Phosphate-bonded ceramics, also known as chemically bonded phosphate ceramics, form through the reaction of magnesium oxide with mono-potas­sium phosphate in water according to the following reaction:

MgO + KH2PO4 + 5H2O ^ MgKPO4 • 6H2O

The reaction product (MgKPO4^6H2O) is Ceramicrete, a rapid-setting phosphate ceramic [ 147] that contains a considerable amount of bound water. The reaction takes place at room temperature, although there is some heat generation from the reaction, to form a hard, insoluble ceramic. Some waste components react to form insoluble phosphates, and others are encapsulated in the matrix. The patented technology [148] has been licensed to treat mixed and LLW and is being used for macro-encapsulation and containerization of uranium. In the US, this low temperature waste form has also been investigated for both micro — and macro-encapsulation of radioactive and hazardous waste streams [141].

The waste treatment process includes neutralizing the waste to a pH of 5; adding sodium sulfide, tin chloride, and silver zeolite to precipitate insolu­ble compounds of Hg and Cr, Tc(Re), and I, respectively; evaporating water to reduce the volume; and adding the binder mix (MgO, KH2PO4, CaSiO3). Adding silica as wollastonite (CaSiO3) or fly ash improves the waste form performance [141] .

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