"Yeah, and umm.... he's the only one who has been allowed to examine it, if I recall... and the only one who knows the "secret ingredients" that he uses as proof that they are the same. He's hidden his real data. "
-- His ingredients are listed... but he does not list proportions.
1. Kaolin Clay is the MYSTERY POZZOLAN added. It may even be cooked (Kaolinite).
2. His experirement uses "French Limestone".
3. He also uses Natron.
4. Davidovits adds an unknown quantity of some mysterious ash. This is to make the blocks acid resistant. As discussed, Ash should be around from the "discovered Kilns" and Gypsum production. This ingrediant is not inherently necessary for creating Roman cement, though.

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[www.romanconcrete.com]
"There is meager information on the chemistry of pozzolan. It can be processed from different sources such as fly ash, rice hulls, and clay to name a few."
...
"the Romans pounded tile mixed with lime to make a mortar for constructing floors."
...
"the ancients used a powdered burnt clay/tile with lime in their concrete process"
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[www.romanconcrete.com]
"In his book Rhodes provides a scholarly discussion on the practice of baking pottery. Broken pottery was used to make Roman concrete according to ancient writers. "
...
"It turns out that the temperature for baking clay pots ( used in a broken state to make concrete in Rome) is in the same range as the temperature used to drive the gases out of limestone in making lime - so the same kiln can be used in both cases."