Black indian limestone - Mortar staining

[Ian Gill]

Hi - excellent site.
I have recently had my garden landscaped. Part of this involved the laying of a new patio in black Indian Limestone.
This has suffered from mortar staining from the pointing. The effect ranges from stains near the edge of the pavers to a “washed� appearance on others – almost as if someone had wiped over parts of the surface with a grey cloth.
There appears to be some natural reaction as well - two pavers that were surplus to requirements and left leaning against a wall seem to have reacted to rainfall.
The builder insists that the pavers will weather with time but in my opinion it has detracted from the overall appearance of the patio.
I would add that I am quite happy with the natural variation of the stone itself, it is the mortar staining that concerns me.
Are there any elements of poor workmanship or is it a common risk. All other aspects of the landscaper’s work were of a high quality and we have reached an "agreement" regarding the effects of pointing.
I have tried (carefully!) jet washing but this has not proved particularly effective.
Reading through the forum I see that acid cleaning is not recommended on the Indian Sandstone. Does this apply to the limestone as well?
Is there anything I can do apart from wait for natural weathering?
PS They look great when wet so they’ll probably look OK for most of the summer!
Incidentally the patio was priced overall at £100 per sq metre inclusive of site clearance, new base, labour etc. It obviously included cost of stone - £28 per sq metre. I live in Surrey as there seems to be quite a variation in pricing throughout the country and peoples expectations of what this work "should" cost.

[Tony McC]

The acid used to remove cement-based stains works by reacting with the Calcium Carbonate content of the cement. Limestone is mostly....Calcium Carbonate!

So: you can see that using an acid-based cleaner on limestone is a bigger potential bollix than using it on sandstone. However, with the black limestone, the effect of the acid is much, much less noticeable than it is with paler limestones, and I've seen relatively fresh cement stains on black limestone cleaned with 5% HCl acid and no effect discernible on the flags themselves. I'm not daft enough to suggest that this means that acid is ok to use on any black limestone - it could be a one-off, but the contractor concerned told me that he's never had a problem cleaning black limestone in this way. He suspects there may be some surface damage and discoloration but is is impossible to spot it without getting down on yer hands and knees.

So - it might be worth testing a very small, discreet corner. Start with a weaker dilution, say 2%, and avoid doing it in direct sunlight as there is some evidence that acid discoloration of stone flags is exacerbated by a photochromic reaction that may be summat to do with intense UV light. Pre-wet and then clean a couple of linear metres of stained joints, making sure you wash off thouroughly after 10 mins max, and then, regardless of success or otherwise, LEAVE IT OVERNIGHT. Some colour reactions aren't immediate but take place over 24-48 hours, so what might, at first, seem like a result, can be highly misleading.

If the cleaning works and the stone is still black the following evening, then it's probably safe to treat the rest of the paving. However, you must treat ALL of the pavement, not just the stained areas or you run the risk of blotching, and it is abso-bloody-lutely critical that you pre-wet the pavement before applying the acid wash. If the stone is 'thirsty', it will soak up the acid and absorb it deep into its structure. Making sure that the flags have had a good soaking beforehand helps keep the acid on the surface, where it's needed, and where it can do least harm.

I'd be very, very interested to hear how it goes, whether it works or not, and, if it does work, how many applications and how much elbow grease was needed - photies would make me go all goosebumpy!

Bear in mind that, if it does all go tits up, you can't blame your contractor, and don't say I didn't warn you. Just becaus eit works for a particular contractor in a particular part of the copuntry at a particular time of year using a particular type of limestone and a particular type of acid, doesn't mean it is guaranteed to work for you. Do the test area FIRST.

[Ian Gill]

Hi Tony, thanks for the prompt reply and advice.
It is probably going to be a week or so before I have enough time to do this but I will update you with the results.
Is there any particular cleaner you would recommend or are they all of a muchness?
Regards,

[Tony McC]

Probably too late, but acid is acid is acid, so they are indeed, much of a muchness.