Hey neighbor! I made my own concrete for the last cabin I built, brought the Portland cement in by sled in the spring. I screened the aggregates from the nearby gravel bar. I use 12" PVC piers 9'+ deep. No heaving at all. My buddies cabin on sonotube piers heave, so I use PVC.
what you used for the form shouldn't make any difference in the chance of it heaving. it's the frost depth. i would suspect that your set of piers were deeper than the frost depth in your area opposed to your buddies piers may not have been deep enough. depending on the size of the structure you will be supporting and the distance between the supports required, another issue a person could run into is inadequate soil bearing for the particular load. a 12" dia pier doesn't have much of a foot print to distribute the loads into the ground. what would be nice is if a guy could bell out the bottom of his piers prior to pouring to give the pier more footprint to bear on. belling it out to 18" to 24" in dia for about 8"-12" in ht before transitioning back to the original bore dia would go a long way in both bearing capacity as well as increased up lift capacity for your wind loads (think of it as the pier being dovetailed into the soil some 10 ft below. my .02 anyway.
