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(SO4)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)12·26H2O, 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°C 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.