Efflorescence

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.

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.

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. 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.

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.

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. 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.
2. Use factory prepared adhesives and additives. Factories 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.
3. 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 installation 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.
4. 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.

Original article supplied by Aqua Mix®