Is the migration of a salt to the surface of a porous material, where it forms a coating. The essential process involves the dissolving of an internally held salt in water, or occasionally in another solvent. The water, with the salt now held in solution, migrates to the surface, then evaporates, leaving a coating of the salt.
In tiling terms, efflorescence is a crystallization of minerals that can commonly appear on the surface of cement grout joints and then spread or leak to the surface of surrounding tiles or surface areas. The term ‘efflorescence’ actually dates back hundreds of years to the Latin word efflorescere, which means to blossom or flower out. It is described like this because the salt crystal are carried upward to the surface through grout joints.
Efflorescence will commonly occur when the salt crystals in cement grout joints or adhesive are carried upwards by moisture. As the moisture evaporates when it reaches the surface, it can leave behind the salt deposits which will give the appearance of chalk, white haze on the surrounding surface. In the same way that your body sweats and water evaporates from your skin and leaves a salty residue, cement grout does the same. The darker the grout and the tile, then the more noticeable efflorescence will be.
What are the causes of grout efflorescence?
How can you prevent grout efflorescence?
How to clean/remove efflorescence from tiles and grout?
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Aqua Mix Eff-Ex®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:
Credit: Scott Worthington