Acid stains on driveway

[Lions3020]

We've had some building work done and it's been very tricky to get the contractor back to do the final bits of snagging/clean up.

One such job was the removal of some cement staining on the drive where the mixer had been positioned.

Never mind, a job well within my DIY competence (or so I thought). I had some mortar satin remover (muriatic acid solution) in the garage and, following the instructions for patios, applied a diluted solution to the stains, scrubbed and rinsed. That didn't do the trick but the instructions said it could be applied neat to brick work, scrubbed, left for 20mins, then rinsed.

The good news is that the mortar stains are away but the bad news is that I now have the discolouration on the treated area (see pic).

This is the driveway dry having been rinsed thoroughly three times after treatement 3 days ago. As far as I can work out three things might have happened here.

- there is still acid residue, so the application of an alkaline solution and another thorough rinse might help?

- the acid has removed any sealant present on the blockwork and this is allowing moisture into the blocks - contributing to the 'wet look'. Application of a breathable sealant might then help?

- the acid has removed the cement and top layer of the block work resulting in the true colour of the drive showing through. In that case leaving it to the elements to become weathered once again would bring back a more uniform finish.

Any thoughts and advice on what to do (if anything) are appreciated.

 

[HERTS P&D]

It still looks wet.

Andy

 

[Lions3020]

It's definitely not wet. That's three days of drying time with little to no rain.

 

[HERTS P&D]

If that's the case, I would acid wash the whole drive and then let it weather.

Andy

 

[accelerator]

I would leave it for 2-4 weeks and then look at the colours again to see what's happening.

What's probably happened is the acid has reacted with compounds formed by weathering and taken the bricks back to a more "pre-weathered" status. In time the treated bricks will probably go back to the weathered look, and that's why I would leave it at first to see what happens.

Sealing now would be a bad idea as you could seal in the colour mismatch.

 

[endecotp]

It’s significant that the blocks look dry in the middle and wet at the edges. Has the size of the “dry” patch in the middle of each block changed? If it’s slowly getting larger, then just wait and see what happens. Don’t do anything in a hurry.

 

[Lions3020]

I think the size of the dry patches are largely the same but can't be sure without taking a picture each day (which I might now do for a week or two and report back)

I did wonder if that pattern was the acid settling in the joints between the blocks and therefore changing their colour in that way.

 

[PEDANTICVINDICTIVEMAN]

Builders didn't put a polythene sheet under the mixer then

 

[Lions3020]

UPDATE - This is the situation after about a month of doing nothing (see pic below).

The wet look patches have receded quite a lot. Possibly through a combination of rain washing away any acid residue and general weathering.

Will leave for another month or two to see if the drive returns to a more uniform colour.

 

[accelerator]

Looks a lot better. Where your acid solution was you can see the bricks are now lighter. The question is whether this will go darker like the rest. In my view, the darker colour could either be from weathering, or it could have been from the firing process, and the solution has removed a fine layer from the bricks. Hopefully it's the former. Doesn't look too bad now anyway.