A Quick Guide to Preventing Efflorescence in Your Concrete Countertops

efflorescence

Severe efflorescence in concrete

Efflorescence. It’s the whitish powdery material that forms on the surfaces of masonry or concrete construction, and also it’s the white blush that can form on sealed concrete floors or concrete countertops. While it poses no threat structurally, efflorescence is an aesthetic nuisance that affects both interior and exterior concrete. This article discusses why efflorescence occurs, how it can be prevented and how to deal with it if it does happen.

There are two kinds of efflorescence: primary and secondary. Primary efflorescence occurs when concrete bleedwater dries on the surface. Secondary efflorescence occurs when soluble mineral salts are leached out of cured concrete.

Efflorescence “growing” on the inside of a concrete block wall.

Primary Efflorescence

Eliminating primary efflorescence begins before the concrete is cast, simply by using basic good concreting practices: Start with a concrete mix that uses well-graded aggregates, a low water-to-cement (w/c) ratio, and fly ash or other pozzolan as a partial cement replacement; use a water reducer to increase workability without adding extra water to the mix.

Extra water in the concrete makes it more porous, weaker, and more susceptible to shrinkage cracking. The extra water is an unwanted internal reservoir that can leach the salts out of the concrete. Concrete made with a w/c of 0.45 or less will produce a relatively strong, dense mix that’s unlikely to have excessive bleedwater.

Making good concrete is just the first step. It does no good to have a well-designed, low w/c ratio concrete if it’s not cured properly. Wet curing under burlap, plastic sheeting or curing blankets allows the concrete to gain strength and density during the critical early days after casting. Well-cured concrete inhibits water movement, and this is one important step to controlling primary and secondary efflorescence. If the concrete doesn’t allow moisture movement, the salts deep within the concrete can’t be leached out.

efflorescence on a polished concrete big-box store floor.

Secondary Efflorescence

All masonry and concrete materials are susceptible to secondary efflorescence, including concrete countertops. Secondary efflorescence is most often caused by moisture or water vapor migrating through a concrete slab, bringing soluble salts to the surface of the concrete. The amount and character of the deposits vary according to the nature of the soluble materials and the atmospheric conditions.

Concrete contains a variety of soluble mineral salts, both from the cement and from admixtures like calcium chloride, and even from chemicals applied to the concrete after it has hardened. It’s those salts that are the “seeds” of efflorescence.

While all concrete has some soluble salts in it, not all concrete will effloresce. Efflorescence will occur only if all of the following conditions exist within the concrete:

  • The concrete must have soluble mineral salts within it.
  • There must be moisture to dissolve the soluble salts.
  • Evaporation or hydrostatic pressure must cause the mineral salt solution to move towards the concrete surface.

If any one of these conditions is eliminated, efflorescence will not occur.

Controlling secondary efflorescence is a more common problem for contractors who have “inherited” pre-existing concrete. The mix itself can’t be changed, so factors that affect water movement into and out of the concrete are where steps can be taken. Identifying and minimizing the sources of moisture are the first step. Reducing the porosity of the concrete to prevent the soluble salts from being leached out is the second step.

This second step to controlling efflorescence is to apply chemical hardeners, also known as densifiers, to existing concrete. These make the matrix less porous by generating calcium silicate hydrates that plug the pores and clog the capillaries.

Curious how repairing efflorescence plays out in a real-world scenario? Read this blog from our backlog that details an example of how to repair a concrete floor with efflorescence that was progressively worsening.

Efflorescence part 3: Example of repairing a concrete floor

Consider the following scenario: A new urban condo has acid-stained concrete floors finished with an acrylic sealer. The unit downstairs is unoccupied and unheated. The owner notices efflorescence starting soon after move-in and progressively worsening over several months.

We use the example of a floor because concrete floors are generally more susceptible to efflorescence than concrete countertops, and because this actually happened in my condo building in downtown Raleigh, NC.

Efflorescence is occurring because the moisture in the slab from construction and from acid staining has been locked under the acrylic. Water vapor leaving the slab is drawing soluble salts to the warmer side of the concrete.

Fixing this floor starts with stripping off the sealer and then physically removing the efflorescence. Common means are scrubbing or washing with a dilute acid solution. However, you can’t just mop dilute acid (with a lot of water) all over the place. Doing so would pump more water into the concrete. The best solution is to use an automatic scrubber that washes, scrubs and vacuums in one step. This minimizes the amount of water that penetrates into the concrete.

