Development of palm oil fuel ash (POFA)-supplemented cement for application as engineered barrier in radioactive wastes disposal facility

Abstract

Environmental sustainability in radioactive wastes management is supported by the application of innovative material in its implementation. Cementitious material formulated with the addition of agricultural and industrial wastes is a promising option for application in the disposal of radioactive wastes. In this study, optimized palm oil fuel ash (POFA)-supplemented cement was developed for application as an engineered barrier, for example, backfill in a borehole disposal facility. POFA-supplemented cement is anticipated to be more environmentally friendly. Most importantly, it contains additional secondary C-S-H phases resulting from pozzolanic reaction beneficial for containment of radionuclide contaminants in the near-field, thus eventually retards the radionuclides' migration to the geosphere finally the biosphere where humans and biota reside. The optimum mixture of 70% Ordinary Portland cement (OPC) and 30% POFA at a water to cement ratio of 0.4 was determined based on the monolith's satisfactory compressive strength and hydraulic performance. The physical and chemical properties of the optimized POFA-supplemented cement were determined. Meanwhile, safety-relevant radionuclides Ra-226, Cs-134 and Sr-88 investigated were derived from the radioactive wastes inventory planned for disposal in the borehole in Malaysia. The containment and retardation properties of the radionuclides in the POFA-supplemented cement were determined based on their sorption and diffusion behaviours and were compared against reference OPC. The sorption mechanisms were mainly attributed to the binding of the radionuclides onto the surface of C-S-H phases. The batch sorption study indicated greater sorption of the radionuclides onto the POFA-supplemented cement than the reference OPC at all parameters investigated due to interaction with secondary C-S-H phases. Pseudo-second-order model best described kinetic rates of the sorption process and its mechanisms were governed by both pore and film diffusion. Langmuir isotherm model best described the sorption process. The sorption process was also thermodynamically spontaneous and feasible with endothermic nature. Meanwhile, the diffusion study resulted in earlier confinement of Ra-226 and Cs-134 in the POFA-supplemented cement compared to the reference OPC. Finally, performance assessment revealed the slightly improved confinement and retention of Ra-226, Cs-134 and Sr-88 in the POFA-supplemented cement. It could be summarized that the optimized POFA-supplemented cement showed promising potential as a backfill barrier for the containment of Ra-226, Cs-134 and Sr-88 in a borehole disposal facility.