Slate tiles on the terrace are loose - how to glue and grout them

Hello - as you can see in the pictures, my terrace has become somewhat dilapidated. However, I don't want to have the entire terrace relaid, but only repair the damaged spots. Here is what I plan to do:

- lift and clean all the loose slabs (especially on the underside)
- remove the joints that are as dilapidated as possible with a chisel
- reattach the loose slabs with assembly adhesive or silicone
- fill the open joints with joint mortar

A few questions about this:

- Do you think the bonding with assembly adhesive or silicone will hold on the substrate (concrete foundation) or the slabs?
- Assembly adhesive or silicone, which is better?
- Which joint mortar is used for this?

Thanks in advance - BG Manuel

Why no tile adhesive? Good ones are also suitable for natural stone and outdoor use. That would be obvious to me

Today I lifted the slabs and I really don’t like how it looks underneath. It’s pretty hard, partly concrete or mortar, but especially around the edges there is a lot of dirt (see photos). What a mess...

The problem is the following. It’s a rental house, but the landlord only wants a small solution, meaning a company should just glue the few loose slabs. Two companies have looked at it and refused, saying that if it’s done at all, it has to be done properly. But the landlord doesn’t want that because it’s too expensive. So I thought OK, I’ll glue them myself and even if it only lasts 2 to 3 years, it doesn’t matter, who knows how long we’ll be here anyway, and besides, I can just reglue them after two years. The materials don’t cost much – glue and a bit of mortar under 30 euros and maybe a day’s work, if at all.

Well, what to do. Of course, I’ll talk to the landlord again – it keeps getting worse and now the slabs are breaking. If she still insists on refusing, what could a small low-budget, self-made solution that doesn’t claim durability and perfection look like?

Thanks and best regards, Manuel

Apply tile adhesive thinly over and sponge the boards.

Rental property: Stick pragmatically - happens every year again..
Own property: Partially or completely redo.