M. Atkins

Zeolite P in cements: Its potential for immobilizing toxic and radioactive waste species

Zeolite P, approximately (Na2O,CaO).Al2O3.2SiO2.4H2O, has been shown to develop spontaneously in appropriate cement formulations at > 40°C, and to be a stable phase. Suitable composites can be made from mixtures containing Ca(OH)2 or Portland cement, with high proportions of the pozzolans, metakaolin or class F fly ash. Alternatively, zeolite P is easily prepared in phase-pure form using laboratory chemicals. The latter method was used to obtain zeolite P of composition 0.9CaO.0.1Na2O.Al2O3.2.66SiO2.4H2O, on which characterization studies were performed for its sorption potential in cement-analogue environments. RD values are reported for the 25 and 85°C isotherms, for a range of initial sorbate concentrations (10-10,000 μmol/l). The sorbates investigated were: Cs, Sr, Ba, Pb and U(VI). In water media, zeolite P shows good selectivity for Cs, Sr, Ba and Pb, at 25 and 85°C. The highest RD recorded was for Pb2+, at > 800,000 ml/g (1000 μmol/l initial concentration). In NaOH media, Ba and Sr sorption values remained high. Cs and Pb show a marked decrease in sorption, although RDs are still reasonable, at ∼750 and ∼400 ml/g, respectively. On account of its large ion size, UO22+ uptake into zeolite P is negligible, remaining in solution or precipitating as soddyite or Na uranate. Cements conditioned to form stable zeolites offer great potential in the treatment of hazardous waste streams.

Application of portland cement-based materials to radioactive waste immobilization

Abstract The immobilization potential of cement may be either physical or chemical. Physically it may act as a barrier and chemically as a selective binder for radwaste species. The latter aspect is believed to assume increasing importance at long ages. The nature of cement and modified cement matrices are explored to explain their chemical binding potential. It is shown that Portland cements are very similar worldwide but supplementary materials such as fly ash and slag are less well-specified. The main immobilizing potential of cement systems comes from their high internal pH allowing precipitation of many nuclides as hydroxides. In slag systems, the low redox potential can also reduce the solubility of some nuclides. In order to establish a predictive capability to determine the time-dependence of the internal state of cements, it has proven necessary to develop physico-chemical models of cement performance. The state of modelling is reviewed. Significant advances have been made in recent years, but there remain several problems, one of which is validation. The present database still requires a considerable input with respect to specific radwaste-cement interactions which can only come from experimentation. Experimental work can also help in verification of the existing database. Findings of experiments on uranium, iodine, and strontium interactions with cement are presented. Iodine is shown to be immobilized by lattice incorporation into crystalline cement phases and by ‘sorption’ into C-S-H gel. Uranium VI however, can be precipitated as low solubility hydrated Ca salts. Much remains to be determined. The impact of higher temperatures is singled out as an important unknown factor affecting the performance of cements. It is known that elevated temperatures cause crystallization of C-S-H gel, thereby lowering the pH at which it buffers. In systems open to the environment, the course and speed of reaction of groundwaters with cement components are only known qualitatively. However, models can be modified to incorporate these interactions, provided the appropriate database is available.

A thermodynamic model for blended cements

Abstract A model is described for predicting the solid and solution chemistry of blended cements for use in equilibrium modelling. A computer program, CEMCHEM, has been written to predict the stable phase assemblage from the composition of the initial cement blend. Solubility models developed for the cement hydrate phases are then used to predict the aqueous solution composition at equilibrium with the cement. Example calculations for mature cement pastes are presented and compared with real pore fluids extracted from aged cements.

A thermodynamic model for blended cements. II: Cement hydrate phases; thermodynamic values and modelling studies

Abstract Blended Portland cements are likely to form a substantial proportion of repository materials for the disposal of radioactive waste in the UK. A thermodynamic model has been developed therefore in order to predict the composition of the solid and aqueous phases in blended cements as a function of the bulk cement composition. The model is based on simplifying cement to the system CaO SiO 2 Al 2 O 3 SO 4 MgO H 2 O, which constitutes 95% of most cement formulations. Solubility data for hydrogarnet and ettringite suggest that they dissolve congruently and that conventional solubility products can be used to model their dissolution. A solubility model for the siliceous hydrogarnet series, based on ideal solid solution on either side of an immiscibility gap, closely matches experimental solubility data. Solubility data for hydrotalcite and gehlenite hydrate are less consistent and indicative of more complex dissolution processes. On the basis of earlier work, an accurate solubility model is described for hydrated calcium silicate gels in the CaO SiO 2 H 2 O system. Together, these solubility models form a relatively complete thermodynamic model for blended cements. Model predictions for fully matured cement blends are compared to the compositions of pore fluids extracted from aged cement blends. Departures from expected behaviour occur in alkali-bearing systems and are discussed.

