I have done several soil/sludge stabilization projects and could provide you with more information if needed (please provide more detail).
This sound similar to a project where we stabilized some contaminated steam sludge (hazardous) so that we could place it in a RCRA lined landfill. I've also done some paper mill sludge, residual soft soil road base and hazardous waste stabilization. I've used several additives (kiln slag, portland cement, fly ash and lime). As with most cases, cost and the proper additive is generally determined be the availability of the additive.
Point to note!!! In almost all of my applications, we were only trying to achieve minimal soil properties so that the material could be handled and/or self-supporting. If you are trying to achieve usable geotechnical properties, you are probably not choosing the most cost effective solution (ie. soil/sludge stabilization is expensive).
Respond if you care for more information.
Good luck, these projects tend to be messy.