The role of the fly ash pozzolanic activity in simulated sulphate radioactive liquid waste

Effects of the very concentrated sulphated salts that accompanied the low (LLW) and medium level wastes (MLW), on the cementitious characteristics of materials employed in the Spanish disposal are discussed. The study was carried out according to the Köch-Steinegger test, at the temperature of 40°C during a period of 365 days. Samples of a specific backfilling fly ash-pozzolanic-cement mortar, were immersed in a simulated radioactive liquid waste very rich in sulphate (2.05 M) and sodium (4.1 M) ions. In addition to the mechanical behavior, the changes of the microstructure characteristics of the mortar were evaluated by X-ray diffraction (XRD), mercury intrusion porosimetry (MIP) and extracted pore solution analyses. The results showed the important role played by the fly ash pozzolanic reactivity which is activated markedly by the ingress of sulphate and sodium ions towards the porous microstructure. Accordingly, the porosity is strongly refined and the mechanical properties enhanced.