Slate floor tiles. Grouting problem

[Mark]

I have just grouted the Brazilian slate tiles and it was a disaster. The grout dried instantly on the tile surface and I had to scrub it off (way before the grout joint was dry

I washed the slates 3 times and let dry for a day before applying matstone tile sealer,2 costs applied with clean cloths and all was fine.
I buffed the sealer as the instructions and left for 3 days before starting to grout

I used brit adhesive super flex grout

I cannot get the grout off the tile with water

How can I clean the grout off and why did this happen, what did I do wrong??

 

[On one]

It looks like 2 coats of sealer wasn't enough, especially for a textured tile. With that type of tile I would be applying one coat of sealer after another until the slate was saturated......knowing what would happen if it wasn't completely sealed.
Unfortunately I'm not too sure what residue remover you could use on the slate without further damaging the slate

 

[One Day]

You should speak to LTP (who make the mattstone) and talk to them.
You'll need at least 3 products which will
1) strip back the mattstone.
2) remove mattstone residues.
3) clean the grout residues.

Washing and waiting for it all to dry 100% can take days I'm afraid.
Then it will need sealing to saturation which can mean multiple coats.

I've not installed brazilian slate for years but the last time it was from Topps and was awful.
I would use Universeal rapid all-in-one before grouting and would expect 3 coats on slate.

 

[Tile Shop]

Sealers break down with acids so in theory just using a diluted acid based grout stain remover, like the ltp one, should do the lot in one or two treatments. The slate doesn't contain calcium carbonate so higly unlikey the remover will damage the stone, but always try a test area first. Ltp do another remover called Cement, grout and salt remover. That is designed for more delicate stones such as unpolished marble and limestone, so could also be worth a try if the normal grout remover is too harsh. If you want to go belts and braces on it, use the ltp powerstripper first, to break down the sealer before tackling the grout with either of the removers. Once the grout is cleaned off, it will need resealing with a further 1-2 coats of the mattstone.

 

[hmtiling]

Did a slate job for the first time in a while. Cleaned, 2 coats of rapid all in one, grouted same day and a coat after. No issues.

 

[Mark]

Did a slate job for the first time in a while. Cleaned, 2 coats of rapid all in one, grouted same day and a coat after. No issues. View attachment 109718

Thanks for you assistance that’s great I’ve finally cleaned the tiles took three goes!!! All clean will go get some3 in one and try again.
[automerge]1568551722[/automerge]

Sealers break down with acids so in theory just using a diluted acid based grout stain remover, like the ltp one, should do the lot in one or two treatments. The slate doesn't contain calcium carbonate so higly unlikey the remover will damage the stone, but always try a test area first. Ltp do another remover called Cement, grout and salt remover. That is designed for more delicate stones such as unpolished marble and limestone, so could also be worth a try if the normal grout remover is too harsh. If you want to go belts and braces on it, use the ltp powerstripper first, to break down the sealer before tackling the grout with either of the removers. Once the grout is cleaned off, it will need resealing with a further 1-2 coats of the mattstone.

Cheers. I have now cleaned the tiles with acid and washed it all off. Will re seal with 3 in one sealer and try again thanks for your very in-depth assistance