Fabrication of B4C hardened body using geopolymer binder with cold reaction sintering and application to neutron shielding

Boron carbide (B4C) is widely used as a neutron shielding material, but it is known to be difficult to sinter. Typically, B4C is sintered at ultra-high temperature (>2000 °C) and high pressure, or hardened using an organic binder such as silicone rubber. In this study, hardening by an inorganic binder, that is, a geopolymer binder was investigated. The raw materials for the binder were fly ash, sodium metasilicate anhydride (Na2SiO3) and sodium metasilicate 9-hydrate (Na2SiO3·9H2O), with the B4C being hardened at only 130 °C by novel “cold reaction sintering” using a warm-pressing device. The process involves a simultaneous reaction involving geopolymer formation (geopolymerization) and solidification. The effects of the geopolymer content and the mixing ratio with B4C powder with different particle sizes on the compressive strength and density of the B4C were investigated. When coarse B4C particles (∼100 μm), and fine B4C particles (∼1 μm) were combined, the fine B4C particles and the binder filled the voids between the coarse B4C particles. As a result, it was found that when appropriate pressure was applied to the raw materials, the binder was completely geopolymerized, and a strong hardened body was formed. The compressive strength of the hardened body exceeded 50 MPa when the content of fine B4C particles was 30 wt% and the content of the geopolymer binder was 20 wt%. Furthermore, neutron transmission test of the hardened bodies were conducted to evaluate their neutron shielding properties, and it was confirmed that the geopolymer-based B4C composites achieved a B4C volume occupancy of more than 70%, which is unreachable for commercial rubber-based B4C composites.