Study On Mechanical Properties And Mechanism Of Expansive Soil Improved By Steel Slag Powder And Cement Under Dry-wet Cycle

Expansive soil,as an unsaturated cohesive soil,is a harmful geological soil due to its characteristics of water absorption expansion and drying shrinkage.Especially in the environment of alternating wet and dry climate,it can cause engineering accidents because of its deformation of wet expansion and drying shrinkage.Chemical improvement method is a common treatment method.It can improve the interaction between soil and water by adding additives and concurrent biophysical and chemical reactions,so as to improve the engineering properties of expansive soil.In this paper,steel slag powder(SSP)is used as a new solidifying agent and combined with cement to improve expansive soil.The physical and mechanical properties and mechanism of the improved expansive soil under dry-wet cycling conditions are mainly studied.The improvement effect is verified by a variety of test indexes,which provides a basis for exploiting the utilization ways of SSP in engineering.Through laboratory tests,the regularity of physical and mechanical properties of pure soil(Es),cement soil(Es-C),SSP-Cement soil(Es-SSP-C)and NaOH-SSP-Cement soil(Es-SSP-C-N)at different curing ages and different cycles of drying and wetting has been studied.The results show that,in terms of mechanical properties,the strength of the three improved soils increases with age,and the all four kinds of soils have different degrees of strength loss under wet-dry cycling.But the strength values always show as Es-SSP-C-N>Es-C>Es-SSP-C>Es,which means that Es-SSP-C-N is better than the other two schemes in the improvement effect.In terms of deformation characteristics,the soil-water interaction is well controlled by the improved soil.The swelling and contraction deformation in the dry-wet cycle is more than 90%less than that of the pure soil.With the increase of the number of dry-wet cycles,the deformation of the pure soil samples increase gradually,while the improved soil sample can maintain a relatively stable state in the first 11 cycles,and the amount of deformation increases after 12 cycles.Mierostructure and phase change of four soils at different ages and cycles of drying and wetting have been analyzed by means of scanning electron microscopy(SEM)and energy dispersive spectrometer(EDS).The mechanism of soil improvement and drying-wetting cycle has been explained.The results show that,in the presence of cementitious materials,the voids in the improved soil decrease and the arrangement of particles becomes more compact.Clay particles and unhydrated materials are closely bonded together by hydration products such as C-S-H and C-A-H.With the increase of curing age,hydration products gradually increase,and the integrity gradually improves.In the dry-wet cycle,the cracks of pure soil samples increase continuously,and the arrangement of particles becomes loose gradually,which affects its strength and other characteristics.Under the dual action of dry-wet cycle erosion and hydration reaction supplement,the cementing substances increase with the increase of the number of cycles in the early stage,and the framework is composed of ettringite(AFt)and other substances,so the integrity tends to increase.In the later stage,Ca(OH)2 and other hydration products around the crack dissolve and recrystallize,which weakens the bonding force between the aggregates and leads to the decrease of the strength of the specimen.