Hi, The sub-base layer of mortar mix is cement and concrete/sharp sand, at a ratio of around 4:1 or 5:1. Then, to get the slab to bond with the mortar, mix 1 part cement to 1 part SBR fluid and apply to the back of the slab using a paint brush. This is called a "priming slurry". You can buy a dedicated slurry product, but cement + SBR will do the same job.