The material characterization and gamma attenuation properties of Portland cement-Fe3O4 composites for potential dry cask applications

Abstract

The effect of nanosized magnetite (Fe3O4) additions on the microstructural, mechanical and gamma attenuation properties of White Ordinary Portland cement (WOPC) pastes was investigated. The microstructure of a set of cement composites with Fe3O4 content ranging from 0 to 50 wt% was examined using X-ray diffraction and Scanning Electron Microscopy (SEM) techniques. Magnetite additions did not influence the hydration products of Portland cement after 28 days of curing. SEM showed uniform distribution of magnetite nanoparticles in the cement hydration products of composites with less than 10 wt% Fe3O4. Aggregation of magnetite nanoparticles occurred when high loading of magnetite was added to the cement (20, 40 and 50 wt%). Compressive strength and stress-strain curves were also measured to characterize the mechanical performance of the composites. A maximum compressive strength of 60 MPa was obtained for cement pastes reinforced with 10 wt% Fe3O4, which represents an enhancement of 50% over the plain WPOC paste. Composites loaded with 50 wt% Fe3O4 yielded to a reduction of compressive strength (35 MPa) of 5% over the reference paste. Mechanical behavior of the material was explained in terms of their microstructure, i.e. uniform magnetite embedded in the paste has a reinforcement effect due to the promotion of hydration process and the filling effect of the particles. Heterogeneous distribution of magnetite particles due to agglomeration acted to weaken the cementitious matrix by creation of pores. Finally, transmission experiments and Monte Carlo simulations were conducted to evaluate the shielding properties of the composites when exposed to a Cs-137 gamma source (0.662 MeV). The addition of Fe3O4 improves the shielding capability of Portland cement pastes with enhancements ranging between 3.1 and 2.6% for samples with 2.5 and 50 wt% Fe3O4 respectively. No simple relationship exists between the attenuation properties and magnetite loading. Changes in the attenuation coefficients of the composites are explained in terms of the chemical composition and microstructural effects.