Trials to solidify a Zn-containing low level radioactive sludge with calcium sulfo-aluminate cement

It is well-known that Zn2+ acts as a strong retarder or even an inhibitor for the setting and hardening reaction of Portland cement. Calcium sulfo-aluminate (CSA) cement was expected to be the best solution to solidify the Zn-containing sludge in this study, because it generates an Ettringite-phase during hydration in which the Zn2+ can be integrated. With a commercially available CSA cement it was possible to develop recipes with a sufficient percentage of wet waste volume and a passable range of consistency for all existent water contents. The compressive strength of these recipes varied between 20 and 40 MPa depending on how much waste was in the recipe and how much water the waste contained. With water and sulfate resistance conforming to the guideline B05, the compressive strength was above the required 10 MPa in all cases. Additionally the linear expansion incl. layers was always clearly lower than the limiting 2%. The disadvantage of the recipes was superficial disintegration of the samples during leaching. One effect is that one can easily scrape off material from the surface of the samples, another is that flakes of material from the leached samples can be found on the bottom of leach bottles. For these reasons the development of a recipe for the solidification of the Zn-sludge is still a work in progress. (author)