Hydroceramics are another concrete-type material that is similar to zeoli — tized rock. It is made by curing a mixture of inorganic waste, calcined clay, vermiculite, and Na2S, NaOH with water under hydrothermal conditions (60-200°C) to form a matrix containing crystalline zeolites embedded in a sodium aluminosilicate matrix [143] . The solidification process occurs as a result of hydration reactions. The NaOH solution dissolves the metakaolin (Al2O3^2SiO2) much the same as in geopolymers, but abundant water or hydroxides provide the water to create crystalline silicates instead of an amorphous matrix. The hydroceramic process takes advantage of the sodal — ite and cancrinite structures in immobilizing oxyanion salts such as nitrate, nitrite, chloride, fluoride, and iodide within the physical cage-like structures of the crystals created.

Hydroceramic waste forms have been shown to be effective on low — activity sodium-bearing waste. The technology is still under investigation with studies focused on optimization of waste pre-treatment (calcination), waste stream-specific optimization of the formulations, and a study of scale — up factors to ensure viability for full-scale operation [143] . In cases where the waste has a high nitrate-nitrite composition, the waste must first be denitrified in some manner, such as calcination, to remove the nitrates and nitrites from the waste. If sodium nitrate-based waste is pre-treated with metakaolin, sucrose, and then calcined, it can be used to make a hydrocer­amic waste form 2 143] . Successful waste forms have been achieved with waste loadings of 40-60 wt% waste [144, 145] . Hydroceramic waste forms have been made with Idaho National Laboratory’s HLW calcine [146].

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