Cottage cheese clog

As mentioned in my other thread (plumbing topology) I'm redoing the drain pipe under my home. About 10 years ago I had a major clog and cut a section of the 2" main out and it revealed a white cottage cheese looking clog all the way through the main, though it's a bit gritty and packed in as hard as clay.

I'm replacing the whole thing with 4" and discovered the 2" old main from the kitchen down just past the bath tub is again completely clogged with the same substance in the shape of a crecent with just a sliver clear on the bottom of the pipe! Were very diligent about not dumping any grease or food from the kitchen. One culprit could be dishwasher granule soap. The washing machine is on another seperate main to the drain field so that is eliminated. Anyone have any ideas as to what can cause such a clog?
TIA

Helwoe!

I found this bit of information (Hard Water Problems ) I wonder if the increased amount of soap because of the effects of hard water are to blame? We definately have hard water. Shower head clogs with lime twice yearly.

Helwoe!

maybe

pan grease is going down the drain

Were very diligent about not dumping any grease or food from the kitchen.

No pan grease goes down the drain.

Improper venting on your DWV system can cause all of these problems, even on vertical drops.

I thought about that. I'll check to see if there is any obstruction in the vent pipes. Thanks