Characterisation of ASR gel: Topical report

This report presents in detail the results of the characterisation of Alkali-Silica gels. One inactive surrogate gel and three active gels sampled from cemented waste packages were characterised by XRD and TGA-MS analyses. XRD analysis confirmed that the ASR gels were mainly amorphous, as a broad peak between 15 and 40° (2) characteristic of amorphous silica was observed. For some samples diffraction peaks that might be attributed to calcite were also detected. A total mass loss in the range of -29 and -36 wt% was measured by TGA-MS analysis. This decrease in mass was mainly due to H2O and O2 release and occurred between 40-170 °C or 40-300 °C. However, it should be noted that an important mass decrease occurred during the purging step, with a higher mass decrease for the active gel D95-0045 and a lower one for the active gel D95-079. C- and N-containing volatiles were detected but quantities were too low to be quantified. Other techniques should be applied to determine the quantities of such compounds. In the future, 29Si NMR will be conducted to know the silica connectivity of the selected ASR gels thanks to chemical shifts, which can be related to different local Si environments (Qn).