The stability of cement superplasticiser and its effect on radionuclide behaviour
thesisposted on 13.08.2012, 09:17 by Amy J. Young
Superplasticisers are used to improve the flow properties of fresh cement and offer undoubted benefits to the construction sector. There is concern in the nuclear industry, however, that organic additives may increase the solubility of radionuclides when in contact with cementitious grouts or backfill. The research presented in this thesis describes the effect of a commercial polycarboxylated poly ether comb type superplasticiser on the behaviour of U (VI), Th (IV), Eu (III) and Ni (II) in blended cements. Both Blast Furnace Slag (BFS) and Pulverised Fly Ash (PFA) blends with Ordinary Portland Cement (OPC) were investigated. Solubility experiments approached from oversaturation were conducted in a range of high pH aqueous solutions that are representative of the respective cement pore waters. Results show that the as received superplasticiser is responsible for an increase in solubility of all the metals investigated, however, the extent of solubility enhancement is dependent on the metal investigated and the nature of the high pH solution. U (VI) and Eu (III) display solubility enhancement over several orders of magnitude while Ni (II) and Th (IV) solubility is enhanced to a lesser extent. Batch experiments of Ni (II) and Eu (III) uptake onto BFS:OPC and PFA:OPC cement were investigated from two view points i. By regarding the superplasticiser as a ligand where batch experiments of metal uptake on crushed cement are carried out with increasing concentrations of free superplasticiser in solution. ii. By investigating metal uptake in cements prepared with superplasticiser already present. Results of the batch experiments show that the presence of free superplasticiser in solution reduces uptake of Ni (II) and Eu (III) by both BFS:OPC and PFA:OPC. Further, metal bound in the presence of free superplasticiser is readily remobilised on exposure to fresh cement solution. Conversely, metal uptake is quantitative and irreversible when exposed to crushed cement prepared with superplasticiser in the original mix. Leaching of U (VI), Th (IV) and Ni (II) from hardened cement with and without the addition of superplasticiser was investigated by the preparation of monolithic BFS:OPC and PFA:OPC cement samples. BFS:OPC samples prepared with superplasticiser suffered from bleed and a significant proportion of the original metal inventory was found in the BFS:OPC bleed water after 48 hours, varying from 19% in the case of nickel to 32% for uranium. Autoradiography of the monoliths showed accumulation of uranium and thorium in the region adjacent to, and in contact with, the bleed water. Therefore, the bulk of the metal inventory had not been incorporated into the cement. The stability of the superplasticiser under conditions likely to be present in a Geological Disposal Facility (GDF) was investigated by exposing the superplasticiser to chemical, thermal and radiolytic attack. Several analytical techniques were assessed for their ability to characterise the products of these experiments. Whereas Infra-Red (IR) spectrometry provided comparative information regarding the functionality of the superplasticiser before and after exposure, Gel Permeation Chromatography with Refractive Index Detection (GPC-RID) was found to be the best technique available to observe changes to the superplasticiser. Under chemical (alkali) attack by exposure to 0.1 mol dm-3 NaOH and 95% saturated Ca(OH)2 , the superplasticiser samples were found to decrease in molecular weight, a result consistent with alkaline hydrolysis of the polymer chains. Little change to the superplasticiser were observed on heating to 80ºC, however, on exposure to gamma radiation (60Co), further polymerisation and cross-linking of the polymer chains was initiated, with a significant increase of the polymer s molecular weight. Solubility experiments on U (VI), Th (IV), Eu (III) and Ni (II) were repeated to investigate the effect of irradiated superplasticiser. As with the as received material, the results show that the presence of irradiated superplasticiser causes an increase in solubility of all the metals investigated. In this case however, Th (IV) and U (VI) display a much greater enhancement in solubility, whereas Eu (III) and Ni (II) show solubility enhancement similar to that observed in the non-irradiated samples. The results presented in this thesis give a representation of the behaviour of metals in the presence of polycarboxylated poly ether comb superplasticiser and highlight the importance of considering the consequences of the use of cement additives in the concept of the GDF. The metals studied encompass a range of oxidation states from Ni (II), through Eu (III) and Th (IV) to U (VI). The behaviour of these metals here is of potentially great significance as it may indicate similar behaviour by other actinide species, most notably the behaviour and apparent mobility of Th (IV) may suggest analogous behaviour of tetravalent Pu (IV) and Np (IV).