This study aims at evidencing the effects of lime treatment on the microstructure and hydraulic conductivity of a compacted expansive clay,with emphasis put on the effect of lime hydration and modification.For this purpose,evolutions of hydraulic conductivity were investigated for both lime-treated and untreated soil specimens over 7 d after full saturation of the specimens and their microstructures were observed at the end.Note that for the treated specimen,dry clay powder was mixed with quicklime prior to compaction in order to study the effect of lime hydration.It is observed that lime hydration and modification did not affect the intra-aggregate pores but increased the inter-aggregates pores size.This increase gave rise to an increase of hydraulic conductivity.More precisely,the hydraulic conductivity of lime-treated specimen increased progressively during the first 3 d of modification phase and stabilised during the next 4 d which correspond to a short period prior to the stabilisation phase.The microstructure observation showed that stabilisation reactions took place after 7 d.Under the effect of stabilisation,a decreasing hydraulic conductivity can be expected in longer time due to the formation of cementitious compounds. This study aims at evidencing the effects of lime treatment on the microstructure and hydraulic conductivity of a compacted expansive clay, with emphasis put on the effect of lime hydration and modification. For this purpose, evolutions of hydraulic conductivity were investigated for both lime-treated and untreated soil specimens over 7 d after full saturation of the specimens and their microstructures were observed at the end. Note that for the treated specimen, dry clay powder was mixed with quicklime prior to compaction in order to study the effect of lime hydration. It is is observed that lime hydration and modification did not affect the intra-aggregate pores but increased the inter-aggregates pores size.This increase gave rise to an increase of hydraulic conductivity .More precisely, the hydraulic conductivity of lime-treated specimen increased progressively during the first 3 d of modification phase and stabilized during the next 4 d which corresponds to a short period prior to the stabilis ation phase. microstructure observation showed that stabilizing reactions taking place after 7 d. Under the effect of stabilisation, a decreasing hydraulic conductivity can be expected in longer time due to the formation of cementitious compounds.