Sewage sludge ash contaminated with radiocesium: Solidification with alkaline-reacted metakaolinite (geopolymer) and Portland cement.

This study contributes toward developing measures for the disposal of radiocesium-contaminated sewage sludge ash (SSA). Here, we prepared two types of solidified bodies containing 30 wt% radiocesium-bearing SSA. The material used for the two solidified bodies were alkaline-reacted metakaolinite (geopolymer) and ordinary Portland cement (OPC). Cement has been used for solidification of low-level radioactive wastes, and geopolymer is a candidate of cement alternative materials. The characteristics of these solidified bodies were investigated by various aspects including mechanical strength, transformation of SSA components during solidification, and radiocesium confinement ability by leaching test. The compressive strength of geopolymer- and OPC-solidified bodies at 30 wt% SSA content was more than 40 MPa. After static leaching test at 60 °C, Cs was hardly leached out from the geopolymer-solidified bodies containing SSA at 30 wt% to ultrapure water (<0.1%), whereas more than 30% Cs was leached from the OPC-solidified bodies containing SSA at 30 wt% even though only ~9% of Cs in the SSA is soluble. These results strongly indicate that geopolymer is far superior to OPC for solidifying radiocesium-bearing SSA.