What is the definition of "ground-up shell"?
I'm not sure this is the best place to ask. 

Ground-up shell means just that - shells that have been ground (reduced to small particles or powder by crushing it). 

Shell tempering is one of the hallmarks of Mississippian cultural practices. It is often characterized by the adoption and use of riverine, usually mussel shells, (or more rarely marine) shell-tempering agents in the clay paste. 

Read more here: https://www.revolvy.com/main/index.php?s=Shell%20tempered%20pottery&amp;item_type=topic