Experimental investigation into effects of lignin on sandy loess

Promoted by the international Helin Airplane project where sandy loess is widely distributed, this study investigates the feasibility of improving the mechanical properties of sandy loess by using lignin, an environmentally friendly material. Considering the climatic conditions in the distribution area of sandy loess, such as strong evaporation and large diurnal temperature difference, extensive experiments including curing test, wet-dry cycle test, freeze–thaw cycle test, triaxial test, XRD test and SEM test are conducted to determine the strengthening effect of lignin on sandy loess. Test results show that 21 days are enough for lignin-modified sandy loess to complete curing and obtain maximum compressive strength, which increases with the increase of lignin content. Lignin has insignificant effect on the resistance of sandy loess to wet-dry cycles, but it can significantly improve the resistance to freeze–thaw cycles. Lignin causes significant alterations on the stress–strain relationship and mechanical response of sandy loess by mainly changing the cohesion while maintaining the frictional angle unchanged. SEM results show that lignin gradually plays the role of cementation, bonding, and fiber reinforcement with increasing content, and it mainly affects fine-grained soils while it has less effect on large particles. Good agreement between different experiments is obtained.