Efflorescence on Limestone

Efflorescence on Limestone: Only an inert white deposit?

Efflorescence is in most cases recognised as a white deposit on masonry, ceramic and stone surfaces. Many people can identify it; few know how the mechanism works however just about everyone believes the deposit is inert and can be removed at any time in the future. However the latter claim and the identification of it as only a white substance are both inaccurate. I want to discuss both these issues with reference to limestone as limestone is not only currently very fashionable but is also the stone most affected by efflorescence.

Efflorescence: Not only a white deposit.

I have visited many job sites where brown below surface staining has been evident on both honed and polished limestone. It can manifest itself as a dark patch or in many cases a lighter patch with a darker perimeter. These have in just about every case been put down to the migration of iron compounds and although this can be the source in many installations it is due to soluble alkaline solutions. These solutions (soluble salts) are referred to as efflorescence and yet are not the white deposits normally associated with the term.

This type of staining by soluble alkaline solutions is triggered in most cases by rainwater. Rainwater tracks down over numerous alkaline bearing surfaces within the framework of a stone installation. It runs over Portland cement walls, floors, through mortar beds, adhesives and grouts as well as down wall cavities. As the water flows it picks up the soluble alkaline solutions and gradually it’s own ph turns to alkaline. Cement is the common thread as it has a high content of Calcium Hydroxide  – Ca (OH) 2- that along with other hydroxides and carbonates react with organic compounds in the stone to form the soluble organic salts we are discussing.

The alkaline water moves throughout the system in many cases much further downstream from the original source, explaining why alkaline stains can be seen in areas away from direct water exposure. As the water evaporates the alkalinity increases and depending on saturation is manifested as the common white deposits (full saturation) or the less understood brown staining. Remove it later as it is inert!

This commonly held view is totally incorrect. Visible efflorescence should be removed as soon as it appears. Moreover steps should be taken to minimise the creation of these damaging alkaline compounds.

I cannot go into every aspect of how these compounds can damage limestone and other stone however I will discuss several of the lesser known ones. Efflorescence build up can create the so-called wet spot look often seen in stones such as granite, marble and limestone. This effect is almost always blamed on moisture from the setting materials, mortar bed or substrate. However in many cases it is efflorescence. When calcium hydroxide combines with atmospheric carbon dioxide a relatively insoluble calcium carbonate is formed. This tends to block the pores and stylolitic seams in the stone and hence traps moisture. The trapped moisture takes the shape of the area of concentrated alkaline hence it looks like a spot. This is most common where an alkaline solution is of low saturation and hence the efflorescence does not come completely to the surface explaining why the moisture is trapped sub- surface. Removal of this is very difficult with surface-active chemicals and usually requires complete re-surfacing of the stone

Another lesser-known effect of alkaline solutions is the part they can play in promoting bacterial growth. The resulting salts have a great ability to absorb moisture from the air. The higher the relative humidity the more saturated the salt becomes. Ultimately the salt becomes saturated and a transition from crystal to liquid occurs. The salt can then migrate further into the stone installation to contaminate other areas. However this ability to absorb moisture from humidity also prolongs the time a stone is wet. This inturn prolongs the perfect conditions for bacterial growth between wet and dry weather and is the reason why many installations exhibiting high levels of efflorescence also exhibit high bacterial levels as well.

Lastly crystallising soluble salts also increase the expansion and contraction of the stone. This is a real problem in stones with large areas of veining such as limestone and marble. The continuous expansion and contraction eventually creates a weakening of the stone surface rapidly reducing their performance and structural integrity. This same mechanism can also deteriorate a stones polish by creating small expansion cracks that result in the surface becoming rough and hence looking unpolished.

So as you can see these alkaline solutions (including rainwater as it changes it’s ph) can create major problems for a stone installation and are therefore best removed as soon as they appear. Traditional methods of cleaning such as acid are well documented however on heavily calcified surface deposits and most of the below surface “brown” staining discussed above, acid is not effective. For these problems only mechanical removal will work.


The bottom line with problems of alkaline exposure and efflorescence is that it is almost impossible to stop totally. However there are many things that can be done to lower the risk of formation and damage. In short anything that reduces the amount of water that gets into a system – latex modified adhesives, mortar beds and grouts, waterproof membranes, washed sand, on grade vapour barriers, and of course sealers – all help in this respect. However design details (soffits, guttering etc) reducing the amount of water that runs across or through a stone system are also very important as they reduce the quantity of alkaline bearing water that is at the root of the problem.

Sealers obviously play an important part in the solution. They will reduce (not stop) the amount of water entering the stone. When applied on all six sides they are even more effective than the traditional single side application. This presumes that the sealer used is not a bond breaker and that it has good vapour transmission otherwise it will become a liability instead of part of the solution. Aqua Mix has amongst it range penetrating sealers that can be applied on all six sides. However even a surface application over the stone and grout is going to greatly lower the risk of problems. So in short using a sealer helps to reduce the problem of alkaline damage and it’s associated problems.

In summary efflorescence is a term that is most often used inaccurately. These alkaline solutions in many cases rainwater create damaging salts that are not restricted to the white deposits commonly identified. They encompass a range of visual stains such as the “brown staining” and wet spots most often attributed totally to other causes. Moreover they are damaging and should be removed as soon as possible from the stone or surrounding installation.


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