Hey, all-
I have a 300 sg ft slab of lightly colored concrete with a exposed aggregate perimeter. They were poured about 6 weeks ago, and appear to be well cured and dry, and it is time to seal them.
I have settled on a sealant, but after talking to lots of finishers & product manufacturers, I couldn’t seem to get a consistent answer on how to prep the surface. The surface has barely any effloresence, but does have several bird-splotches (dang birds). I settled on using phosphoric acid at a 8:1 dilution, brushed it on for 5 minutes with a little scrubbing, then rinsed with a strong jet of water. Upon the advice of a concrete product supplier, I used TriSodium Phosphate (TSP) to neutralize it- about two pounds in 3 gallons of water, washed over the slab for a few minutes, then rinsed off. After a couple of days to dry, I am hoping it is ready for sealing.
Try as I might, I could find very few people who use TSP to neutralize phosphoric acid, and therefore could not determine how to mix it. I am concerned that the acid would still be present, not allowing the sealant (which is water based) to adhere properly, or worse yet, the acid would continue to degrade the concrete. Anybody have any experience with TSP as a neutralizer? Did I just screw up this slab for good?
Replies
I'm confused. My experience with concrete sealers is that once it's clean and dried, go ahead with a sealer. The real issue is how glossy or slick it should be. I'd go to a dealer who supplies professionals for advice with a particular product.
Barmil- That's pretty much what I did, and got a lot of varying answers. Many of the product reps or retailers did not appear to have significant field experience (I was surprised at how many said as much). The strategy I used was based on the best cumulative solution I could gather...but I'm still not confident about it.
The acid and TSP are overkills.
Wash off the bird poop and you're good to go.
Note: we neutralize acids using either ammonia or sodium bicarb. Dilute either, apply and wash off the salts created. Then rinse again. Let dry and seal.
Too late- it's already done. Cleaned with phosphoric acid (rinsed), followed by TSP (2lbs:3gals water) for a few minutes. Question is: will the sealer hold, or will the slab surface slowly disintegrate?
Edited 10/22/2008 1:58 am by jjapogee
Take a look at what you bought. It is likely less than 50% TSP.
http://en.wikipedia.org/wiki/Trisodium_phosphate
If the acid wash and rinse are already done, the acid will not continue to eat away at the concrete. It is done. The alkalinity of the concrete and acid have completed the reaction and you only have some salt formation remaining. A thorough rinsing should remove that.
Neutralizing acid washing is done to stop the reaction that is taking place by substituting a base that is more reactive to the acid than what the acid was trying to react with. In eather instance the acid and base (be it concrete or the neutralizing agent) form a salt, and that is what you need to rinse off. Once it is gone you can seal the slab.
A simple test would be to let some water stand on the slab for a few minutes and then test it with a strip of litmus paper (the same strips you use to test pool swiming pool water will work if they will indicate a neutral PH) On a standard litmus test a PH of 7 indicates it is neutral, and your good to go.
If the sealer fails, it won't be because of a contaminated surface.
I just went through this on a walkway I poured. You are good to go. The acid is neutralized by the concrete.
Don't over think this. I put down a sealer that has no sheen. I think it is a silicone based product but I'm not sure.
The one thing I would change is to have applied the sealer in one continuos application with roller. So there was a wet edge the whole way. But I'm being picky.
I got some blotching because I used a sprayer and didn't do it in one continuous wave across the slab. But with this sealer I can seal it again in a few years.
"Don't overthink this." Yep, that pretty much nails it- all of my efforts to do it right are creating 'analysis paralysis'!My lack of confidence comes from not being able to find anyone who uses TSP commonly to neutralize the phosphoric, and being unable to get clear answers on how to prepare the correct TSP mix, or how to apply it. Thanks for your help on this.