Borate Substituted Ettringites
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BORATE SUBSTITUTED ETTRINGITES
LASZLO J. CSETENYI AND F. P. GLASSER Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen AB9 2UE, Scotland
ABSTRACT The setting of cement is adversely affected by soluble borates. To reduce interference, the extent to which borate can be insolubilized has been investigated. One specific mechanism of insolubilization is by inclusion into ettringite. Ettringite, Ca6Al2(SO 4 )3(OH)12-26H2O, is a normal and stable constituent of Portland cement. It has an open but non-zeolitic framework. Borate can substitute partially or fully for sulfate. Formation conditions, solubility and stability of borate ettringites, Ca6Al2(BO4)2-4(OH,O)12"26H20, are characterised using XRD, IR, DTA, and SEM. The potential durability of borate ettringites in a repository environment have been assessed by exposing it to Na-sulfate and Na-carbonate attack at different concentrations. Ion exchange occurs; back substitution of borate by sulfate is incomplete; high carbonate concentrations can, however, decompose borate ettringite. On heat treatment up to 85 0C the crystalline morphology and the OH arrangement of the structure are altered, but the X-ray powder pattern, and hence its structural framework are largely unaffected. It is concluded that ettringite has potential to reduce the solubility of borate.
INTRODUCTION The presence of soluble borates in waste streams intended for cement-based immobilization processes can lead to technical problems: borate interferes with cement set. Various conditioning processes have been proposed to mitigate the interference. However, one concern is the nature of the precipitated products obtained by reaction between cement and borate: can these products be characterized and their properties determined? One such product which acts as a host for boron is ettringite. Ettringite, Ca6A12(SO4) 3(OH) 12 .26H 2 0, has an open, low density framework. Tunnels running through the structure accommodate much of the molecular water. The framework, while physically open, is nevertheless not zeolitic. Therefore ions incorporated into the framework may be relatively rigidly held. Substitution of borate ions into the framework has been investigated. Partial substitution is known to occur in natural analogues of ettringite, e.g. charlesite, Ca 6 (A1,Si)2(SO4)2(B(OH 4 ).(OH,O)12 .26H 2 0 [1], and sturmanite in which (A1,Si) are also partially substituted by Fe(III) [2]. Oxyanion substituted ettringites (in which sulfate is replaced eg. by arsenate, borate, chromate, molybdate, selenate or vanadate) have been synthesized at pH 11.512.5, a range typical of cements [3]. In charlesite, only one third of the sulfates are replaced by borate, but it is relatively easy to prepare the sulfate-free borate analogue of ettringite Ca6A12 (B(OH) 4 )2 -4(OH,O)12 .26H2 0, collectively termed "borate ettringite". Depending on the preparation conditions and time, borate ettringite with variable boron content between two and four boron atoms per formula unit are formed. The end members are des
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