Basement slab powdery after grinding

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Old 09-25-20, 10:03 AM
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Basement slab powdery after grinding

Hello all - I just had my basement slab grinded by an asbestos abatement contractor to remove asbestos mastic. They did a great job, all of the mastic is gone. But the slab that is left has a powdery surface, and no amount of mopping and vacuuming seems to remove all the powder; in fact any sort of light abrasion just seems to create more powder. I plan to install a 6mil vapor barrier and floating LVP floor over the slab. Wondering if there is anything I should do to the slab first, like seal it or coat it in some way?

Here's some background, with pictures attached. The house is about 70 years old, and we moved in 1 year ago. The contractor was hired to remove asbestos black mastic from what they and I assumed was a concrete slab that was painted red. The grinding would have removed the mastic and paint. When they got into grinding, they found that the slab was not painted red, the actual concrete was dyed or colored red. They said they had not seen this before. Anyway, when they finished grinding, I was left with a light red slab, lighter color than what we had originally though was paint under the mastic (see picture of the floor after grinding, contrasted with the other picture of a section of closet floor that did not require grinding). After they left, I noticed that after walking on it I was tracking red dust in the house. I went down and wiped with a dry paper towel, see picture (left side) for the massive amount of dust that came off. So I called them back the next day, and they HEPA vacuumed once and then wet mopped the entire floor 3 times. After they finished it was better, you can see the paper towel swipe I took on the right side of the picture. Better, but the dust was not gone. They said no amount of mopping would get it all, all you would do is generate more dust with more cleaning. So I tried to vacuum with my shop vac and a plastic brush attachment (not very stiff brush), and sure enough it seems that the portion I vacuumed is actually worse, as if the brushing abraded the concrete and generated more dust.

So what to do? Seal it? Mop it some more? Leave it and just install the vapor barrier and floating floor on top?

It just seems odd that there would be so much red powder left behind, I had the impression that a floor after grinding would be very hard and polished. Is this unusual? Maybe this colored concrete is just softer/weaker than normal? Why did the old floor have a hard/darker colored "skin" on top, was it sealed in some way?

Appreciate any help on this!!
Steve


Floor after grinding

Old floor in an area that did not need grinding

Left is swipe of floor after they first left, right is a swipe after they came back and mopped 3 times.
 
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Old 09-25-20, 11:21 AM
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If you are going over the floor as you describe I would not treat the floor with anything. Go ahead and install your vapor barrier and floating floor.
 
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Old 09-25-20, 01:28 PM
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So what is the red material?
 
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Old 09-25-20, 02:11 PM
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I assumed the original people did a colored colored concrete and didn't like the look and covered it.
 
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Old 09-25-20, 06:17 PM
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Thanks Dane for the input, sounds like the best (and easiest) way forward.

Yeah, colored concrete. The asbestos abatement contractor said they had never seen colored concrete used in residential. Came across it a few times in commercial. The house was built in 1950. Kind of an oddball situation.
 
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