As it is with many industries the concrete industry has its own terminology when it comes to selling their products.
First and foremost is to understand the difference between cement and concrete.
Cement is a product made of limestone, clay and gypsum, the most common blend for all construction, including mortar, is Portland cement. It is a powder, that without the addition of moisture does little.
Concrete is a product made of cement, one or more aggregates and water. What you require, if you desire to have a slab or column is concrete NOT cement.
You can create your own concrete by purchasing suitable quantities of cement, aggregate and adding the appropriate amount of water. Or, you can purchase bags of pre-mixed concrete, which have a defined quantity oc cement and aggregate in the bag and just mix in the recommended amount of water; or you can order ready-mix concrete, which is delivered ready to pour, as shown in Figure 1, from a local concrete supplier.
Figure 1 - Concrete delivery truck
Bags of concrete are sold by the pound. The most common bag sizes are 60 lb and 80 lb. These bags have the correct proportions of cement, sand and gravel.
Ready-mix concrete is sold by the
yard. A yard of concrete is actually a cubic yard of material (27 cubic feet). It is delivered to the location already mixed with water and only needs to be hauled from the concrete delivery truck, Figure 1, to the location on your property where it is required.
Their are advantages and disadvantages to both. With bagged concrete you can work at your own pace, whereas concrete delivered by a truck requires you off-load it immediately and work to finish it before it cures. Bagged concrete (if your project requires more than a few bags), generally means buying or renting a portable concrete mixer as shown in Figure 2, and at the end or the day is much harder on your back!