Abstract：The experimental investigation was carried out with the purpose of making a contrast of strength effects of two clay soil species stabilized by Portland cement(PC)，portland cement-gypsum(PC+G) and compound modifier(PC+G+Al)，which consist of Portland cement，gypsum and alumina expansive agent. The result illustrates that the strength effect of stabilized soft soils differ strikingly due to their respective addition. PC+G+Al modifier in both stabilized soil samples produces more significant improvement in strength development than the other two modifiers. The PC+G modifier produces positive effect on strength，compared with PC modifier，only in one stabilized soil sample，which is in high water content ratio and porosity and contains active soil mineralogical composition. Both PC+G+Al and PC+G modifiers，in both soft soil samples，form ettringite in their hydrates. The formation of ettringite provides pronounced solid-phase expansion to fill up the porosity of soft soils. Whether the formation of ettringite enhances the strength of stabilized soil depends on the concordance in formation of ettringite and calcium silicate hydrate(CSH) gel. The strength of the stabilized soil samples increases markedly when the formation of ettringite is prior to the formation of CSH gel structure，which benefits the pore filling as well as mitigates the expansion damage to the CSH gel structure.
