Statistical evaluation of laboratory mixing methods for improvement of clays

Soil improvement involves mechanical mixing of cementitious binders into natural soils. To reduce carbon dioxide emissions, considerable research efforts are made on supplementary cementitious materials to replace the use of cement. However, different laboratory mixing methods are used, and few studies have compared how these methods affects the strength and stiffness development. This paper presents comparative results of the wet and dry mixing methods, that is mixing a binder in slurry and in dry form, respectively. A natural soil was improved with mixtures of cement, ground-granulated blast-furnace slag, and paper sludge ash (PSA), designed and statistically analysed using a response surface methodology and hypothesis testing. Results showed insignificant differences at low final water to binder ratio (wbr). At higher wbr, the dry method with high PSA proportions yielded 2.4–4.4 times higher strengths than the wet method. The differences were attributed to varying binder dispersion, that is mixabilities and effects of low binder concentrations. The findings shows that the type of laboratory mixing method must be considered in soil improvement studies, particularly when using supplementary cementitious materials in low quantities.