Abstract
The diammonium hydrogenphosphate (DAP, (NH4)2HPO4) reaction with calcite has been extensively investigated. The availability of free calcium ions in the reaction environment has been acknowledged as a crucial factor in the crystallization of calcium phosphates with a high (hydroxyapatite, Ca/P 1.67) or low Ca/P molar ratio (dicalcium phosphate dihydrate, Ca/P 1.00; octacalcium phosphate, Ca/P 1.33). On the contrary, no data are available on the DAP interaction at room temperature with dolomite in terms of reaction mechanism and composition of the reaction products. Here, a multi-analytical approach based on scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectrometry (EDS) and X-ray powder diffraction before and after heating treatments is proposed to explore how the formation of calcium phosphates occur on Mg-enriched substrates and if the presence of magnesium ions during the reaction influences the crystallization process of calcium phosphates. The DAP reaction with polycrystalline dolomite gives rise to the formation of struvite and of poorly crystalline hydroxyapatite. Calcium and magnesium ions mutually interfered in the crystallization of magnesium and calcium phosphates, respectively, whose effects influenced the properties (size, micro-morphology, composition and crystallinity) of the newly-formed phases.
Highlights
Diammonium hydrogenphosphate (DAP, (NH4 )2 HPO4 ) is a promising inorganic consolidating treatment for decayed carbonatic stones [1,2,3,4]
The DAP reaction with calcite (CaCO3 ) of carbonatic stones has been extensively investigated over the last years, [7,8] and recent findings showed that the DAP reaction with polycrystalline calcite of carbonatic stones is non-stoichiometric [9], which implies that many crystalline phases may form in complex mixtures with a nanocrystalline and/or partially-substituted HAP [1,5,6,9,10,11,12]
The micro-morphology of this coating, which resembles almost the features of polymeric products used for the conservation, is quite different from the crystalline shell observed on calcite-based lithotypes [9,10], even where the reaction occurred in presence of
Summary
Diammonium hydrogenphosphate (DAP, (NH4 ) HPO4 ) is a promising inorganic consolidating treatment for decayed carbonatic stones [1,2,3,4]. The DAP consolidating treatment aims at restoring the stone matrix through a partial transformation of the original material in newly-formed phosphate phases. Crystalline hydroxyapatite (HAP, Ca5 (PO4 ) OH) is formed by combining calcium ions of the substrate with phosphate groups of the reagent [5,6]. The DAP reaction with calcite (CaCO3 ) of carbonatic stones has been extensively investigated over the last years, [7,8] and recent findings showed that the DAP reaction with polycrystalline calcite of carbonatic stones is non-stoichiometric [9], which implies that many crystalline phases may form in complex mixtures with a nanocrystalline and/or partially-substituted HAP [1,5,6,9,10,11,12]. It has been demonstrated that the composition of the new phases depends on several variables, as e.g., the reaction condition (pH, ion strength), the stone substrate (lithotype, microstructure) and the treatment protocol (duration, application method, DAP molarity) [10,13,14,15,16].
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