Thermodynamic investigation of the CaO-Al[sub 2]O[sub 3]-CaCO[sub 3]-H[sub 2]O closed system at 25 C and the influence of Na[sub 2]O

The solubilities of calcium hemicarboaluminate, calcium monocarboaluminate and calcium tricarboaluminate have been determined and the equilibrium phase diagram for the CaO-Al[sub 2]O[sub 3]-CaCO[sub 3]-H[sub 2]O closed system at 25 C has been calculated. Six isothermally invariant points have been located involving six stable hydrates: CH, C[sub 3]AH[sub 6], AH[sub 3], calcium hemicarboaluminate, calcium monocarboaluminate and calcite. Calcium tricarboaluminate, the carbonate analogue of ettringite, does not appear to be stable at 25 C. This study was part of a larger study on radioactive waste solidification.

Solubility properties of ternary and quaternary compounds in the CaO Al2O3 SO3H2O system

Solubility measurements are reported for four hydrated cement phases. A technique employing repeated dispersion and filtration is described which enabled the solubility (activity) products of ettringite (C6AS3H32) and hydrogarnet (C3AH6) to be calculated with confidence. Free energies of formation are also given. These phases dissolve congruently, whereas monosulphate (C4ASH2) and tetra-calcium aluminate hydrate (C4AH13) exhibit incongruent dissolution, and can therefore not be described by a conventional solubility product. The results represent additional data for inclusion in thermodynamic databases currently being applied to the modelling of cement systems.

Sulphate attack on concrete: limits of the AFt stability domain

Abstract The compositions of the solutions at the four invariant points that define the AFt equilibrium surface, have been calculated at 25°C with and without sodium ions. The results indicate that AFt is stable over a large range of sulphate concentration depending on both the calcium and aluminium concentrations. The presence of sodium modifies the AFt stability domain but it remains stable even at high sodium concentrations.

Encapsulation of Radioiodine in Cementitious Waste Forms

Risk assessment models applied to radioactive waste repository design disclose that iodine is one of the nuclides causing most concern. Computer calculations for these scenario studies assume that iodine, in the form of I − , is poorly sorbed on most geological materials. Therefore it is important that iodine be retained at source, ie. within the vault, for as long as is practicable. In the UK context, cements are likely to form a major part of the waste package for low and medium active wastes, and of engineered vault structures. These cements are likely to be blends of one form or another, including Portland cement blended with blast furnace slag, or fly ash. It is therefore important to assess the effect of Portland cement and blending agent on iodine speciation and uptake by the constituent solid phases. Data are presented on the uptake of I − on specific phases: Ca(OH) 2 , calcium aluminate sulphate hydrates, hydrotalcite and calcium silicate hydrogel (C-S-H). Combining this with a model for predicting phase assemblages in well-aged slag cements, yields an optimum blend for immobilising radioiodide, on ageing. Precipitation of I as AgI during cementation and radiolytic effects on I are also discussed. The production of gaseous radioiodine (I 2 ) is potentially a serious problem.

Phase Development and Pore Solution Chemistry in Ageing Blast Furnace Slag-Portland Cement Blends

Blast furnace slag (BPS) cement blends have certain advantages for the encapsulation of low and intermediate radioactive wastes. However, their hydration reactions are more complex than for Portland cement because Portland cements fully react within several years whereas slag hydration takes considerably longer. Empirical testing of blends matured for short periods, up to several years, are not therefore adequate to characterise the chemical immobilisation potential of such systems. A modelling approach is required to predict long—term properties. This paper is concerned with defining the internal environment within slag-cement matrices. The important parameters for definition are pH, Eh, solubilities and speciation in the aqueous phase, and solid phase equilibria. Compatibility studies in the CaO-CaSO 4 -Al 2 O-SiO 2 -MgO-H 2 O system are described and compared with the results of X-ray diffraction on pastes aged up to 2 years. In addition, pore water compositions for slag-rich cements are given, and the potential for predicting their redox level(Eh) by extending the present model is discussed.

The use of silver as a selective precipitant for 129I in radioactive waste management

Abstract 129 I is one of the more hazardous nuclides occurring in radioactive waste. In the form of I − , its most likely speciation, it is poorly sorbed on most geologic media. Several workers have suggested the use of silver to precipitate I − as the insoluble AgI, in a cemented waste form, or as a “getter”. The efficacy of this procedure is examined by experiment, in conjunction with thermodynamic predictions. The addition of AgNO 3 to Portland cement leads to coprecipitation with C-S-H, with low Ag solubilities (∼ 10 μ mg/L); 2–;3 orders of magnitude lower than predicted (from Ag 2 O). AgI is stable in these matrices, with low aqueous I concentrations ( − passing into solution; concentrations up to 900 mg/L were observed. It is shown that repository conditions, on closure, are also likely to induce solubilisation of I − from AgI. It is concluded that the use of Ag is unlikely to significantly improve the immobilisation properties of the near field for radioiodine.