I wanted to reply to this earlier however I've been very busy the last couple of weeks with facilitating the 'Sculptural Kitchens and Bathrooms' and 'Garden Art' classes given by Rob O'Donnell. Those of you who were unable to attend those classes this year missed two, amazing classes. The wealth of information and direct experience given during these classes is phenomenal and Rob is a highly competent and well liked instructor.
Regarding diatomaceous earth: I remember reading about it some time ago [15 years or more] but do not remember the results of using it. There are three papers on Pozzolans on the FEN home site at www.ferrocement.net
Go to the site and select resources, then online publications to access that material as it's possible there is more information there
Please do keep everyone posted on any information that you may find in your search.
On sourcing flyash: Boral is the largest source for industrial flyash. They only sell in large, bulk quantities however you may be able to track down a local source by searching Boral+Flyash+ your city and state. I've done this and with one web-search and one email I was able to get the name of a company that bagged flyash and sold it in smaller, manageable quantities.
Your local ready-mix, concrete plant uses flyash in their mixes and most likely has flyash in bulk. It's possible you can get some from them but will need to bring your own containers. One fellow I know lined his pick-up bed with heavy duty, 5 mil plastic, leaving enough extra to fold over and cover it so it didn't blow away as he was driving home.