The present study is an attempt to estimate how the use of fly ash (from a local electric power plant at Lippendorf, South of Leipzig city, Saxony, Germany), hydrated lime, and lime/fly ash could improve the geotechnical properties [including consistency limits, compaction properties, unconfined compressive strength (qu), elasticity modulus (Esecant), durability, California bearing ratio (CBR), indirect tensile strength (σt), and the hydraulic conductivity (K-value)] of three different soft finegrained soils [tertiary clay, organic silt, and weathered soil] collected from Halle-city region, Saxony-Anhalt, Germany. One of the most significant objectives of the present study is to use the ultrasonic pwave velocity (Vp-value) testing as non-destructive method to evaluate the improvement of the geotechnical properties of the stabilized soils and to correlate the p-wave velocity values of the stabilized soils with the other geotechnical parameters (qu-, Esecant-, CBR-, and σt-value). In addition, the study is designed to evaluate the effect of lime-, fly ash-, and lime/fly ash-stabilization process on the microstructures and on the mineralogical composition of the three studied soils using scanning electron microscope (SEM)- and X-ray diffraction (XRD)-analysis, respectively. Furthermore, one of the objectives of this study is to estimate the heat flow of the soil-chemical additive mixtures and their hydration reactions using calorimetry-analysis. The results of the present study illustrated the following findings: The addition of lime, fly ash, and lime/fly ash to the three tested soils led to a reduction of the plasticity index and contributed to an increase in the optimum moisture content and a decrease in the maximum dry density. The addition of lime, fly ash, and lime/fly ash together to the three studied soils increased the qu-, CBR-, ót-, and the Vp-values. The increase due to the addition of lime/fly ash together is greater than the increase due to lime and fly ash separately. All the tested stabilized mixtures passed successively in the freeze-thaw durability test. Additionally, the SEM-micrograph of fly ash- and lime/fly ash-stabilized weathered soil showed rod-like crystals (ettringite) and XRDanalysis confirmed the formation of ettringite. The calorimetry-analysis illustrated that the high value of CaO-content and the presence of calcite mineral in the natural organic silt contributed to an acceleration of the hydration reaction of the optimum lime- and the lime/fly ash-organic silt mixtures.