Highly troweled floor finish
Our planned finished floor is high troweled concrete. We are opting for this over polished concrete. We’re unsure what we should ask for when it comes to sealers, densifiers or curing agents that is low in toxins for someone who’s chemically sensitive, but that will also help seal the surface to protect from staining. Maybe something like radon seal is enough? The other thing I wonder about is would a coating prevent the concrete from breathing and drying? Maybe that’s not a concern like it is for moisture management in walls where you’d intentionally use smart vapor barriers and vapor open paints? Would appreciate thoughts and guidance.
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The concrete does not need to breathe or dry. Concrete cures by a chemical reaction where the water is incorporated into the material, it does not "dry". Concrete can remain wet forever afterwards with no concern for drying. The example I like to use is hydroelectric dams, where the concrete is quite literally in contact with water forever with no problem.
What you DO want is a vapor barrier UNDER the slab, typically in the form of a polyethylene sheet. This will prevent ground moisture from wicking up through the slab and adding humidity to your indoor space.
BTW, I recommend you put remesh in the slab to help minimize future cracking since that concrete is going to be your finished floor. BE SURE the concrete crew supports the remesh properly on chairs or other supports that will ensure the remesh is held in the approximate center of the slab DURING THE POUR. I commonly see the remesh supported incorrectly, resulting in it being pushed towards the bottom of the slab during the pour. Remesh UNDER the slab does nothing to stregthen the slab -- the remesh needs to be in the middle of the cured slab to be able to do its job properly.
Bill
Emil,
Our house has a power troweled concrete floor on the main level. I cut it into three foot squares, and grouted them to both avoid cracks and give it a tiled look. If you don't want visible cut lines I'd still suggest expansion joints under some interior walls to relieve internal stresses
As Bill said, you don't need to make allowances with the finish for the concrete to dry, but you do need to wait for the concrete to cure before applying most sealers to get good adhesion. There are lots of low-VOC sealers now. The big box stores stock them. You can also apply a polish over them which needs periodic renewal.
Of course the concrete needs to dry; there is always water left after curing. Less so with super-low slumps and mechanical placement, but that's not how houses are built. Concrete doesn't mind being damp but it WILL dry to whichever side of the slab is drier. Fortunately sealers generally don't need the concrete to be fully dry, just cured for 28 days and surface-dry.
Here is one of many options that claim they're safe for chemically sensitive people: https://www.greenbuildingsupply.com/products/green-building-supply-penetrating-concrete-sealer. I don't have experience with that product so I can't vouch for it.
Thank you. I'm wondering if a highly troweled concrete floor needs a sealer? Looking at this sealer and many others, they all say that a highly troweled floor may not absorb the sealer well due to it not being porous? Maybe just using a densifier instead would be enough? Or maybe nothing?
The more you trowel a floor, the more fine aggregate is brought to the surface and the weaker the surface is, often resulting in spalling--sections of concrete flaking off over time. I'm sure there are fine points I'm not aware of but I don't recommend relying on troweling alone. Densifiers can definitely help, as would using a mix with extra Portland cement, but they can also affect the working characteristics of the concrete so I would either find a flatwork contractor who REALLY knows their stuff, or simply trowel the surface smooth and apply a non-toxic sealer. Either way the surface will be a mottled gray color. For more visual appeal, you can grind the surface down--called polishing--which removes the surface layer of fine aggregate and exposes the larger aggregate below. Then seal that.
Corinne Segura, who writes here occasionally, has an excellent blog all about materials for chemically sensitive people. Her article on concrete sealers is older but is probably still accurate: https://www.mychemicalfreehouse.net/2020/10/non-toxic-concrete-sealers-paints-stains.html.
Emil,
I second Mike’s recommendation to look at Corrine Segura’s site.
You need a sealer for two reasons. Without one the surface of the slab will dust off constantly - much as you have no double experienced in garages. It will also stain anytime you drop or spill something like food or drink on it.
Random liquids that get spilled will also soak in, often resulting in lingering issues in those spots. The sealer helps keep dust down, and also keep spills from being a long-term problem.
Bill
Why would you choose this option over polished concrete? Polished concrete seems like it would be the better option long term and if you are sensitive to chemicals or simply don’t want them in your house
freyr-design,
The upsides of just using a power troweled slab are cost and a visually calmer surface.
The downsides are that there are still imperfections in the surface.
Wouldn't the two be fairly comparable for people with chemical sensitivities as they both need surface treatments?
You do not need to seal a polished concrete floor. While it is optional the process of polishing removes much of the porosity of the finished surface and makes them much more durable. While they of course may stain (everything can stain) the variation of concrete can often make it more of a patina than a flaw. If you want any building material to stay the same as day one you are out of luck. At the same time you could spill a bit of lemon, wine, etc on acrylic sealed concrete and still have staining. And it will arguably be more noticeable. Lastly, you can polish to whatever finish you like. I think if I was going the concrete route I would opt for polished unsealed to sealed, but maybe I just haven’t found the right sealer yet.
feyr-design,
For the chemically sensitive unsealed polished concrete does sound like a good solution.
Having lived with a concrete floor for 30 years I don't think I could manage an unsealed one, although maybe the wax based finishes would alleviate the problems with staining?
For really good sealers you need to go back to the days when they were xylene based. Application meant a splitting headache - and probably a permanent diminution in IQ for the installer!