When I first started working for my wife’s grandfather, we would load sand and gravel on the truck every morning, and then pick up sacks of portland cement on the way to the job. We would mix the 3 ingredients in a wheel barrow on the job-site and make our own cement.
Nowadays, we use pre-bagged ready-mix in 80 pound bags. Most will simply pour the bag in the hole, add water, and mix in the hole. The biggest advantage to ready-mix is the time savings. Even if mixing in a wheel barrow, we still save the time of not having to load the sand and gravel (and then unload).
There are many companies that manufacture ready mix, from TXI to Quickcrete. TXI sells ready-mix to Mccoy’s Corporation (a smaller lumber company) in Texas and Quickcrete is sold to the big chains (Home Depot and Lowe’s). I prefer the TXI, it seems to mix better. The Quickcrete is too gravelly and doesn’t set right.
Ready-mix comes in many sizes and varieties, from fast setting to anchoring cement, so be careful when purchasing and make sure you are getting the right product for your needs.