[en] The thermal decomposition of simulated Magnox highly active waste and of HARVEST feed slurries which include the glass forming chemicals has been studied by (i) weight loss, (ii) physical appearance, (iii) bulk density, and (iv) apparent modulus or strength of the resulting products as a function of decomposition temperature. Two different feed slurries were used, one (SN) containing a precipitated silica with some sulphate impurity and the other (SG) containing a micronised silica gel. The waste on its own is almost completely calcined by about 500⁰C. A further 0.7% weight loss occurred up to 915⁰C. In addition to the waste the feed slurries contain alkali (Na and Li) boric oxide and silica. The calcination of the complete feed is virtually achieved by 500 to 550⁰C. This is at first surprising because it is known that the nitrates of Na and Li are not decomposed significantly at this temperature. It seems likely that the silica has sufficient surface reactivity to act as a weak acid liberating nitric acid. The bulk density of the intermediate solids is fairly low (0.3 to 0.8 g/cm³) until 650 to 700⁰C is reached. This temperature seems to mark the onset of fluxing and sintering for beyond it the density rises quickly to 2 g/cm³ at 700 to 800⁰C and to the full glass value of 2.6 g/cm³ by 1000⁰ C. The colour of the solids varies little until about 900⁰C when it darkens considerably. The sulphate impurity in SN causes a yellow sulphate/molybdate/chromate phase to separate above 750⁰C. The apparent modulus (or strength) of the intermediate solids decreases with temperature up to 500⁰C. It then rises as the solids begin to sinter/flux. Below 500⁰C the SN solids are the stronger, suggesting that the sulphate impurity renders this silica more reactive. (author)