Applications of Geopolymers in Concrete for Low-Level Radioactive Waste Containers

Abstract

The purpose of this mini-thesis research paper is to investigate the applications of geopolymer concrete as a replacement for ordinary Portland cement (OPC) concrete currently used for the containment and transportation of low-level radioactive waste in South Africa. Geopolymers are materials which are high in aluminosilicates and use polymerisation reactions to bond together, as opposed to the conventional calcium-silicate-hydrate hydration reaction that occurs in traditional cement. The nuclear waste applications of geopolymers are investigated in the South African context – using locally available fine and coarse aggregates and class S fly ash originating from Kriel Power Station in Mpumalanga. Three geopolymer mix designs are tested – all of which use the same 1:2:2 mix proportion but with differing aggregate sizes, a common water/cement ratio of 0.5 and an 8 M or 12 M NaOH and Na₂SiO₃ activator. The aim of this research is to reach a density of greater than 2400 kg/m³, compressive strength of 50 MPa and a tensile strength of 4.5 MPa using readily available industry materials. The mixes were cast and cured in an oven at 80° for 1 day and 3 days, respectively, after which the 8 M NaOH-only mix produced the best results at 11.9 MPa compressive strength, a 1.0 MPa tensile strength and a density of 2070 kg/m³. These results have not met the South African Nuclear Energy Corporation (NECSA)’s standards as of yet, and the mixes will have to be further developed as a possible precast low-intermediate radioactive waste container solution.