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How to remove efflorescence

You may have just had new concrete installed at your property and over the next couple of weeks, you notice that the concrete is dusty. Usually, this is seen when you have a patio and walk dust into your house… That’s bound to start irritating everyone!

A customer of ours laid some very expensive 32Mpa plain grey concrete that they wanted to hone and seal. The idea was to match their polished concrete floors and create an indoor-outdoor modern flow.

The problem was that the concrete was dusty and they were walking dust into the house. So they called Barefoot Concrete to help them to stop the dust and to then hone and seal the concrete with an eco-friendly water-based sealer.

So we explained to them that the issue was most likely efflorescence, a fact of life with most concrete.

When you use concrete as a retro or decorative area you may tend to notice that concrete is not perfect! We getting more and more efflorescence related issues because of the popularity of hone and seal or polished concrete floors.

As this modern trend to have exposed concrete floors grows, we are facing more incidences of efflorescence being noticed and becoming problematic. So let’s take a look at the major efflorescence issues. As you read through this blog we will suggest realistic measures to control the conditions that encourage efflorescence and we will offer remedial suggestions. We will hopefully offer useful explanations of this complex phenomenon in practical terms without a lengthy detailed chemical analysis.

What often happens is that a contractor may perform work on concrete but is unaware of the preexisting conditions that caused the efflorescence. As contractors and property owners, we are all affected by issues related to efflorescence.

We hope this blog gives everyone interested in it a greater clarity of some of the issues so that efflorescence is better understood and the appropriate measures are taken to prevent its occurrence and can confidently treat it.

The good news is that the costs associated with efflorescence prevention are minimal when compared to the cost and inconvenience of the remedies, especially when the space is occupied. It’s also important to note that efflorescence in itself is not a health problem although it might indicate moisture levels sufficient to support mould. Efflorescence is also not a structural issue. It is an aesthetic issue, however, it’s an issue we would all like to avoid.

So, now we get to the big question:

What is efflorescence?

Efflorescence is the white powdery substance on the surfaces of unsealed concrete and the white blush seen with sealed floors.

Efflorescence is caused by vapour migrating through the slab bringing soluble salts to the surface of the concrete.

Efflorescence is normally worn off or washed away on unsealed concrete surfaces. In stubborn cases, a mild acid rinse or even a light grind may be necessary.

Efflorescence that becomes trapped under a sealer is unsightly and is even more conspicuous on darker floors.

In the worst-case scenario where the vapour cannot pass through the topcoat hydrostatic pressure can build to up to create water blisters that can delaminate decorative concrete coatings.

What causes efflorescence?

Efflorescence requires the movement of moisture. Without moisture movement, there would be no efflorescence on the surface to create the problem.

Unfortunately, too many concrete finishers routinely introduce large amounts of unnecessary “water of convenience” to the mix in order to facilitate concrete placement.

Primary efflorescence is caused by the water in the concrete evaporating from the slab leaving behind the soluble salts on the concrete surface.

The fact that these salts are actually more soluble in colder temperatures coupled with increased bleed water in cold weather increases the likelihood of efflorescence showing up after winter concrete placement.

Contributing further to efflorescence with cold-weather pours is the use of calcium chloride to accelerate the set time. High slump concrete and the addition of calcium chloride are major contributors to efflorescence.

Secondary efflorescence is often described as water coming from underneath the slab or water that is introduced from the surface.

Likely sources of secondary efflorescence would be a saturated base material, an improperly drained site or excessive amounts of water used by the decorative flooring contractor during his cleaning process – rinsing off the acid stain residue, for example.

When extra mix water and extra soluble salts from calcium chloride are added to concrete placed in cool weather followed by more water from the decorative processes, some degree of efflorescing is bound to occur.

How to remove efflorescence?

Ok, so your concrete has efflorescence, what are your options?

A fast fix, which we don’t recommend purely from a health perspective and being environmentally responsible, and our preference for water-based sealers and solutions, is to use xylene or solvent-based acrylic.

