Mixture of dredged soil and steel slag (hereinafter referred to as "MDS") has been studied as one of the applications of dredged soil for geomaterial. Recent studies revealed that MDS had environmental safety and purifying effect for bottom sediment if it was employed as geomaterial in marine area. Mechanical properties of MDS have been studied; nevertheless the dominant factors affecting strength of MDS have not yet been clarified. It happened that the in-situ strength of MDS was smaller than the strength of MDS made at site for quality control with same materials.
　In this study, several series of tests were conducted to evaluate the relationship between mixing condition and strength of MDS and to identify the dominant factor affecting the strength. Test results showed that dredged soil was solidified as far as about 1cm from the surface of a steel slag particle. Moreover, it was found that the chemical equilibration related with the elution of silica from dredged soil and calcium from steel slag was one of the factor affecting the strength of MDS. This result implied that the optimum mixing ratio between dredged soil and steel slag would be exist. The optimum mixing ratio was detected by the results of unconfined compression tests of MDS with various mixing ratio organized by dry mass ratio of dredged soil and steel slag. It was also found that the strength of MDS was significantly affected by the initial water content of MDS, which was the water content measured immediately after mixing. MDS specimens were observed by X-ray CT not only before, but also during and after the unconfined compression tests. Based on the observations, the failure process of MDS were discussed and marshalled.