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Concrete floor finish

Polishing a concrete floor

There are different options to mechanically polish a floor, depending on the final look and strength needed. In both options the concrete will be ground back to the desired exposure level: nil, minimum or full exposure of aggregate.

grind and seal (topical coating) After grinding back the concrete slab, a seal is applied. This option is cheaper, but not as strong as the second option. It needs to be sanded back and applied again after around 5 years, depending on the intensity of usage.

mechanically polished (MPC) the top of the concrete slab is densified after grinding back to the desired exposure of aggregate.

here’s an Australian video explaining the two types.

Why seal a concrete floor?

Raw concrete is very porous and will stain easily. A concrete floor will also release dust through contact with the air. This process is called carbonation Ca(OH)2 + CO2 → CaCO3 + H2O.  

Silicate densifiers are applied to the top coat of the concrete, and reacts with the concrete, so that it becomes harder and less porous.

Stronger concrete:

Silicate densifiers will make concrete stronger, and add between 1,000 to 2,000 PSI to the compressive strength. As a result, it is more resistant to chipping than it would be with just a finishing sealer.

A densifier will protect concrete from impact damage. Applying a densifier is all that is ever needed for a garage floor; any additional finishing sealer will wear off if it sees a lot of heavy use. Applying an additional sealer is an option for residential floors, to make them less susceptible to staining. 

Terrazzo floor © Alain Hamel

Concrete densifier product options:

There are three types of silicate densifiers, sodium, potassium and lithium silicates. Each silicate densifier will penetrate to a different depth;

  • sodium silicates penetrate the deepest but burnish up to the lowest sheen,
  • potassium silicates penetrate up to 2 mm and burnish up more than sodium,
  • lithium is the shiniest but only penetrates to the thickness of a sheet of paper.

The ones that penetrate the least come out shinier but you can’t restore them as many times.

How to choose between concrete densifiers:

For the shiniest floor: definitely use a lithium densifier, and what can make an even bigger difference is choosing stronger concrete with a higher MPa (megapascal) for the initial pour. 25-28 MPa concrete is typical for a house but you could instead specify 35-38 MPa concrete as it generally polishes better.

For the most hassle-free floor: choose a potassium densifier rather than lithium. The deeper penetration means you will be able to buff out any etching caused by acidic materials, many more times over the years than you could with lithium.

For the healthiest floor: after densifying don’t use further sealers, as most sealers will contain some level of VOCs (volatile organic compounds). Many densifiers have no VOCs; they will seal the floor, so additional sealers aren’t absolutely necessary. A surface sealer will also wear out and require re-application in as little as a year, depending on how much use they see. Any sealer will be subject to etching as well. 

For the cheapest floor: sodium densifiers are the most affordable, and they are the choice to make if you aren’t polishing a floor to a final finish. Warehouses, garages, workshops for example, or an interior floor that will be covered by a flooring material.

You will probably find that residential floor finishers prefer to work with potassium or lithium densifiers for one main reason – the deeper penetration with sodium densifiers means that it will not be cured by the heat generated during burnishing. Instead, you have to wait 72 hours before burnishing it. That simple fact means a return trip for installers, whereas potassium and lithium densifiers can be burnished immediately, so they can finish a job in a day and not have to return 3 days later.

How  to apply silicate concrete densifiers:

Of course you should read the manufacturers’ recommendations before getting down to business, but here are the steps to a finished floor in a nut shell:

  • 28 days after the pour, concrete is sufficiently set to begin the finishing process.
  • If the floor is really out of level you can grind it with a 70-80 grit metal bond diamond abrasive to even it out, then continue through the grit levels to achieve the desired shine. Note: Only start as low as 80 grit if absolutely necessary as that will add a lot of cost. In most cases there is little need to start lower than 200 grit or go beyond 1,500 grit. Every pass costs you money in pads and labour. So don’t get up-sold to a 3,000 grit finish by a charming contractor with lots of time on their hands. You probably won’t see the difference in the finish but you’ll definitely see it in the bill.
  • Some manufacturers recommend applying the densifier after the first leveling pass for greater penetration, but there is a risk that it will be removed if there are many more grit stages remaining. Other manufacturers recommend waiting to apply densifiers until right before the final pass at the highest grit to ensure not too much of the product is removed. Use your own judgment here.
  • After applying the densifier, squeegee or hose-off the excess material before you start burnishing it. The heat from the burnishing will cure it at the same time as it polishes and you can walk on it immediately. 

Sealing cracks in concrete:

A certain amount of hairline cracks are natural; much larger than that could be an indication of a structural problem. First apply the densifier and see how it looks; the glass crystals should fill cracks up to .7mm wide. Any remaining cracks can be repaired with a cementitious filler, any excess will be removed with the grinder. For cracks much larger than that, install a backer rod first, and then the cementitious filler.

Alternatively, you can use an epoxy crack filler, but as they never completely harden, they have a tendency to leave a bit of a mess during polishing.

polished concrete floor – image credit homestolove.com.au

Choosing a sealer:

You don’t need a sealer on a mechanically polished concrete floor (MPC). However when you opt for the grind and seal method, you will have to choose a sealer.

Concrete coatings (both densifiers and sealers) are for the most part alkaline in nature, while a lot of the foods we consume (coffee, citrus fruits, tomatoes, etc) are acidic in nature. So when these foods spill and are left for too long a period of time, the acids will neutralize the alkaline and leave etching on the floor.

Repairing: When there is enough etching to get on your nerves, you can burnish the floor back to a nice sheen again. But if you remember that bit mentioned earlier about depth penetration and hardness – the harder the silicate crystal formed in the concrete by the densifier, the higher the sheen that is possible when the floor is polished or burnished, but the less times you can restore it.

Waterproof sealers: Some concrete sealers are breathable and will let moisture pass through, others are not. If a concrete floor has no vapour barrier below it or if the concrete has not sufficiently dried, ‘waterproof’ sealers such as most epoxies are, can bubble and peel as moisture rises from below.

If you don’t know if there is vapour protection under a concrete slab, be sure to use a breathable sealer. It’s probably a good idea to go for a breathable product anyway, unless you are positive you have continuous and well-sealed vapour protection below the slab and that the slab has had sufficient time to fully dry. If you have in-floor radiant heat, running it after you pour will speed up the drying process.

Wax The safest surface finish would be a natural wax with no toxic additives.

Applying a concrete sealer:

Different products and manufacturers will have specific instructions for the application of their products, so we won’t get into too great detail here. But the short story is – a day or so after completing the densifier, you can apply a final sealer. Normally you would just apply it to the floor and buff it with a high-speed burnisher and 3000-grit or hogs hair pad. A second and third coat can be applied if desired. 

Concrete floor environmental impact

CO2 impact Cement is a big contributor to CO2 emissions worldwide. This means that a concrete floor is not the best option for the environment. Concrete has an embodied CO2 cost of 200 to 400 kg per m3, depending on the type of mixture. So if the floor is not structural, it is a good idea to look at how the thickness can be reduced, and how the amount of cement can be reduced without compromising the quality of the floor.

Earth floor If you want to build a truly eco friendly house, an earthen floor, stabilized with lime would be a better option. This option can look fantastic as well.

earth floor- image credit: lehmtonerde.at

About this article: this article is part of a series of technical articles to help clients understand the materials and techniques used. They help the client make decisions about technical parts they are less familiar with. Some of the information can be used directly by contractors that are not familiar with new sustainable building methods. That’s why I try to provide clear information and instructions as possible, to be used on site regardless of the location (Africa, United States, Europe or China). Some parts are extracted from other websites, the sources are mentioned at the bottom of the article.



Circular Ecology

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