Is there any way to get cement/mortar marks off indian sandstone slabs?

johndoe

Hi,

I bought some reclaimed indian sandstone paving and realised that the previous owners had laid the slabs the wrong way! So now the correct face has mortar marks round it. See photos.

Is there any way to remove the marks without it looking as obvious as it does now? What's the best way to do it without damaging the flag please?

This is a dry lay showing both sides of the slabs (on the left is the dark 'wrong' side)


This shows the marks of mortar and cement on the 'correct' side of slabs. As you can see the edges have the mortar lines on them

johndoe

Leakyboots said:

You could try brick acid and a stiff non ferrous brush brass or stiff synthetic bristle and elbow grease as they are not polished, or look at getting them cleaned with sandblasting, more expensive but no chemicals or effort.

Thanks for your helpful reply!

I'm going to buy https://www.toolstation.com/bostik-concentrated-mortar-brick-cleaner/p98902 and this https://www.screwfix.com/p/roughneck-soft-grip-brass-wire-brush/379jf
and see what it does.

Pete.8

I don't think hydrochloric acid should be used on sandstone.
Check that it is suitable before buying

johndoe

You're right. It appears that it might not be good for sandstone but I don't think I have a choice. I'm going to buy it but use it diluted on spare slabs and see what damage, if any, it does.

On a side note, can anyone tell from the pictures if the slabs are indian sandstone or something else? I just took the sellers word for it that it's indian sandstone but have no idea.

Pete.8

It usually looks rather like slate in that it's made up of layers.
There can be hues of red and brown and many have fossils in them that are visible on the surface.
The acid will likely stain them black

johndoe

Pete.8 said:

It usually looks rather like slate in that it's made up of layers.
There can be hues of red and brown and many have fossils in them that are visible on the surface.
The acid will likely stain them black

Thanks. But I still can't tell whether they are indian sandstone or not   I'll take some close up pics of a slab tomorrow to see if you can confirm?

Leakyboots said:

It will stain them if a steel brush is used, do test on a piece and then come back when its turned black, in the meantime, while I wait, I will just plod on spouting absolute garbage about things I know nothing about.

I don't know if you're being sarcastic or not but I am following your advice lol

gondor

I would have tried a pressure washer first.

JoeX

johndoe said:

Thanks. But I still can't tell whether they are indian sandstone or not   I'll take some close up pics of a slab tomorrow to see if you can confirm?

It’s not that it’s Indian that makes it fancy or quality, it’s that it is sandstone and looks good.  It’s imported from India because they have that natural resource and lots of very cheap labour.

Pete.8

I have been to many houses that have Indian Sandstone patios.
The most obvious feature I've noticed is that they all seem to have visible fossils here and there and the surface appears to be 'layered'- rater like slate.
I'd not be able to confirm if you have Indian Sandstone or Welsh Granite tbh
But do post some pics and I'll offer my thoughts, but see what others think.

I'm aware of quite a few homeowners who wished they had used something else as I've heard that it can 'flake' after heavy frosts, and stains really easily.
I went for a porcelain patio - a stiff brush, a bucket of warm water with a squirt of fairy liquid and plenty of elbow grease and it comes up like new.

Nollie

What you consider the ‘wrong’ side might have been the right side for the previous owner, some prefer the rough-hewn finish. If it has been specified that way you may have a problem. The concrete/mortared face is by necessity unfinished and unsealed for adhesion purposes, so the mortar will have sunk into and bonded with the pores of the stone to a certain extent. The upshot is that you may not get it all off.

By all means, give it a go, using a specialist alkaline stone cleaner like one of the Lithofin products. If you can’t get the smooth flipside sufficiently clean, however, you may have to live with the rough-hewn face or take it out completely and lay something else.

But please don’t ever use acid cleaners on porous alkaline stone like sandstone as it dissolves the surface, causing it to roughen, pit and break up. Once the surface of the stone is ruined by acid cleaner, it will be more difficult to clean as the pits (small holes) will collect dirt and the slabs will attract algae quicker, turning green and slippery.

johndoe

Pete.8 said:

I'm aware of quite a few homeowners who wished they had used something else 

I'm starting to wish I had bought something else lol.

Nollie said:

What you consider the ‘wrong’ side might have been the right side for the previous owner, some prefer the rough-hewn finish. 

I understand. I was just quoting what was said on paving expert https://www.pavingexpert.com/stonpv03 in the section "Which way is up".

Even though you and others have warned me about the acid, I had to try as I have nothing to lose. Results are in my next post...