How to clean concrete blocks prior to applying adhesive

[Mmathis]

On the tube, it says "all surfaces must be clean & dry, and free of dust." Define "clean," and explain how to get there.

This is cinderblock that's been in contact with clay soil -- I'm going to attach a capstone. I've scraped off all the soil and brushed it pretty well. But it still looks dirty to me....

 

[Big Lou]

According to how many block you need, buying new blocks may be cheaper and less laborious. If not, clean with a cleaner made for concrete driveways. If what you see is just soil then use a stiff brush along with good cleaning liquid. It is good you noticed the blocks remained dirty because motar would not have adhered well.

 

[Mmathis]

According to how many block you need, buying new blocks may be cheaper and less laborious. If not, clean with a cleaner made for concrete driveways. If what you see is just soil then use a stiff brush along with good cleaning liquid. It is good you noticed the blocks remained dirty because motar would not have adhered well.

These blocks are permanently in place so can't be replaced. I was just doing more research myself, and discovered that there are commercial concrete cleaners, so I'll check at HOME DEPOT tomorrow.

It's funny, but every site that I visited regarding surface prep for concrete, all just said, "be sure it's clean & dry."

Muriatic acid was recommended, as was TSP, oxygen bleach, and others....but this was for cleaning garage floors & stuff like that.

 

[RobAmy]

A wire wheel on a drill would work if real extreme but just a wire brush by hand should work. Then add a bonding agent in the mortar (if using mortar vs caulking) for the cap stones. No need for harsh chemicals to clean it.

 

[dieselplower]

I agree on the wire brush. Compressed air works good on dust.

 

[Mmathis]

Thanks for the suggestions, but maybe I should explain a little more.....

This is the collar on one side of my pond. Normally, the dirty cinder blocks wouldn't be an issue, since it's all going to be covered by the liner, BUT I need to raise the level of the collar by 4" -- thus the addition of the cinder block caps.

The "exposed" sections of cinder block are packed with soil. There was also soil on the block above [solid section]. I scraped all the surface dirt off, and dug out about 1/2" to an inch into the block holes. I brushed with a stiff plastic brush, but of course, the dirt is imbedded into the concrete....

Would any of the above-mentioned options you guys have suggested still work for this [cleaning only the "blue" highlighted surfaces in my drawing]?

 

[Big Lou]

I would simply go with a stiff or wire brush. Swept or lightly water spray it off. Much faster and easier. I initially thought you were building a garden low wall. All should go fine.

 

[Mmathis]

I was reading about oxygen bleach. What the heck! I poured on some peroxide, straight out of the bottle, and let it sit a few minutes. Man, did it fizz! Then I lightly rinsed away the residue. And guess what -- majority of the dirt bubbled out and it's fairly clean!!!!