Another black lime stone problem

[blacklimestoneproblem]

So another customer who purchased some indian black limestone slabs and now dealing with the problems.

Firstly we have 3 different types, one batch are several years old admittedly faded to light grey, which came from the old patio which was laid on sand and didnt display any of the problems which many have reported on here and we are experiencing with the newly laid slabs. 2nd batch came from India but UK supplier went bust so couldnt get any more of the size we wanted, eventually sourced another local UK supplier.

So the slabs have been laid on some well dug out Type 1 hard core, then a rough concrete base (where water can pool in places which makes it an ice risk that can then break apart the materials as it can get down to -12C here sometimes and ice is excellent for splitting apart things) and then mortar which wasn't entirely uniform so areas which dont touch the underneath of the slab now become dark from the moist patches that build up from the different temperature gradients caused by the overnight damp and lowered dew point.

The mortar wasnt cleaned off the slabs at the end of each day's laying so this has left its mark, dirty boots have also contributed their own mix of dirt with sawn slab dust, concrete dust and mortar on the slabs to varying degrees.

A mild diluted mix of Bostik's Patio & Driveway cleaner (hydrochloric acid with no surfactant to disperse chemical reaction in suspension) was used to clean up the dirty marks and this has left a general lightening of the slabs with some slabs displaying definite brush marks.

Easyjoint was then hand fed into the gaps, but not watered first into a slurry, and then surplus brushed and water washed out but not enough water was used as pools of rainbows were left on the slabs to dry out. This in turn has left its own darker patches which contrasts against the black/grey and white parts of the slabs. According to Azpects this is rainbow effect is caused by the linseed oil used in their product I guess to prevent the mix from curing into a hard mass when in transit. Azpects also confirmed the course sand mix is naturally coloured sand so no additional dyes, stains or any other type of colour treatment used to get the easyjoint into the different shades thats its available in.

Tried washing down with just water and brush not much change to the slabs, saw a bit more of the rainbow effect appear as the water penetrated deep into the easy joint and anyone who has seen an oil leak will know oil floats on water, hence the linseed oil being used as this is a light oil.

I tried a couple of samples of LTP Power Stripper (a benzene sulphur derived acid with negatively charged surfactant) and had mixed results but generally promising from the limited samples, then the ground works experts used some Persil Bio mixed with Soda Crystals on the slabs which was scrubbed in using a wire brush which has left its own unique brush marks in places and for a day a slightly more uniform greyish white appearance to the slabs before rain washed it off more thoroughly. Then finally I used some more LTP Power Stripper samples applied to various stones again with mixed results.

The mixed results from the LTP power stripper have ranged from a using 1:4 mix where some slabs have come back somewhat to their original black/dark grey colour but its not removed the darker stains left by the easyjoint linseed oil thats soaked & dried into the slabs as we'd hoped. It should be noted linseed oil will degrade in sunlight over about a 6 month period.

I've also tried neat/undiluted for 15mins then washing off with warm and also hot tap water (60 C we have that set to), generally in this situation you just see a line where it was applied neat but again some slabs its cleaned up well and other not.

At the same time, where I've applied it neat in a line and left it overnight and then washed off with hot tap water, its cleaned the section where the hot water was applied with the deck brush but you can also make out where the original line was allowed to dry out.

From what I have learnt from spending some time online probably no more than a few hours.

As limestone is just limescale in a different form with different impurities (iron for shades of red, carbon for shades of grey) and is highly reactive to acids, where some sites have recommended a high alkaline wash, this can mean lye, sodium hydroxide (bleach) or even caustic soda.

Now I suspect this is why the ground works experts who laid the patio used washing powder, but didnt understand the difference between a normal detergent and a bio, because bio's are an enzymatic liquid (use enzymes like you find in your body to digest chemicals) that works in a limited temperature range which is very hard to control and manage on patio slabs. Enzymes also target the type of dirt normally found on clothes which also make it largely ineffective against the type of dirt found on top of slabs having been recently laid.

I also found out, I think mentioned on this site, that these slabs are best laid on sand because the cement is also a calcium product which generates effervescence which comes up through porous stones like these ones and between the gaps on laid bricks. I should note that only a bit of washing up liquid was used when making the mortar, no other additive was used like SBR or a water proofing agent.

Anyway have done some more samples and tests to find the most effective way to get the LTP powerstripper to get these slabs darker, I've not really been able to find the magic procedure probably because where the slabs have been cleaned with acid, whilst the power stripper does leave them a tiny bit darker, its not really having an effect like we hoped and would need lots and lots of the powerstripper on the worst acid etched slabs.

However where the slabs have the dried left overs from the mortar, dust and I guess effervescence coming through from the mortar below, it is cleaning the slabs up reasonably well to a standard we could live with. It was even surprising to see the powerstripper also brought back somewhat the faded years old slabs we've had relaid but again these would need quite a lot more ie several applications, not just one or two applications so the costs then make us consider alternative.

We have also tried some LTP stone oil and Azpects Back to Black, both do a good job of leaving the stone much darker like when they are wet, but where we have seen the back to black applied to another patio, in the wet the water pools on top and leaves the stones quite slippery. Now Azpects have suggested wiping on and then wiping off immediately in order to prevent a gloss like effect on the slabs.

Has anyone else done this so they have ended up with a matt finish even when its rained on the slabs to stop them becoming slippery?

Subjective I known considering the different types of footwear that exists, but these were with merrell outdoor footwear which is Vibrams soles.

Another train of thought, is to simply acid etch all the new slabs so they all go back to the same faded white grey like our years old slabs and then use something like back to black or the stone oil.

My other half contacted a stone cleaner/restorer who may post on here who suggested the same thing, but to use the miracle 511 product.

I also note that Fife Lass on here has suggested leaving the stones for a year so the effervescence & mineral salts can be allowed to come up, in a way, giving the stones a chance to react now before a "final" treatment.

This seems logical seeing as these slabs are highly reactive, it would make sense to let the stone get the most easily exposed and reactive parts "out of them" as we are seeing mineral salts coming up even a day later after sections have been treated with the ltp power stripper, much like you see with the white stuff in the close up photos of the sets describing some of the problems with black limestone and then doing a comparison of products.
http://www.pavingexpert.com/stonpv05.htm
Top photo of this link for example of white stuff coming up.
http://www.pavingexpert.com/stonpv05a.htm


So whats the general consensus to remedy these slabs?

Give it a year, then clean and treat with a sealant/oil that will soak in?

Or acid etch them to a uniform-ish light grey and then treat them in the next few weeks subject to weather, knowing the most exposed reactive elements of the stone will still react with moisture coming up from below and general damp getting into them?

TIA

[blacklimestoneproblem]

Solved. Posted on 04 Oct. 2017,09:40 on this thread http://ext.pavingexpert.com/cgi-bin....y118310

"New install black limestone patio - cleaning, Cleaning new black limestone patio"