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About three years ago, I had a decaying wood porch removed during a major exterior remodel. The new porch is larger and has a concrete floor and stone steps. The concrete floor has one of those "broom swept" non-skid surfaces.
Every time I sweep the porch, I see a lot of gray/white concrete dust. After three years (and the annual pollen wash with a garden hose), I'm thinking that this concrete floor is still too... porous?
I had the brilliant idea this morning that it might need some type of waterproofing sealer to prevent me from sweeping my floor away in a few years.
What products do you suggest -- something that an unskilled homeowner could apply? I don't want a paint or anything that would leave a visible film.
I wonder why are you seeing grey/white concrete dust if this was a quality concrete floor??? This would concern me a lot.
Your local home improvement store will have concrete sealers and, there is another solution, paint it with paint designed for such flooring.
However, before doing anything, I would double check the floor. If you can pick it and more dust or particles come up, sealing or painting it won't do much good. I would seek out a professional.
I'm pretty sure what you're seeing is harmless efflorescence. It's caused by moisture beneath your porch, or from within the concrete, leaching salts from the concrete as it moves through it. I'm not sure how much help it will be to seal it as the moisture will then be trapped beneath the slab instead of moving through it.
What do you know about the construction method?. Was a plastic vapor barrier used under the slab? Does it sit directly on soil? Or is there a gravel base? How thick was the slab.
I would suspect you have a spalling of the fine sand particles in the finish of the concrete. When concrete is placed it is screeded to level it, this process also brings up a mix of cement and the finer sand particles to the surface to allow a smooth finish, this layer is known as the cream. So on top of the concrete slab is a layer of 1/8 - 1/16" of mostly sand / cement mix, this allows it to be troweled smooth.
If the original mix was too lean in cement then you can get a problem like described. This cream layer will start to break up and flake off. It is normal with all concrete exposed to weather. You really notice it on things like sidewalks. Rain is like mini-jack hammers that eventually takes off the upper most layer. The better the original mix the better the slab will resist. Most of those home projects are probably something like 300# mixes. It is even worse if the concrete plant "Recycles" old poured cement. Some places they pulverize stuff like old sidewalks or recovered cement pourings and reuse it as part of the sand / gravel for a new mix. These type mixes tend to have a lot of fine dust which does not make a good cream layer. They scale off more than normal.
It is not a big deal in most cases. The slab turns from a white color to a sand brown and the surface starts to show embedded aggregates, it all happens to most exposed concrete after a while.
The salts if they are leaching can hasten the process.
If the concrete was going to be sealed it should have been sealed shortly after being poured. Usually it is the next day after it sets up. Sealing it at this point is debatable if that will help. Longer term I would think not. The process has already started and the concrete has cured. 28 days is the normal cure to near max strength, tho the process goes on for just about ever, in 28 days you have something like 95% cure.
The reason they seal concrete after pouring is to trap in extra moisture for that 28 day curing period and to retard the break down of the cream layer longer. The resulting slab is stronger than an unsealed one.
In very poor original mixes this process of shedding the upper cream layer can be severe and if the surface is exposed to weather / freezing eventually even the slab itself will start to breakdown. This takes a number of years but a lot depends on the quality / additives to the original mix as well as the placement / finishing techniques.
What Cosmic described sounds closest to what is happening (Cosmic, you really should write a book!).
The porch is covered, gable roof with a nice overhang. I don't actually see this dust on the surface, but I see it in the air when I sweep the porch, no matter how often I sweep and I just don't believe that should still be happening.
I need to post a close-up picture of the concrete. It has a lightly textured surface, sort of like a sidewalk.
We are on our third tiled porch. I love it. The first one was salsalito tile which I would not recommend. The annual cleaning and waxing was a full day project. It also got very slippery with just dew or a slight rain. Ceramic tile with a slight texture has served us well in AL/GA. Keeps the dust out of the house, looks good and does not heat up as bad as concrete in summer.
Concrete mix is critical. You want sand of various sizes, cement in proper proportion and proper gravel / aggregates.
