Efflorescence

Aqua Mix Eff-Ex®

By Scott Worthington

Since the introduction of Aqua Mix® Eff-Ex® I have had numerous questions regarding the ability of this product to clean and inhibit efflorescence on cement mortars, grouts and renders. More importantly if it would be prudent to apply Aqua Mix® Eff-Ex® to a mortar bed or concrete slab prior to installing tile? The same question has been directed towards sealers and their effectiveness at preventing efflorescence. To answer this let’s have a quick revision regarding the mechanism of efflorescence and especially how it pertains to cement and mortar.

A dictionary (Webster) definition of efflorescence is a white deposit appearing on the surface of stone or masonry formed by the crystallisation of soluble salts carried to the surface by the evaporation or drying of moisture. You need a source of soluble salt, water (as a catalyst) and heat or sun to dry or evaporate the moisture so the water can deposit the efflorescence on the surface. In past articles I have detailed many of the different sources of soluble salt including the stone itself however the biggest contributor in most cases is the cement or mortar used to install, support or grout the tile or stone.

Concrete or mortar can contribute efflorescence by way of it’s components as well as from a by product of the process of hydration or curing. If we look at the components first:

Cement:

This is perhaps the biggest contributor to efflorescence in concrete; specifically it is the alkali content of portland cement. You can predict with reasonable accuracy the tendency of a mortar or concrete slab to show efflorescence by looking at a chemical analysis of the cement. Cements that are high in alkalis are more prone to produce efflorescence than those of lower alkali content. All cements contain some water soluble alkalis. Those common in mortar and concrete are sodium and potassium. (Note some commentators suspect that the sulphate content of cement may be as significant as the alkali content in contributing to efflorescence). American Standards recognise the contribution of alkalis in cement to efflorescence by providing provisions for specifying low-alkaline cements.

Lime:

Lime is seen by some as a contributor to efflorescence although this is a contentious issue. As an additive it acts as a plasticiser as well as enhancing the bond between the mortar and tile or stone, in doing so increasing the water resistance of the installation and hence lowering the risk of efflorescence. It can also tend to dilute the deleterious effects of high alkali cement. However in certain conditions where acid has been used to clean the installation unbuffered hydrochloric acid can react with the lime to produce soluble calcium chloride which can migrate to surface forming efflorescence.

Sand:

Sand is not water-soluble in itself however it may be contaminated with material that is soluble and can contribute to efflorescence. Sand should be taken from sources free of contamination from things such as soil runoff, plant life, salt water, and decomposed organic compounds. Using clean washed sand should eliminate sand as a contributor to efflorescence. Sand and specifically its particle size and distribution has a large bearing on the cured density, hence water resistance and ability to set off efflorescence.

Admixtures:

I mention admixtures because these are often blamed as a contributor to an efflorescence problem generally through ignorance. Contemporary admixtures in general help to reduce the risk of efflorescence. Most modify the cement so that hydration is improved and controlled, resulting in a denser mortar and concrete which in turn absorbs less water. However I have come across several situations where calcium chloride was added as an accelerator to a mortar and was later analysed to be a contributor to the resulting efflorescence. However these are isolated and rare cases.

The second way cement can create efflorescence is from a by product of the process of hydration. Analysis of efflorescence on the surface of concrete and mortar indicates that the predominant salt is calcium carbonate which is not water soluble. During curing (hydration) and the formation of calcium hydrosilicates reasonable quantities of calcium hydroxide are emitted as a by product of the hydration process. Being soluble the calcium hydroxide dissolves in the excess water. During the drying phase this water, a calcium hydroxide solution, migrates to the surface where the water totally evaporates leaving the calcium hydroxide behind. Once on the surface the calcium hydroxide reacts with atmospheric carbon dioxide forming the insoluble carbonate, calcium carbonate. This type of efflorescence can occur over a long period of time. The catalyst for the initial efflorescence is the water used in the mortar or concrete. However subsequent water from rain etc can dissolve more calcium hydroxide further prolonging the reaction.

In summary efflorescence from mortar and concrete is caused by a combination of the components and from a by product of hydration. This means that in most cases a mortar bed or concrete slab has a reasonably high chance of forming its own efflorescence regardless of other possible sources of soluble salt. There are numerous ways of reducing this risk most of them directly related to the formulation of the mortar or concrete itself. The use of washed sand and admixtures (to reduce the quantity of water for hydration thus creating a denser mortar) as well as low alkaline cements are amongst the most common. However when confronted with a slab or mortar bed prior to tiling that was installed without your knowledge it is almost impossible to tell if these were constructed using any or all of these measures.

