The leaching behaviour of Cs and Sr is dependent on how they are retained within the cement. The project found that Cs remains as CsNO3. Sr(NO3)2, however, was not present in the hardened cement and retained within the hydrogarnet and a nitrated AFm phase. Secondly the impact of superplasticisers on the leaching behaviour of the cement matrix containing cerium was also explored using ADVA cast 551. Cerium was chosen as opposed to caesium or strontium as the main concerns in industry due to superplasticisers is regarding the complexion of uranium and plutonium, and Ce is a good simulant for plutonium. It was found that the introduction of ADVA cast 551 increased the fluidity and workability of the cement, did not alter the cement crystal structure however did increase the leached amounts of Ce by approximately 30%. In storage conditions this complexed Ce would come into contact with the cement backfill. A flow through leaching test was used on an NRVB cement sample and synthesised solutions of the leached Ce including the ADVA cast 551 to monitor how much Ce would pass through. It was found that similar amounts of Ce had passed through the NRVB whether the solution contained superplasticiser or not. This project found that the leaching behaviour of ions was dependent on how the ion is present and in which phase. Due to Cs and Sr being incorporated differently the Cs leaches much faster than the Sr, as it is not incorporated into any phase. The Sr is associated with several phases, however is mainly found to be associated with AFm type phases. The most efficient leaching accelerant method is the electrical method; therefore with further investigation this could potentially be used to explore the optimal cement compositions for cementitious waste immobilisation. This could also be used in cement samples containing superplasticiser, given that this increases mobility over short durations, the next step would be to investigate the long-term impacts, via simulation using electrical leaching. List of Abbreviations Cement Nomenclature AFm Alumino-ferrite-mono phase AFt Ettringite C-S-H Calcium silicate hydrate C3S Tricalcium silicate, Ca3SiO5 C2S Dicalcium silicate, Ca2SiO4
C3A Tricalcium aluminate, Ca3Al2O6 C4AF Calcium aluminoferrite, Ca4Al2Fe2O10 Abbreviations A Acidically leached samples A551 ADVA cast 551 BFS Blast furnace slag BSE Back scattered electron imaging Cement sample containing 3wt% caesium nitrate CE Cement samples containing cerium nitrate CEA Cement samples containing cerium nitrate and ADVA cast 551 Cement sample containing 3wt% CsNO3 and 3wt% Sr(NO3)2 D Days E Electrically leached samples EDS Energy Dispersive X-ray Spectroscopy ICP-MS Inductively coupled plasma mass spectroscopy ILW Intermediate level waste HC Hydrotalcite cement phase HLW High level waste LLW Low level waste M Months NRVB Nirex reference vault backfill PC Portland cement Cement sample containing 3wt% strontium nitrate SEM Scanning electron microscopy SP Superplasticiser TGA Thermo gravimetric analysis W Statically leached samples XRD X-ray diffraction Y Years Contents Acknowledgements, Publication & Award ii Abstract iii List of Abbreviations v Contents vi Chapter 1: Introduction 1 1.1 Introduction……………………………………………………………...............2 1.2 Project aims……………………………………………………………...............3 References (Chapter 1).........................................…………………….................6
of the argon plasma, where temperatures reach nearly 10000oC. At these temperatures chemical compounds of the mist dissociate into the component atoms and then ionised towards the end of the plasma. The ions are then brought into the mass spectrometer via the interface cones, where they are separated by their mass-to-charge ratio. The ions then exit individually and come into contact with a dynode of an electron multiplier, which detects the electrons released by the impact of the ions[8]. At the end of each leaching test a 20 ml sample from the leachate was collected in a centrifuge tube. The samples were filtered and acidified with nitric acid to ensure stability and comparability with calibration standards. The samples were submitted to the Sheffield Assay Office where the ICP-MS was performed. Table 3.5 summarises the solutions analysed. Table 3.5: Solutions analysed using ICP-MS. Solution Elements Analysed Blank (B) calcium, aluminium, magnesium, sodium Caesium Nitrate (C) calcium, aluminium, magnesium, sodium, caesium Strontium Nitrate (S) calcium, aluminium, magnesium, sodium, strontium Caesium Nitrate and Strontium Nitrate (CS) calcium, aluminium, magnesium, sodium, caesium, strontium 3.4 The impact of on leaching behaviour Introduction The aforementioned complexing behavior of superplasticisers and its impact on leaching of radionuclides, was investigated by comparing the concentration of cerium in a leachate from two static leaching tests. Furthermore in the storage environment any leached radionuclides from the cement would then come into contact with cement backfill, this project also tested the permeability of the Nirex Reference Vault backfill (NRVB) used in industry to assess how much of the leached cerium will actually escape to the surrounding
removing them from their leaching containers. These crystals could be due to the barrier formed during leaching. Fig 4.78: Surface of leached samples 28DB (left) and 6MB (right) Given that the solution was replaced regularly with fresh water, it was assumed that this re-crystallisation of C-S-H would be more prominent in the static samples as there is a higher concentration of ions. Therefore the TGA of exposure surface layers of the W-B series were examined and it was found that C-S-H amounts decreased up until 28 days after which the amounts of C-S-H seem to increase as shown in figure 4.79 which compares the weight loss curve for both W-28DB1 and W-3MB1. This is much earlier than was noticed in the semi-static tests and therefore suggests that removal of solution slows down the C-S-H reformation. This is only an indication as the weight loss could be due to the higher amounts of entrapped water in porosity. Fig 4.79: TG curves of W-28DB1 compared with W-3MB1 The increased weight loss as a result of higher C-S-H amounts could also be due to the increase in hydration or pore structure modification within the sample. To further investigate this the TGA profiles were compared for 0D samples with varying curing periods. Figure 4.80 shows TG curves for the original 0DB sample (cured for 42 days) alongside samples with the same composition but cured for 6 months, 1 and 2 years. Fig 4.80: TG curves of 0DB samples at different curing periods