What do I use to add an eighth of an inch to an inch to level out an area of existing concrete patio?

A patio needs to be sloped for rainwater to flow downhill away from the home. It is possible to pour new concrete over the top of an existing concrete slab, but you should drastically rough up to top surface first so that it is like jagged rocks to secure the slab and prevent it from moving around, especially during freezing winters. Thin layers are susceptible to cracking. If your old slab prevents you from pouring at least 2 inches, then you should break up enough existing concrete to allow for that minimal thickness to be poured. No need for gravel for the thin pours: stick to sand and high PSI concrete (4000-5000 PSI). Add lignosulphonate or 0.5% molasses to increase the curing time.