However, if vapour continues to come up through the slab though, the condition will most likely reappear. Therefore, be aware that if this is inside a home with children, pets and anyone with health conditions, there can be health issues because of the toxicity of solvents like xylene.

The most prudent course of action is to conduct a test to determine the rate of vapour transfer and then develop a remedial strategy. It is important to diagnosis the causes as best you can with the information available.

One way to do this is with a moisture meter, it’s a valuable tool to get a moisture reading. Once the moisture levels are established a sealer can be selected based upon the manufacturer’s recommendations.

If you treat the concrete with a densifier, and the slab continues to have efflorescence problems even after the application of a densifier contractors have used low build, low solids micronized acrylic water-based products as the final treatment. These finishes and polishes can also be used over film-forming sealers to add abrasion resistance.

In most efflorescence cases, the contractor most likely inherited the problems that contributed to creating the efflorescence. Diagnosing the causes of efflorescence after a concrete floor has been sealed can be difficult.

Therefore, it is important to determine how much moisture exists in the slab, the source of the moisture and also weather conditions, like seasonal groundwater might contribute more moisture in the future. Consider vapour testing and resist the quick fix, find the problem and treat accordingly!

How to prevent efflorescence

Efflorescence reducing measures are:

  • site surface drainage
  • a well-graded concrete mix with a water reducer to minimize paste
  • concrete not exceeding a 4″ slump
  • that the concrete is well consolidated
  • placed directly on a vapour retarder and cured in some fashion

All these factors contribute to concrete that has a minimum amount of bleed water with a lesser pore and capillary network that will resist rather than facilitate absorption and movement of moisture… in other words, a dense and relatively impermeable concrete slab.

Ok, for those needing to be more technical… An option to the vapour retarder is a waterproofing admixture to help prevent efflorescence. This is added to the concrete at the plant, but it has a downside, it can pose real problems for the sealing or resurfacing contractor since a common ingredient, stearic acid, is hydrophobic in the same way that powdered release agents for stamped concrete are so the waterproofed concrete does not readily accept coatings.

Helpful mix design factors include ordering a well-graded mix from the ready-mix producer. The advantage gained from a well-graded mix design is the reduction of the weakest part of concrete, the cement paste (cement and water), making for a denser concrete than with a standard mix.

Another important mix design consideration is the replacement of 15% to 20% of the Portland cement with fly ash which contributes significantly to lessening efflorescence.

Fly ash brings three important benefits to reducing efflorescence. Fly ash reduces the amount of Portland cement and free lime as well as chemically binds up a portion of the free lime and salts that cause efflorescence.

In addition, fly ash requires less water again resulting in a denser paste which aids in keeping moisture from travelling up through the concrete and down through the concrete from the top. 

Water reducers are also helpful at minimizing the amount of water and a reduction of cement (paste).

The closer a mix design gets to the desired water-cement ratio of approximately 20kg of water to kilograms of cement, the fewer efflorescence issues arise, especially with a mix including fly ash. 

Keeping multiple pours consistent with the mix design, subgrade conditions and finishing practices will produce consistent results.

Remember that in colder climates cold concrete and cold ambient temperatures encourage efflorescence because the salts are actually more soluble in colder temperatures plus concrete tends to bleed more in cooler weather further encouraging the upward transportation of moisture and its passenger, soluble salts.

If you are in a cold climate follow cold-weather concreting procedures whenever possible including raising the placement temperature of the concrete and then covering overnight to retain the heat. This will encourage a more dense pore and capillary structure and help close down the moisture transportation routes.

Curing becomes an important factor as we recognize that moisture moves much more slowly through denser concrete from either direction.

When concrete is kept moist for a longer period, especially the first few days, more capillaries and pores fill partially or completely to form a denser and more impermeable matrix that discourages the migration of moisture and soluble salts.

Conversely, concrete that is placed at a high slump and not cured acts as a sponge, full of miniature raceways allowing easy movement of moisture from the bottom-up and top-down.

