To provide an overall functional evaluation of buffer materials, this study attempted to investigate the relationships among the engineering properties, plastic index (PI), compaction efficiency, sorption properties, and distribution ratio (Rd) for some buffer materials composed of quartz sand and bentonite. Th and U were nuclides of interest, and both synthetic groundwater (GW) and seawater (SW) were used for batch sorption experiments, while the deionized water (DIW) was used for engineering property tests. SW and GW were also used to evaluate the effects on PI. The results show that the maximum dry density was reached when bentonite content was 30% with the same compaction energy by the ASTM D698 method. PI and bentonite content of tested buffer materials consisting of bentonite and quartz sand demonstrated a linearly proportional relationship regardless of the solution used. The following sequence of PIDIW > PIGW > PISW is due to coagulation and flocculation effects. The buffer materials of lower PI value could decrease swelling potential and increase permeability. The Rd observed in GW and SW of U increased linearly with PI measured in DIW, although the Rd of Th remained relatively constant above a PI of 88. From the viewpoints of associated engineering and sorption properties, the buffer materials containing 30-50% bentonite are probably the most favorable choice. Another result shows that U has a better additivity with respect to Rd than Th in both synthetic GW and synthetic SW. These results will allow a determination of more effective buffer material composition, and improved estimates of the overall Rd of the buffer material mixture from the Rd of each mineral component.