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

Abstract

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 CO 3 exchange between the hydroxide and the atmosphere has important negative consequences for the radiocarbon dating of ancient mortars.

The Cannero Castle (Italy): Development of Radiocarbon Dating Methodologies in the Framework of the Layered Double Hydroxide Mortars

“…Anions such as CO 3 2stabilize the positive charge and recent studies demonstrated that carbonate anions within the interlayers of LDHs undergo dynamic exchanges with atmospheric CO 2 even under ambient conditions. This high capability in capturing large anions is a characteristic of the structure and chemical properties of LDHs minerals (Ishihara et al 2013;Artioli et al 2017;Mishra et al 2018;Ponce-Antón et al 2018). As reported in literature, LDH phases have been observed as products of pozzolanic reaction between lime and clays in ancient mortars, in modern pozzolanic cements, as well as in dolomitic lime mortars (Brandon et al 2014;Artioli et al 2017;Ponce-Antón et al 2018;Secco et al 2018).…”

“…Mg-carbonates, main constituents of ancient magnesian mortars, decompose endothermically releasing water and CO 2 over a temperature range approximately between 220 and 550°C (Hollingbery and Hull 2010;Bhattacharjya et al 2012). The transitions involved during LDH thermal decomposition have been widely investigated (Stanimirova et al 1999;Roelofs et al 2002;Pérez-Ramírez and Abelló 2006;Bhattacharjya et al 2012; The Cannero Castle: Double Hydroxide Mortars 621 Artioli et al 2017), and, in general, a three-step mass loss behavior has been defined: dehydration (25-280°C), dehydroxylation (280-400°C) and anion expulsion (>400°C), leading to the progressive collapse of the double-layered structure. Furthermore, considering other LDH phases, the layered structure of hydrocalumite collapses when heated above 250°C, turns into an amorphous phase at 300°C, and it transforms into calcium oxide (CaO) and mayenite (Ca 12 Al 14 O 33 ) at 600-700°C (Vieille et al 2003).…”

“…Moreover, the situation is further complicated if Mg-rich phases are present, either derived from the aerial reaction of lime binders obtained from the calcination of dolomitic carbonate compounds, or after pozzolanic reaction processes between the lime binder and reactive silicates. The different kinetics of carbonation between the Ca-phases (lime, portlandite) and the Mg-phases (periclase, brucite) induce different setting times, also enhancing the delayed formation of carbonate-containing double layered hydroxides (LDH) of the hydrotalcitetype, with severe effects on the radiocarbon dating of the structures (Artioli et al 2017;Ponce-Antón et al 2018).…”

Selecting the Most Reliable 14C Dating Material Inside Mortars: the Origin of the Padua Cathedral

“…In addition, the mineralogical analysis can help to recognize the presence of hydrotalcite—a mineral of the family of double-layered mixed-metal hydroxides (LDH or AFm)—that is a dangerous and poorly known contaminant related to hydraulic lime mortars (Secco et al 2016). In fact, LDH phases possess exchange capabilities for a variety of organic and inorganic anions including carbon dioxide (Artioli et al 2017), so that they could modify the original atmospheric CO 2 signal of the mortar. The 14 C dating performed on LDH-containing mortars often results in younger age determinations.…”

Hydrotalcite and Hydrocalumite in Mortar Binders from the Medieval Castle of Portilla (Álava, North Spain): Accurate Mineralogical Control to Achieve More Reliable Chronological Ages

“…LDH phases have been observed in ancient hydraulic mortars, in modern pozzolanic cements, and in dolomitic lime mortars [57][58][59][60]. Nevertheless, until the current study, neither the shape nor the size distribution of hydrotalcite and hydrocalumite in the lime mortars has ever been described in detail.…”