Fixing indian stone slabs to thermalite block?

Is this an internal or external wall? If it’s internal you could use plasterboard adhesive.

You can stick the sandstone to blockwork using o.p.c and a p.v.a additive or with a strong outdoor tile adhesive, no need to seal blockwork.

No sealant needed Just loads of exterior tile adhesive as they come different thickness ‘s and make sure the slabs are thoroughly soaked because this product is prone to “popping”

If outside , I would tight coat the thermalite with a 6;1;1 washed sand cement lime scratched render ,then sbr adhesive in a ratio of 20% to 80 % water painted on surface. Leave to dry Recoated as work proceeds , and use a recommended exterior tile adhesive,grout .Then seal with a grangers waterproof sealant.consult a tile supplier for advise on tile adhesive and grout.