Selection of suitable surfactants for the incorporation of organic liquids into fresh geopolymer pastes

Abstract

• Torque monitoring during emulsification of OL into fresh GP allows to have one-line viscosity. • Torque evolution allows to predict if OL will be successfully incorporated in GP or not. • Quaternary ammonium salts such as CTAB are well adapted to adjust the viscosity of fresh GEOIL. • Capillary forces between OL and MK were shown using Zêta potential measurements and Washburn tests. Geopolymers (GP) have recently emerged as suitable binders for the conditioning of radioactive organic liquids (OL). The condition for suitably incorporating high amounts of OL into GP is assessed in this research. A series of OL has been selected, comprising alkanes (dodecane and 2 paraffinic mineral oils), an industrial gear oil and tributylphosphate (TBP), in order to cover a wide range of viscosities (0.0018–2.33 Pa.s). The incorporation of viscous OL (>0.2 Pa.s) into fresh GP results in a regular torque increase, indicating a suitable process with the dispersion of the OL in the form of fine micrometric droplets. When the torque remains stable, for OL of low viscosity (<0.2 Pa.s) like TBP/Dodecane mixtures, the incorporation is not possible, or only partial in the form of large droplets quickly coalescing and leading to an organic supernatant above the hardened GP. For low viscosity OL, surfactants are required. The use of quaternary ammoniums (QAs) like cetyltrimethylammonium bromide (CTAB) has been studied. Torque monitoring shows that the OL is incorporated similarly in the GP slurry, whether the surfactant is introduced at the beginning, through the course or at the end of the mixing process. The torque evolution also provides the optimum surfactant concentration, by evidencing that beyond a certain concentration, the medium hardens too quickly, preventing the incorporation of OL. Finally, torque monitoring highlights that QAs with shorter alkyl chains, such as hexamethyltrimethylammonium, are able to replace CTAB with the same efficiency. Therefore, the powerful action of CTAB on the incorporation of alkanes into GP is attributed to its positive charge rather than to its surfactant effect.