Role of hydrotalcite-type layered double hydroxides in delayed pozzolanic reactions and their bearing on mortar dating

Double-layer hydroxide minerals are part of a very interesting group of natural and synthetic compounds with trigonal or hexagonal symmetry and a flexible layered crystal structure. They are formed extremely frequently in geologic, industrial, and synthetic processes. The ease of formation is related to the possibility of accommodating divalent and trivalent cations in the structure, together with a range of anionic species. Some compounds of the group, namely those based on hydrotalcite chemistry, are invariably found as products of the pozzolanic reaction between lime and clays in ancient mortars and modern binders that serve as alternatives to Portland clinker. The present review wishes to relate the structural properties of hydrotalcitetype compounds to the crystal-chemical mechanisms taking place during long-term pozzolanic processes. The kinetics of CO3 exchange between the hydroxide and the atmosphere has important negative consequences for the radiocarbon dating of ancient mortars.