cost of slab foundation

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!