What's Causing This - Indian Sandstone

[rossyl]

Hi All

We have a sawn and smoothed Indian Sandstone patio. We have cleaned it (using chemicals) but these marks remain. The marks almost look like the slabs are retaining moisture, but can't see how that could be the case.

The slabs were laid last year.
Each is on a full bed.
Each slab was primed with a slurry mix.

NORMAL LOOKING SLAB
Which is how the majority look


SLAB WITH STAINING
There is one large area where multiple slabs look like this.
The marks don't brush of there is no residue.

http://[img]https://forums-images.p...images.pistonheads.com/206506/202505062329178

 

[Odds]

Were they laid using 'dot n' dab' method?

 

[rossyl]

Were they laid using 'dot n' dab' method?

Hi, no, as above, laid on a full bed and primed on their back with a slurry mix.

 

[Odds]

Maybe some mortar was dropped on the slab during installation, leaving a residue behind ?

 

[rossyl]

Maybe some mortar was dropped on the slab during installation, leaving a residue behind ?

It's a large area with this issue

 

[ellal]

Do the chemicals you used to clean the slabs contain acid?
Because it may well be that you have effectively 'bleached' the colour in some areas.
Made that mistake myself!

 

[rossyl]

Do the chemicals you used to clean the slabs contain acid?
Because it may well be that you have effectively 'bleached' the colour in some areas.
Made that mistake myself!

There are multiple areas of patio, with the same slab. Only one area has this issue. The same process for laying, the same slabs, and the same cleaner has been applied to all and in the same way.

I've shown one area where it looks fine.

I've shown another area where is does not. That's what I'm looking to resolve.