slab foundation

Concrete slabs should be designed with expansion joints to allow the slab to move freely. If a slab is too large without expansion joints, it will crack.

Adding cement to concrete does indeed make it stronger. cement is a hydraulic binder, meaning it binds well with water. This makes it ideal for use in concrete, as the water helps to activate the chemical process that makes concrete harden. The more cement you add, the stronger your concrete will be. 

That being said, there is such a thing as too much cement. Too much cement can actually weaken concrete, as it causes the paste to become too fluid and accelerates the hydration process, leading to cracks and shrinkage. So while adding cement will make your concrete stronger, be sure not to add too much or you could do more harm than good!

The answer to this question depends on a few factors, including the type of concrete, the weather conditions during and after the pour, and the thickness of the reinforcing bars (rebar). In general, however, a minimum thickness of 4 inches is recommended.

If you are looking for a more specific answer, it is best to consult with a professional engineer.