I am wanting to tile over an existing concrete slab but need to raise it by about 10mm before laying the tiles. I am wanting to do this with a mix of sand and cement but do not know what the ratios are. Can you help

What are you reasons for wanting to raise the height? To level it? To smooth it out because of imperfections?

Is it exterior or interior?

Inside you could use 'Thinset'(to a certain height) Standard mortar(premixed) otherwise.

Exterior you could use mortar(premixed)

As far as making your own,I have used 2 spades full of mortar(type N),7 spades full of sand,and one spade full of lime.(this was a historically correct match for barn work,it is strong,but not a grey color like standard mortar)

Hope this is helpful,

Ken

It may be easier to just put down a 1/4" hardibacker sub floor board. Modified thinset and the backer will give you 10 mm plus it will help to level the floor.