For the lime modified expansive soil

two series of tests are carried out. The first ones are water vapor adsorption tests

which are carried out using water vapor adsorption apparatus. The results indicate that with the increasing of lime percentage

the hydroscopicity of expansive soil decreased first then started to increase. And the minimum hydroscopicity is obtained at the point when the content of lime is 6%. In order to study the main reason causing the change of hydroscopicity

the second tests

i.e. the nitrogen adsorption tests

are performed on modified expansive soil. Analysis of the change of porous structure is made. It is found that with the increase of lime percentage

BET microporous specific surface area changes as the fold line; and the minimum value is obtained when the content of lime is 6%. With the increase of lime percentage

accumulating mesoporous volume decreased. With the increase of lime percentage

mesoporous diameter increases first

then decreases

and the maximum value is obtained when the content of lime is 6%. That the hydroscopicity of expansive soils is inversely proportional to the mesoporous diameter is summarized; and the optimal lime percentage is 6%.