Adding a layer of concrete and tiles in the garage
I would acid etch the old cement with a strong muriatic acid solution to clean and make the surface rough. Then add a adhesive promoter like Moos milk painted on the slab that will help the cement bond and reduce cracking. I have done this on floors as thin as 3/4" and as thick as 2" with good results. With a thin slab a fine aggregate like 1/4" will also help.