The role of sulfate groups in controlling CaCO3 polymorphism

Abstract

The nucleation and growth of CaCO 3 phases from aqueous solutions with SO 4 2À :CO 3 2À ratios from 0 to 1.62 and a pH of $10.9 were studied experimentally in batch reactors at 25°C. The mineralogy, morphology and composition of the precipitates were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and microanalyses. The solids recovered after short reaction times (5 min to 1 h) consisted of a mixture of calcite and vaterite, with a S content that linearly correlates with the SO 4 2À :CO 3 2À ratio in the aqueous solution. The solvent-mediated transformation of vaterite to calcite subsequently occurred. After 24 h of equilibration, calcite was the only phase present in the precipitate for aqueous solutions with SO 4 2À:CO 3 2À 6 1. For SO 4 2À :CO 3 2À > 1, vaterite persisted as a major phase for a longer time (>250 h for SO 4 2À :CO 3 2À = 1.62). To study the role of sulfate in stabilizing vaterite, we performed a molecular simulation of the substitution of sulfate for carbonate groups into the crystal structure of vaterite, aragonite and calcite. The results obtained show that the incorporation of small amounts (<3 mole%) of sulfate is energetically favorable in the vaterite structure, unfavorable in calcite and very unfavorable in aragonite. The computer modeling provided thermodynamic information, which, combined with kinetic arguments, allowed us to put forward a plausible explanation for the observed crystallization behavior.

Characterization of vaterite synthesized at various temperatures and stirring velocities without use of additives

“…Traditionally, the XRPD patterns of vaterite have been indexed using some of the existing structural models, although some Bragg reflection peaks could not be modeled [13,45]. That is the case of the broad peaks observed at 38.5 and 85°in Fig.…”

Vaterite Stability in the Presence of Chromate

“…A similar evolution of the solubility of sulfate-bearing vaterite has been proposed to explain the stabilization of vaterite with respect to calcite precipitated from aqueous solutions with increasing concentrations of sulfate anions. [4] The preservation of the original shape of the vaterite aggregate after its transformation into calcite implies that this transformation has the characteristics of a mineral replacement process. This process is characterized by an interface coupling between vaterite dissolution and calcite precipitation.…”

“…[3] In such a case, the presence of certain ions in the growth medium can determine the stabilization of unexpected polymorphs since their incorporation into the crystal structure of these polymorphs is energetically more favorable. [4] A recent experimental study has shown that high concentrations of Cr(VI) promote the formation of vaterite in a porous medium. [5] OBJECTIVE In this work, Raman spectroscopy analyses of crystal aggregates grown in a silica gel bearing a range of Cr(VI) concentrations were conducted.…”

Characterization of alkali activated geopolymer mortar doped with MWCNT

“…It is likely that a significant proportion of the Na + is consumed in the activation of slag mortar constituents to form a sodium aluminosilicate-type gel, reducing the availability of Na + species in the pore solution [53]and therefore reducing the alkalinity from the high levels which would favor zeolite growth, even with the higher alkali dosage added to these samples, also the increased intensity of N-C-S-H results in the decrease in the available balancing cations for geopolymer structure and so favour the Faujasite formation. It can also be noticed a slight increase in calcite content with time which may be probably resulting from reactions involving atmospheric CO 2 [54].…”