This mix sounds like to me that it had way to much of what is called dust. This is powder materials about like face powder and could be entrained in either the sand or gravel. They are supposed to use a process to get most of it out of the stock piles. This dust is sold as a separate product for various uses, like as a base material under paving, concrete, etc. Usually it comes from the gravel not being cleaned enough during the crushing phase or excessive handling of the gravels after it is cleaned, especially if shipped by methods like railroad cars. A grinding type action occurs, creating more dust in the rail cars. Or some other improper materials used in the mix.
If you have excessive amounts of the dust included in the mix, not only will the top cream layer scale off badly but the entire concrete mass can also flake off as it is exposed. The cement is unable to bind the super small dust particles the way it does properly sized sand particles.
In a lot of these bad mixes you will see this dust as a film on what shows like bubbles in the mix. These bubbles are air, that show up like a soap bubble, the dust will almost have like an oil sheen to it. It can appear to be very easy to work and the initial surface finish will look nice. One tip off is the scaling will occur very quickly after the slab sets up. You will be able to cause dusting just by moving your foot back and forth across the slab.
If you have improper materials in a mix, sometimes it can be fixed by adding more cement, not what usually happens in cheaper residential type jobs.
Regardless of the concrete mix problems described so well by Cosmic, you may yet be able to effectively seal the concrete with a single component waterborne silicate floor sealer.
This type of product is intended to penetrate the concrete surface, then seal, harden, and densify the concrete surface. It's what all the big box stores use on their sealed concrete floors, with several coats to penetrate the concrete and then build up mil thickness on the floor.
One of the companies I rep sells this product commercially to the flooring trade, but I believe that you'll also find a comparable product in the box stores. If not, try calling on a wholesale epoxy/industrial flooring company in your area to see if they have a small quantity they'll sell to you. It goes a long way, you'd probably only need a gallon per 150 sq ft per layer.
It's easy to install, just spray it on with a low pressure garden sprayer (hudson type) and keep the floor wet with it for about 30 minutes. Respray or broom the product around to keep the floor wet for this time so the product can saturate the floor, then allow to dry.
This will densify concrete by about 50% in strength, and hold all the surface fines in place.
You must remove the surface latience and dust from the surface before applying; a good sweeping and vacuuming should be OK.
Being water based, there's no VOC's (no solvent smell!).
Regardless of the concrete mix problems described so well by Cosmic, you may yet be able to effectively seal the concrete with a single component waterborne silicate floor sealer.
This type of product is intended to penetrate the concrete surface, then seal, harden, and densify the concrete surface. It's what all the big box stores use on their sealed concrete floors, with several coats to penetrate the concrete and then build up mil thickness on the floor.
One of the companies I rep sells this product commercially to the flooring trade, but I believe that you'll also find a comparable product in the box stores. If not, try calling on a wholesale epoxy/industrial flooring company in your area to see if they have a small quantity they'll sell to you. It goes a long way, you'd probably only need a gallon per 150 sq ft per layer.
It's easy to install, just spray it on with a low pressure garden sprayer (hudson type) and keep the floor wet with it for about 30 minutes. Respray or broom the product around to keep the floor wet for this time so the product can saturate the floor, then allow to dry.
This will densify concrete by about 50% in strength, and hold all the surface fines in place.
You must remove the surface latience and dust from the surface before applying; a good sweeping and vacuuming should be OK.
Being water based, there's no VOC's (no solvent smell!).
The basic problem in those residential type jobs are no core samples are taken so if you get a bad job, you also have no real way to know the exact cause. Do you have a bad mix, do you have a water problem, do you have some type of placement problem? It could have been any of those or a combination.
There is no core sample to break or analyze. Those sealers can work fine to make certain areas better for certain applications. Like if the area gets spills of liquids, lots of types of wear, etc. I do not believe it is the solution for basic problem jobs. Some of those the concrete actually starts to break down inside. Most problem jobs you want to catch it early, fix some type of blame, determine who is going to pay for it, and solve it with a jackhammer.
Sealers just are not the type of glue needed to hold bad concrete together for any period of time. The original cement was supposed to do that job.
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How thick is the slab.....
