The composition and structure of a sludge sample from ahigh-level waste glass melter were studied using optical and scanning electronmicroscopy, and x-ray diffraction. At isothermal heat treatments between 1050_Cand 1350_C, spinel crystals partly dissolved to form on cooling tiny (~ 10 mm)star-like crystals or dendrites. The shear stress in sludge was measured at aconstant shear rate (from 0.005 s-1 to 1.0 s-1) and temperature (from 1050_C to1350_C) during repeated deformation and after idling. The initial thixotropiccharacter of the loose structure of the settled sludge turned on subsequentdeformation (and idling) to rheopectic behavior. As the spinel concentration inthe sludge decreased from 28 mass% (sludge as received) to 15 mass% at 1300_C,the sludge turned into a Newtonian suspension.
