Characterisation of Geopolymers for the Immobilisation of Intermediate Level Waste

Geopolymers are made by adding aluminosilicates to concentrated alkali solutions for dissolution and subsequent polymerisation to take place. Their physical behaviour is similar to that of Portland cement and they have been considered as a possible improvement on cement for several applications including as media for the encapsulation of hazardous or low/intermediate level radioactive waste. We studied in detail a commercial geopolymer to try to get a better understanding of geopolymers in order to enhance its leach resistance for immobilisation of intermediate level radioactive waste. We also briefly investigated two types of experimental geopolymers, one made with a metakaolinite and another from fly ash as the aluminosilicate source. The commercial geopolymer paste had an apparent porosity of 26% and it was possible to reduce it to 17% by adding ∼ 30 weight % foundry sand. The apparent porosities of the geopolymer made from metakaolinite and from fly ash were 13% and 26% respectively. X-ray powder diffraction showed in the three geopolymers, an amorphous phase (deduced by the presence of a very broad diffuse peak centred at a d-spacing of ∼ 0.32 nm), quartz and other minor phases. The energy dispersive spectroscopic analysis under the scanning electron microscope confirmed these. Magic angle spinning nuclear magnetic resonance data from the samples showed Al to be mainly in 4-fold coordination and Si sites varying from Q0 to Q4 coordination as also found by other researchers. ²³Na spectra indicated that the Na was mainly in the pore water. The ¹³³ Cs spectra showed a strong possibility of Cs being mainly bound in the structure while a small amount could still be in the pore water. The initial leach tests showed alkalis were leached out at rates of several orders of magnitude more than the Al and Si network ions. The most likely reason is that a significant alkali inventory is in the pore water. To remove pore water and incorporate simulated radionuclides such as Cs in the network the commercial geopolymer was heated up to 1200°C. Differential thermal/thermogravimetric analysis showed the loss of water occurs in three stages and most had been lost by 700°C. These results are in broad agreement with the Infra red spectra obtained for samples heated over the temperature range 30–900°C. The broad water band intensity in the range 2600-30-900°C. The broad water band intensity in the range 2600–3800 cm ⁻¹ decreased steadily with temperature although a small fraction remained even after heating to 500°C. The silanol band had disappeared at 800°C, and the 3619 cm⁻¹ band (due to OH) virtually disappeared by 900°C.