Grout between slabs

[hammer]

Hi - I have made a hearth for a woodburning stove from sandstone paving slabs. The gaps between the slabs are up to 25mm wide (vary in width as the slabs have a rough edge).
What should I use to fill the gaps
- grout
- cement or mortar with a coloured dye
- cement the gaps until a few mm below the top of the slabs, then grout on top?

Any other ideas would be appreciated. The slabs will get quite hot, although there is a heat guard under the stove, preventing the hearth getting above 100 C

Thanks

 

[White Room]

Sand and cement to finish the joints..