Moisture curing a screed is good practice. AS 3958.1 recommends moisture curing a screed for 7 days, followed by 2 weeks drying prior to tiling, to exhaust initial curing and shrinkage. Polymer additives in the screed can help to provide effective moisture curing.
- If applying a membrane/ second membrane over the screed, prime and caulk movement/control joints using a backing rod. Tamsi Hydrostop is a suitable primer for green screed so that the caulking can be installed along the joints early to prevent water penetrating the open joints. Keep the rest of the screed open to allow moisture to evaporate. Moisture test the screed to confirm a satisfactory moisture content for the membrane before applying the remaining sections of waterproofing over the tile screed. N.B: A second membrane is entirely optional for a screed that contains Efflock, but many builders opt for both. Putting Efflock in a screed is advantageous to repel rainwater for the duration that your screed is open to the weather, even when it is going to have a membrane.
- Use 1% Efflock in the gauging water of cement based tile adhesives. Notch the adhesive in the direction of the shortest dimension of the tile to minimise air entrapment. Tiles greater than 400mm should be back buttered achieving minimum 90% contact coverage. Avoid voids.
- Use 1% Efflock in the gauging water if using cement based grout. Install grout. N.B: If using Mapei Ultracolor Plus, Efflock is not required. If it has rained, never grout the water in! Rainwater should be extracted and the tiling given ample time to evaporate dry before grouting and caulking. Any water trapped under tiles is almost guaranteed to bring the salts and polymers out!
- Keep movement joints free of tile adhesive and grout. Movement joints are required at all perimeters, at all post and columns, and intermediate joints at maximum 4.5m centres, and all of them need to be caulked! The type of tile, colour of tile and width of joint will influence the frequency of intermediate movement joints. NB: Proprietary aluminium/neoprene expansion joint profiles are not recommended externally - from the point of keeping water out. A tile butting into an aluminium extrusion does not provide a seal.
- Install extruded polystyrene backing rods into all movement joints prior to caulking. This provides a compressible convex ‘formwork’ to keep the caulking thin enough in the middle to enable it to stretch, without detaching from one side or the other (see manufacturer’s technical data sheet for guidance on depth/width ratios). A caulked joint that becomes detached, functions as a strip drain instead of a sealed joint.
- Be sure to install movement joints at wall perimeters before installing skirting tiles. This detail is often overlooked!
- For balconies that drain to an edge, a metal balcony trim with a drip angle that sheds water away from the balcony fascia is helpful.
- Consider the expansion rates of metal trims and lineal drains in outdoor tiling. Consider breaking these accessories into shorter lengths and incorporating their own compressible movement joints between lengths. Caulking between tiles and these accessories is designed to stretch and compress across a joint, but it has very little tolerance in absorbing lineal shear forces. Remember – a failed caulked joint becomes a strip drain!
- When installing stormwater grates and lineal drains, always provide bleed points for water that may penetrate the screed/glue/grout to freely drain discreetly into the stormwater pipe. If a secondary membrane is installed on top of a screed, the second membrane should not lock off water from being able to escape from the bottom of the screed/primary membrane. I.e. Water that can potentially get in, must be allowed out!
- Seal porous tiles that are vulnerable to water absorption, salt attack or staining with an appropriate sealer for those tiles.
Understanding how efflorescence occurs is the first step in knowing how to control it. Hopefully our suggestions above give some helpful guidance to preventing problems on your next project. Our staff and suppliers at MBS are always willing to assist you with advice for any specific job.
I would also like to thank both Ben Burdett from Eco Liquids P/L and Colin Cass from Techtile Consulting P/L. Without their help and input this post would have not been possible and a big “thank you” must go to both of them for being so generous with their time.
Techtile Consulting P/L : 0414 539 549
Eco Liquids P/L: 0414 730 736
The advice in this blog post is general advice for a typical tiling installation and is given in good faith. Customers should always undertake their own assessment of methods and materials, materials compatibility and confirm the best approach required to completing a job seeking the correct advice from the material/product’s manufacturer.Fernando de Sousa GM