Phase Formation and Evolution in Mg(OH)₂–Zeolite Cements

The mineralogy and structure of cements in the system Mg(OH)₂–NaAlO₂–SiO₂–H₂O are investigated, with a view toward potential application in the immobilization of Mg(OH)₂-rich Magnox sludges resulting from historic United Kingdom nuclear operations. The reaction process leading to the formation of these aluminosilicate binders is strongly exothermic, initially forming zeolite NaA (LTA structure), which is metastable in low SiO₂/Al₂O₃ binders, slowly evolving into the more stable sodalite and faujasite framework types. Notable chemical reaction of Mg(OH)₂ was only identified in the formulation with SiO₂/Al₂O₃ = 1.3 (the lowest molar ratio among those tested) after extended curing times. In this case, some of the Mg(OH)₂ reacted to form an Mg–Al–OH layered double hydroxide. These results demonstrate that encapsulation of Magnox sludge waste streams could be carried out in these alternative binders but that the binders would encapsulate rather than chemically incorporate the Mg(OH)₂ into the wasteform unless low SiO₂/Al₂O₃ ratios are used.