Cement is just one of the ingredients in concrete. Concrete is a mixture of some type of aggregate material (crushed rocks or sand) and cement. The cement is the binder that holds all the materials together once hardened. Quick-drying concrete, typically found at your local hardware store, is great for setting the posts of a pergola or a deck. I personally prefer concrete with sand as opposed to gravel/rock. I have found that the sand concrete will secure the post better.