<HTML>Anthony
Large amounts of salt destroy concrete, in time the concrete formation becomes soft and fragments.
I do not believe that shells would behave in the same manner as an aggregate additive to the concrete mix, the shells are brittle and would no give added strength, also they would not act in the same manner as a steel fabric reinforcing.
you asked
>How important is the pressure to the formation of limestone, and what part of Davidovits' recipe would take the place of the pressure?<
My simple answer is, limestone will very likely form without pressure, but the result would be a very soft weak form of limestone. Pressure creates density and density results in inherant strength.
As to what part of Davidovits recipe would replace the pressure aspect, I'm not sure the ancient people would have imediate access to anything that could.
Some might argue baking the blocks(sunbaked). The problem with that is it results in a false cure, this results in rapid shrinkage of the compound(block) and this creates areas of weakness and fractures within the structure.
The molecular bonding that takes place in a lime/cement based substance is not the same as sunbaked mud bricks. If you apply excessive amounts of external heat to a lime/cement substance, you create this false cure, which actually tears appart the molecular bond. Due to the rapid evaporation of the contained water, which results in this rapid shrinkage.</HTML>
