Kinetics and Mechanism of Calcium Hydroxide Conversion into Calcium Alkoxides: Implications in Heritage Conservation...

Nanolimes are alcohol dispersions of Ca(OH)2 nanoparticles used in the conservation of cultural heritage. Although it was believed that Ca(OH)2 particles were inert when dispersed in short-chain alcohols, it has been recently shown that they can undergo transformation into calcium alkoxides. Little is known, however, about the mechanism and kinetics of such a phase transformation as well as its effect on the performance of nanolimes. Here we show that Ca(OH)2 particles formed after lime slaking react with ethanol and isopropanol and partially transform (fractional conversion, α up to 0.08) into calcium ethoxide and isopropoxide, respectively. The transformation shows Arrhenius behavior, with apparent activation energy Ea of 29 ± 4 and 37 ± 6 kJ mol(-1) for Ca-ethoxide and Ca-isopropoxide conversion, respectively. High resolution transmission electron microscopy analyses of reactant and product phases show that the alkoxides replace the crystalline structure of Ca(OH)2 along specific [hkl] directions, preserving the external hexagonal (platelike) morphology of the parent phase. Textural and kinetic results reveal that this pseudomorphic replacement involves a 3D diffusion-controlled deceleratory advancement of the reaction front. The results are consistent with an interface-coupled dissolution-precipitation replacement mechanism. Analysis of the carbonation of Ca(OH)2 particles with different degree of conversion into Ca-ethoxide (α up to 0.08) and Ca-isopropoxide (α up to 0.04) exposed to air (20 °C, 80% relative humidity) reveals that Ca-alkoxides significantly reduce the rate of transformation into cementing CaCO3 and induce the formation of metastable vaterite, as opposed to stable calcite which forms in untransformed Ca(OH)2 samples. Similar effects are obtained when a commercial nanolime partially transformed into Ca-ethoxide is subjected to carbonation. Such effects may hamper/delay the strengthening or consolidation effects of nanolimes, thus having important implications in the conservation of cultural heritage.