Geopolymeric solidification is an effective process for treating engineering waste slurry, yet its efficacy varies with source of slurry. This study examined four shield tunneling spoil types for geopolymeric solidification. The unconfined compressive strength of the best sample was 17–126% higher than the least effective one. A significant negative correlation between strength and bentonite indices was found. Subsequently, the addition of bentonite to the shield tunneling spoil for experiments, followed by various analyses, comfirmed bentonite's negative impact on strength. Bentonite forms agglomerates during geopolymerization, adsorbing hydration gel, weakening the direct linkage of the hydration gel; they also adsorb elements such as Al, Ca, and Na, hindering the formation of geopolymer products. However, the final adsorption ratio was not within the optimal range for geopolymer reactions, impeding the improvement in curing strength. This study offers the theoretical guidance for improving engineering waste slurry treatment processes.