Then the concrete must be allowed to dry thoroughly before lithium silicate densifiers are applied. Commercial dehumidifiers can speed drying.

The final step to finishing the floor depends upon the floor’s water vapor transmission rate, the aesthetics and the desired level of durability. The simplest solution is either to leave as is or to apply a renewable “wax” to the floor. Unlike acrylics, wax is unlikely to trap efflorescence. If it does, wax can easily be stripped and replaced. This process may not halt 100% of the efflorescence, but it will allow everyday cleaning to remove the slight residue that occurs. In areas such as under a bed that don’t get cleaned regularly, efflorescence may still occur.

For floors that require more protection (like restaurants), first conduct tests to determine the rate of vapor transfer. If the moisture levels are low enough, choose a vapor-pressure-resistant sealer based upon the manufacturer’s recommendations. Impermeable sealers that are able to resist the existing vapor pressure will not develop efflorescence because the water vapor cannot pass through the sealer.

Efflorescence part 2: Secondary efflorescence in concrete countertops and floors

All masonry and concrete materials are susceptible to secondary efflorescence, including concrete countertops. Secondary efflorescence is most often caused by moisture or water vapor migrating through a concrete slab, bringing soluble salts to the surface of the concrete. The amount and character of the deposits vary according to the nature of the soluble materials and the atmospheric conditions.

Concrete contains a variety of soluble mineral salts, both from the cement and from admixtures like calcium chloride, and even from chemicals applied to the concrete after it has hardened. It’s those salts that are the “seeds” of efflorescence. Some types of salts simply get dissolved and precipitated onto the surface, while others react with atmospheric carbon dioxide to form mineral crystals.

efflorescende

Efflorescence on a polished concrete big-box store floor.

While all concrete has some soluble salts in it, not all concrete will effloresce. Efflorescence will occur only if all of the following conditions exist within the concrete:

  • The concrete must have soluble mineral salts within it.
  • There must be moisture to dissolve the soluble salts.
  • Evaporation or hydrostatic pressure must cause the mineral salt solution to move towards the concrete surface.

If any one of these conditions is eliminated, efflorescence will not occur.

Knowing that concrete contains soluble salts and that water and the movement of water through the concrete are the cause for efflorescence, then solving the efflorescence problem really boils down to controlling the movement of moisture into and out of the concrete. How this is handled varies depending upon a variety of factors, such as whether the concrete is new or old, cast on the ground or is an interior application.

Since moisture movement into, through and ultimately out of the concrete is how efflorescence forming salts move to and accumulate on the surface, the first step to controlling efflorescence started with the concrete itself, as discussed earlier in the primary efflorescence section.

Managing moisture is the next step to controlling, minimizing or even eliminating efflorescence. Dry concrete is far less likely to effloresce, so identifying moisture sources and then controlling the movement of that moisture into and out of the concrete becomes the key to shutting efflorescence down.

Consider a basic concrete floor slab cast directly on leveled, compacted ground. The concrete itself contains moisture, and if it’s made properly the concrete won’t generate primary efflorescence due to the bleedwater evaporating and leaving salts behind. But secondary efflorescence can, and probably will, occur. That’s because the ground beneath the slab is a moisture reservoir. If water was added to the sand or gravel to assist compaction, that added moisture will eventually migrate through the slab, carrying mineral salts with it and forming efflorescence. The reason vapor barriers are installed beneath concrete slabs is to isolate the concrete from the ground, which represents a large reservoir of moisture.

If the concrete floor is then sealed with an impermeable coating like a urethane or epoxy, vapor pressure or even hydrostatic pressure can cause blistering or sealer failure. Breathable coatings like acrylics allow water vapor to pass through the sealer, preventing blistering. But migrating water vapor can slowly cause salts to accumulate beneath the acrylic sealer, causing unsightly blushing or even sealer failure due to accumulated mineral deposits.

Commercial floors in elevated structures are not cast against the ground, so there’s no moisture reservoir beneath the floor to drive efflorescence. Yet these floors can effloresce too. That’s because water can enter the slab from the top during the course of finishing, acid staining and routine cleaning. Porous concrete absorbs water during mopping, and copious amounts of rinse water pump large volumes of water into the concrete. This moisture then leaches the salts out of the concrete, creating efflorescence.