This brings us back to our original question regarding the use of products such as Aqua Mix® Eff-Ex® on mortar beds and concrete slabs prior to the installation of tile or stone. Are they worthwhile? The short answer in my opinion is YES. It is almost impossible (unless you were the architect and constructor of the mortar bed or slab) to verify how they were made or what precautionary elements were incorporated in them. Therefore the best response to an unidentified slab or mortar bed is to treat it as if it will contribute to the overall level of efflorescence. This means that you should consider implementing all or some of the following measures:

1. Treat concrete slab with Aqua Mix® Eff-Ex®. An application can be done as soon as the slab or bed can be walked on. The Aqua Mix® Eff-Ex® will not interfere with the hydration of the cement. However to get the best penetration of the Aqua Mix® Eff-Ex® it is better to wait until the cement is dry. In most cases the slab or mortar bed should be at least 28 days old before installing tile or stone hence an ideal time to apply Aqua Mix® Eff-Ex® is just before tiling. A fairly liberal coat should be applied and scrubbed into the surface to promote penetration. Any surface residue should be removed (wiping or preferably rinsing with clean water) prior to installation. We have done numerous applications to both slabs and mortar beds and it is no coincidence that the quantity and frequency of efflorescence that has occurred on these treated installations is greatly reduced compared to the untreated ones.
2. Apply a waterproof membrane. Waterproof membranes seem to be almost exclusively used to protect living or occupied spaces from water. However this neglects other uses such as the reduction of efflorescence. With efflorescence occurring only in the presence of water it stands to reason that the drier the slab or mortar bed remains the lower the risk of efflorescence. If you can keep them totally dry then the only time efflorescence is a risk is during construction and hydration. In most cases the use of a waterproof membrane will replace the need to treat the slab or mortar bed with Aqua Mix® Eff-Ex®. However if there is a reasonable time lag between the installation of the slab or mortar bed and waterproofing then a treatment of Aqua Mix® Eff-Ex® may be prudent before waterproofing to reduce the risk of efflorescence that can in turn create a problem with the bonding of the membrane (especially water based waterproof membranes). I have experienced three jobs where membranes delaminated due to the presence of unnoticed efflorescence prior to application. Although these are very rare cases a simple low cost application of Aqua Mix® Eff-Ex® would have solved the problem.
3. Use factory prepared adhesives and additives. Factories like Custom Building Products go to great lengths to formulate and manufacture adhesives that contribute as little as possible to efflorescence. The use of washed sand, polymer additives (to improve and regulate hydration) and strict quality control of raw materials insure the adhesives are almost efflorescence free. With cement being in most cases the largest contributor to efflorescence it is imperative that factory prepared adhesives are used as this is in many cases the only cement product a tile or stone installer can totally select and control.
4. Seal grout joints and surface. The application of a good sealer such as Aqua Mix® Sealers Choice® Gold or Aqua Mix® Ultra-Solv® can also lower the risk of efflorescence. These can also be applied to just the grout joint on a glazed tile installation. A sealer works by lowering the amount of the catalyst water that enters the instillation and in doing so reduces the risk of efflorescence. It is important to note that sealers are not waterproof, hence they will still allow some water to enter and cannot be relied upon to totally stop efflorescence. Sealers must also be maintained (with compatible cleaners) and re-applied periodically to maintain their performance. Although some manufacturers give specific periods of time for life expectancy this should be seen as a general guide. No sealer will necessarily last the same amount of time on different surfaces.
5. Install flexible sealants. We are all familiar with the standards and the need to install flexible (not hard cement grout) sealants. However proper application also has an effect on the risk of efflorescence by way of controlling water. The joints at the junction of walls and floors are exposed to large quantities of water. If this water is not transported away correctly then it penetrates the horizontal surface where it can then set off future efflorescence. In other words proper placement and use of flexible joints further protects the adhesive, mortar bed and slab from getting wet and creating efflorescence.

In conclusion cement and the resulting mortar bed or concrete slab can and do contribute to efflorescence. The contribution is a duet made up of the components and the process of hydration. The best way to minimize the amount and risk of efflorescence is to implement the complete system comprising all of the above measures. I have no doubt that Aqua Mix® Eff-Ex® is a very important (new) part of this system. In many cases you are not able to control or implement any of these measures because the concrete slab or mortar bed has already been installed. This is where the application of Aqua Mix® Eff-Ex® prior to the installation of tile or stone comes into its own.