Which grout for internal limestone slabs over UFH please?

[Huskeeter]

Hi there very helpful people

I am looking for the best advice to grout the following. I'm a competent DIYer and this is in my own self-built outbuilding.

Size: 9 x 5m internal, heated office
Base: 70mm insulation, then DPM then reinforced concrete then UFH
UFH: Water-fed, closed system, then
Screed: with fibres, laid about 18 months ago

Tile bed: Sand-cement solid bed
Tiles: 21mm Bradstone limestone, varying sizes up to 1200x600, all laid about 12 months ago
Joints: varying between 7 and 20mm

Cleaned once, let dry for one month
Sealed twice with Mattstone limestone sealer 4 days ago

So now I'm hopefully ready to grout.

QUESTION: Must I use a flexible wide joint compound OR I have also been advised to use a fairly dry sand-cement mix, 4 parts to 1 part, wash off using very little water and then seal twice again?

Would find your advice and opinions incredibly helpful. Thank you.

 

[Alan.P]

I would use Mapei ultra colour plus in limestone. Fast setting so watch out for washing time, maybe split the floor in sections if your not confident, plenty of clean water.