Globally, a significant portion of waste is still disposed of in sanitary landfills. Aeration is considered an effective strategy to accelerate waste biodegradation, enhancing waste stabilization and bringing end of aftercare forward in time. Waste consolidation and subsequent landfill settlement are indicators for waste biodegradation and enable monitoring of the progress of waste stabilization. Different methods, such as topographical measurements, drone-based technology, and computer simulation, can be used for this purpose. Borehole extensometers enable monitoring of depth-dependent settlement or heave in rock, soil, or other types of structures. This system is capable of detecting waste consolidation, provided that the extensometer is tightly coupled to the waste using a suitable grout mixture. In this study, the geotechnical properties of such a grout to be used for extensometers on an aerated landfill site in the Netherlands were investigated. Portland Cement CEM I and activated bentonite were used for laboratory tests. The consistency limits, unconfined compressive and bending tests were performed for samples with different recipes to find adequate grout composition.