Cement compatible materials with potential application in the immobilisation of toxic wastes
Cements and cement-based systems have excellent potential as immobilising matrices for toxic inorganic wastes. This potential is a result of the unique chemical and physical mechanisms which operate on particular waste species to prevent their release into ground waters. These mechanisms are reviewed in this thesis. Some wastes are suitable for direct cementing, such as waste streams form nuclear fuel reprocessing, where cement technology is applied. Other wastes are less well characterised and may be unsuitable for direct cementing. This often arises because waste species concentrations are higher than maximum permissable levels, but are still too dilute for economical cementing. In such cases, a preconcentration step prior to cementing is the favoured strategy. Arsenic and lead wastes present an additional challenge because they are expected to exist as soluble oxyanions under alkaline pore fluid conditions. Both arsenic and lead have also been shown to affect setting characteristics by interfering with normal cement hydration reactions. The materials studied in this thesis were chosen because of their known, or suspected, potential as anion exchangers. It may be possible to preconcentrate arsenic or lead in the anion exchanging solid before incorporation into cement. A further advantage of this method is that an additional barrier against re-release of the toxic species is created. The aim of this project was to determine the uptake capacity of different ion exchanging materials which are compatible with cement systems for hazardous oxyanion species. Three types of material were studied, hydrotalcite (a clay mineral), thaumastic (a phase which is isostructural to ettringite) and phosphate-modified zeolites (a new type of material derived from zeolites). The first two are known to occur in cements, the last is rather an unknown quantity, but many zeolites have been shown to be compatible with cement environments.