Controlling secondary efflorescence is a more common problem for contractors who have “inherited” pre-existing concrete. The mix itself can’t be changed, so factors that affect water movement into and out of the concrete are where steps can be taken. Identifying and minimizing the sources of moisture are the first step. Reducing the porosity of the concrete to prevent the soluble salts from being leached out is the second step.

This second step to controlling efflorescence is to apply chemical hardeners, also known as densifiers, to existing concrete. These make the matrix less porous by generating calcium silicate hydrates that plug the pores and clog the capillaries. Done properly, they offer a threefold benefit. Reduced porosity is the first. The second is that silicate hardeners consume free lime in the concrete. Thirdly, the silicate gel binds other soluble salts, making them difficult to leach out.

While chemical hardeners seem like the ideal solution, not all hardeners are effective. In fact, using the wrong type can actually cause efflorescence. Explaining this involves a little chemistry, so bear with me.

Three common forms of chemical hardeners are sodium silicate, potassium silicate and lithium silicate. Lithium silicate hardeners are the most effective and least likely to effloresce due to a combination of factors. Lithium ions are smaller, and compared to sodium or potassium silicates, there are fewer lithium ions for each silicate molecule. It’s the silicate portion that actually does the work, so in effect lithium silicates are more concentrated. Once the silicate molecule reacts with the available calcium or free lime in the concrete, the sodium and potassium ions are freed and become soluble. In contrast, lithium ions are not freed. Free ions can react with other substances in the concrete to form salts. These salts can then leach out and form efflorescence.

Finally, high concentrations of carbon dioxide (CO2) can cause or accelerate efflorescence. Concrete located in areas with gas, wood or oil-fired heaters will develop efflorescence faster than concrete stored in low CO2 concentrations.

Preventing Efflorescence in concrete countertops and floors: part 1

Efflorescence. It’s the whitish powdery material that forms on the surfaces of masonry or concrete construction, and also it’s the white blush that can form on sealed concrete floors or concrete countertops. While it poses no threat structurally, efflorescence is an aesthetic nuisance that affects both interior and exterior concrete. This article discusses why efflorescence occurs, how it can be prevented and how to deal with it if it does happen.

efflorescence on a concrete wall

Efflorescence “growing” on the inside of a concrete block wall.

There are two kinds of efflorescence: primary and secondary. Primary efflorescence occurs when concrete bleedwater dries on the surface. Secondary efflorescence occurs when soluble mineral salts are leached out of cured concrete. This post will cover Primary Efflorescence.

Primary Efflorescence

Eliminating primary efflorescence begins before the concrete is cast, simply by using basic good concreting practices: Start with a concrete mix that uses well-graded aggregates, a low water-to-cement (w/c) ratio, and fly ash or other pozzolan as a partial cement replacement; use a water reducer to increase workability without adding extra water to the mix.

Extra water in the concrete makes it more porous, weaker, and more susceptible to shrinkage cracking. The extra water is an unwanted internal reservoir that can leach the salts out of the concrete. Concrete made with a w/c of around 0.45 will produce a strong, dense mix that’s unlikely to have excessive bleedwater.

Fly ash adds workability and replaces some of the cement. Since it’s a pozzolan, it consumes the calcium hydroxide produced during cement hydration. Calcium hydroxide, also known as free lime, is a key efflorescence-producing compound. Pozzolans consume the calcium hydroxide and produce calcium silicate hydrates, which make the concrete stronger, denser and less porous. This reduces the likelihood of efflorescence by shrinking capillaries and plugging the pores within the concrete.

Making good concrete is just the first step. It does no good to have a well-designed, low w/c ratio concrete if it’s not cured properly. Wet curing under burlap, plastic sheeting or curing blankets allows the concrete to gain strength and density during the critical early days after casting. In young concrete, the capillary and pore structure is open and well-connected. As the concrete cures, the pores and capillaries get filled in and closed off, yielding a dense, more impermeable matrix. Well-cured concrete inhibits water movement, and this is one important step to controlling primary and secondary efflorescence. If the concrete doesn’t allow moisture movement, the salts deep within the concrete can’t be leached out.

Conversely, concrete that dries out quickly soon after finishing is sponge-like, filled with cracks and interconnected pores that allow moisture to move into and out of the concrete. Rapid evaporation of moisture draws efflorescence-causing salts to the surface through the porous, micro-crack filled, weak concrete matrix. Not only will efflorescence happen, it will continue to happen because the concrete never had a chance to cure into a dense, solid mass.

Next post: Secondary efflorscence

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