Ants mining slate patio - Which sand/grout

[chris66]

Hi, I'm renovating my slate patio which was laid 7 years ago. Some of the 3-4mm grout joints have blown so i decided to remove all of them and re grout the lot. Mapei ultracolour was used previously and seemed to have held up fairly well. Noticed that ants have been mining under the 60cm x 60cm slate slabs as my trowel can drop 50cm in between the tiles in some places. Also noticed that water is pooling under some of the tiles in places. Is it possible that the was the water underneath the tiles evaporates it is blowing the joints? Considering filling through the joint gaps with (polymeric?) sand which will harden and resist further ant mining. Gaps between joints are quite tight 3-4mm and wondering what the best solution would be? Ant bait traps have also been considered. The slate tiles are all stable but I'm concerned further mining will destabilise and I will have to re lay the whole patio at some stage. Thanks in advance for any help.

[MikeG]

Have you checked that the slabs were laid on a full bed ? The gaps you talk about could be due to ‘spot bedding’ (not the way it should be done) which will leave voids.
Is there obvious signs that the ants are mining their way into the bedding mix ? A decent bedding mix should not be prone to ant mining after 7 years.

[chris66]

Thanks for your reply. Yes, the slate tiles were laid on a full bed of 50cm sand/cement 5:1

[chris66]

Hi MikeG,

You were right some of the slate tiles were laid on full bed and some were laid on almost full beds. I've had to take up x5 to re bed them in.
Should I use 4:1 sand cement with plasticiser? Or will 6:1 be adequate, bearing mind it is being laid on a 100mm concrete slab? Still considering either the GfTK vdw 815 or Mapei ultracolour plus for grout. The Gftk states that the sub base needs to be permeable - as I am laying on a slab, do you think this discounts the GfTK in my case?

Thanks in advance for any help.

[Tony McC]

The Mapei is a better option in your case.

You can 'flood' open joints with a 6:1 sliry mortar absed on building sand to help fill any sub-surface voids, and then, after 3-4 hours, use a jet of water to clear any excess mortar from the joints, levaing the voids beneath completely filled, and plenty of space within the joints to accommodate the Mapei.

[chris66]

Thanks Tony, thats really helpful. My concern about using a cement based slurry grout is potential staining on the slate and it being difficult to clean afterwards. Do think this be an issue?
Also, how much water do you mix with the 6:1? is it by eye and very soupy? Can't seem to find a slurry ratio for water as well on the site.

Many thanks in advance for your help.

[Tony McC]

You have to try very hard to stain a good quality slate! Becaus eit is practically impermeable (that;'s why we cover our roofs with it!) stains, even cement stains, have a job getting a hold on it.

As long as you are thorough and wash off carefully, attentively, every square millimetre, you won't have a problem.

If I had to choose one type of stone paving to use when demonstrating the virtues and benefits of a cement slurry mortar, it would be slate - it's so hard to cock it up!

[chris66]

Thanks for the reassurance Tony- much appreciated. Still not sure about the water volume to make up the slurry with the 6:1 building sand and cement? Can't seem to find a guide on the website.

Many thanks

[Tony McC]

I can't quote a definitive water volume because I can't possibly know the moisture content of the sand being used, nor its grain sizing.

As stated on the site, you add water until the slurry has the consistency of a thick-ish soup - that's as definitive as anyone can be unless you were working with perfectly dry sands.

[chris66]

Thanks for your help and advice Tony, the slurry worked a treat. No staining at all. This is a really good support service that you provide, especially for us have -a- go amateurs!

Many thanks