Simple testing for vapour transmission

We should not make assumptions, as we know how that can work out… Therefore the contractor may like to do some independent fact-finding to determine the current vapour transmission rate.

One of the oldest and simplest test methods is the plastic sheet test which is taping down a clear sheet of poly and checking 16 hours later for condensation or for a darkened concrete surface for indications of vapour transmission.

Another surface moisture test is the calcium chloride test which quantifies the rate of vapour transmission. This is a covered dish that is weighed before and after a twenty-four hour period. 

Both of these methods are simple and cost-effective measures in determining whether vapour is active.

A note of caution when using these testing methods… the plastic sheet and chloride tests will track moisture movement near the top only. When the atmospheric conditions are similar to the slab conditions the tests might not indicate significant vapour transmission because movement happens when the ambient conditions differ from the slab conditions.

Moisture migrates and moves toward cooler temperatures. Vapour emissions migrate and move toward heat.

Here is a scenario that happens all the time: Imagine the consequences when the project is completed, the contractor has been paid, and then the owner takes possession and turns on the HVAC causing vapour in the slab to move towards the warmth or the lower humidity of the conditioned space bringing the whitish minerals with it.

If the sealer is acrylic the vapour will pass through leaving the efflorescence. If urethanes or epoxies are on the floor, hydrostatic pressure may build and possibly cause delamination. This is not the fault of the contractor! It’s simply efflorescence.

Efflorescence can occur months or even years after the contractor has left the job due to circumstances similar to those just described or from seasonal groundwater seeping under the slab.

Testing the surface may not be enough to guarantee an efflorescence free project for years to come. Therefore, it is important to determine the source and the rate of vapour transmission before prescribing a remedy.

Given the financial risk, not to mention damage to a contractor’s reputation it makes sense for a contractor to explore procedures that more clearly indicate subsurface conditions in order to avoid the efflorescence problems caused by moisture.

We have mentioned in this blog that a well-drained site and a vapour retarder are deterrents to efflorescence. The case can be made for placing the concrete directly on the vapour retarder or over granular material on top of the vapour retarder.

The argument against the granular material is that it may become a saturated “blotter” before the slab is poured adding even more water that has only one way to leave. Finishers argue that there will be too much bleed water resulting in dusting, but a 4″ slump with water reducers and a well-graded mix will show very little bleed water.

Elevated slabs are quite common in commercial construction and they are not placed on a blotter material. The really important factor here is minimizing water in all the important areas… the subgrade, the concrete and the procedures performed by the decorative contractor. These all require some amount of water, but keeping the water to a minimum helps ensure that efflorescence is controllable.

What are the options?

Technology that might offer double relief from efflorescence related issues involves the use of chemical liquid densifiers and hardeners.

Chemically hardened floors do not require a membrane sealer. This eliminates the trapped efflorescence problem and also greatly reduces maintenance costs.

The chemicals include silicates, silicaonates, polysiliconates, silanes, siloxanes, and lithium, to name a few. Claims vary from product to product and from manufacturer to manufacturer about how far the chemicals penetrate, their permanence and the levels of shine and slip resistance.

Barefoot Concrete suggests using a densifier like Forti-Kal or Forti-Col, or TED, the Efflorescence Destroyer to help remove efflorescence. TED removes efflorescence and rust marks from porous materials, including stone, concrete, brick and grout.

  • Highly concentrated formulation. Contains 50 to 60% Urea Hydrochloride
  • Powerful but much lower fuming than hydrochloric acid
  • Effectively removes mineral residues, including efflorescence, grout haze, rust marks, soap scum, limescale, mortar mess
  • Acidic Cleaner can also be used for removing fireplace soot
  • Temporary contact does not immediately burn skin – you have some time to rinse off the Acidic Cleaner (however, rinse immediately after skin contact)
  • Almost no corrosion of metals, except aluminium

Contact Barefoot today to help you with an efflorescence problem.