Addressing a critical challenge in the nuclear industry, this study contributes to the effective and sustainable management of radioactive liquid organic waste by studying the effect of the viscosity ratio between the geopolymer grouts and the incorporated organic liquids (OL) on the efficiency of the solidification process. This research provides a practical pathway for industries to manage their liquid organic wastes more effectively, economically, and in alignment with regulatory standards. Three mineral oils are used to simulate OL wastes with an incorporation rate fixed at 30% vol. which represents a considerable proportion from an industrial point of view. To ensure adequate conditioning, the focus of this work is on the effect of the viscosity difference between geopolymer grouts and incorporated OL on the rheological, mechanical, and microstructural properties of geopolymer/OL composites. In total, 9 viscosity ratios, ranging from 0.4 to 430, are tested by varying the viscosity of both the geopolymer grouts and the incorporated OL. First, the results obtained show that OLs with a viscosity higher than 0.05 Pa.s can be efficiently encapsulated in MK-based geopolymer grouts, without requiring any surfactants. This study also confirms that the viscosity ratio is a key factor controlling the formulation and the rheological characteristics of fresh geopolymer/OL emulsions. Accordingly, increasing the geopolymer grout viscosity is an efficient technique for the preparation of geopolymer/OL composites with finely encapsulated OL droplets. In addition, based on the experimental results obtained in this study, a good correlation is established between the rheological parameters of the emulsions and the viscosity of the two phases (geopolymer and OL). This enables the proposition of empirical models that allow estimating the final rheological parameters of the fresh emulsions with an accuracy exceeding 90%. Finally, in terms of mechanical properties, this study confirms that MK-based geopolymers can safely be used for the solidification of a high amount of OL wastes.