Structural Changes upon Dehydration of Amorphous Calcium Carbonate

Abstract

Amorphous calcium carbonate (ACC) is a common transient precursor to biogenic crystalline calcium carbonate, but factors controlling the amorphous to crystalline transformation remain unclear. We present a structural analysis and comparison of hydrated and partially dehydrated, synthetic ACC samples. Thermogravimetric analysis showed total H 2 O losses of 46% with heating to 115 °C and 75% for heating to 150 °C. The 1 H NMR spectra of hydrous ACC, obtained both directly and indirectly, via 13 C-detection, contain signals from four principal hydrogen environments: translationally rigid structural H 2 O, a restrictedly mobile H 2 O environment, fluidlike mobile H 2 O that is decoupled from rigid H and C, and hydroxyl. The retention of some restrictedly mobile H 2 O and lack of change in X-ray total scattering and absorption spectroscopy data for dehydrated ACC suggest that thermal dehydration does not significantly disrupt the calcium-rich ACC framework. NMR results and thermal analyses of samples dehydrated isothermally for extended periods indicate that the H 2 O loss mechanism is kinetically hindered and occurs in three stages: simultaneous loss of fluidlike mobile, restrictedly mobile, and rigid H 2 O → loss of restrictedly mobile and rigid H 2 O → loss of hydroxyl and trapped rigid and mobile components that cannot be removed without transformation to crystalline calcium carbonate.

Direct Experimental Measurement of Water Interaction Energetics in Amorphous Carbonates MCO3 (M = Ca, Mn, and Mg) and Implications for Carbonate Crystal Growth

“…Recent thermal and structural studies have characterized these three steps in ACC dehydration as loss of fluidlike mobile → restrictedly mobile and rigid H 2 O components → final loss of hydroxyl and trapped rigid and mobile water. 44,46 Further, these studies suggested that the fluidlike mobile water behaves more like physically adsorbed water and is lost in early stages of dehydration, while restrictedly mobile and rigid H 2 O components are lost at intermediate temperatures. The loss of the final fraction of hydroxyl/trapped water above 300°C is found to be crucial as it triggers the crystallization.…”

“…The loss of the final fraction of hydroxyl/trapped water above 300°C is found to be crucial as it triggers the crystallization. 44,46,49 Similarly, Ihli et al 44 have identified four stages in the mechanism of dehydration of ACC leading to crystallization as stage-I: loss of surface-bound water at ∼40°C, stage-II: loss of water from the interior of the ACC with shrinkage of ACC particles around 140°C, stage-III: expulsion of the most deeply located water around 290°C, and stage-IV: crystallization to calcite at 315°C. In this work, to avoid any structural changes upon heating or crystallization, the first set of water adsorption calorimetric experiments was carried out on amorphous samples degassed 6−8 h under vacuum at 25°C.…”

“…6,44,46,49 The water adsorption enthalpy profiles of ACC in Figure 3a shows the existence of two different energetic regions for both 25 and 100°C degassed samples. The first region corresponding to the adsorption enthalpies for the initial doses of water (coverage up to 3 H 2 O/nm 2 ) is more exothermic and shows a sharp decrease in magnitude from −100 to −60 kJ/mol.…”

Using simulation to understand the structure and properties of hydrated amorphous calcium carbonate

“…However, although our model ACC Ik shows the most segregation of water within the structure, it does not give the slowest diffusivity for the ions. While the more mobile water will be more likely to leave the system during dehydration, full dehydration is likely to require further structural changes 26 .…”

“…Proton solid-state nuclear magnetic resonance showed that two different types of water are present within the structure. One is largely immobile and labelled as structural, but the other showed an increased mobility 22,25,26 . The location of this water appears, again, to be dependent on the sample origin.…”