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Proceedings of ICEM ‘03: The 9th International Conference on Radioactive Waste Management and Environmental Remediation

September 21 – 25, 2003, Examination School, Oxford, England

icem03-4554

CEMENTITIOUS SYSTEMS FOR ENCAPSULATION OF INTERMEDIATE LEVEL WASTE

J. H. Sharp, University of Sheffield, Immobilisation Science

Laboratory, Department of Engineering Materials, Mappin Street, Sheffield, S1 3JD, UK

J. Hill, University of Sheffield, Immobilisation Science


N. B. Milestone, University of Sheffield, Immobilisation Science


E. W. Miller, BNFL Plc, Sellafield, Cumbria, CA20 1PG, UK

ABSTRACT

Encapsulation in cement is the favoured method in the UK for disposal of intermediate and low level radioactive wastes. It is usual to use composite cement systems incorporating blast furnace slag (BFS) or pulverised fuel ash (PFA) as these offer several advantages over Portland cement, notably a lower heat of hydration. The use of these mineral additions utilises a waste product which would itself need a disposal route and, because of the decreased amount of Portland cement used, provides a reduction in cost and energy consumption. Cementitious systems have many attributes which make them suitable for encapsulation and immobilisation, including:

Inexpensive and readily available Assist immobilisation of radionuclides by:

a) acting as a diffusion barrier b) providing sorption and reaction sites c) maintaining a high pH which in turn decreases radionuclide solubility

Provide radiation shielding which is not degraded by the radiation

Controllable permeation and diffusion characteristics over a wide range via selection of constituents and components.

Where physical adsorption is a significant factor for immobilisation, the calcium silicate hydrate gel (C-S-H) formed on hydration of a Portland cement is advantageous as it has a high surface area and large micropore volume. Composite cements based on blast furnace slag will produce a higher proportion of C-S-H than ordinary Portland cement

increasing the sorption capacity, and reducing the capillary porosity so that the diffusion resistance is increased.

Intermediate level waste covers a wide range of materials, for example, metals and ion exchangers, each with differing chemical properties. It is, therefore, necessary to access the ability of the cementitious system to immobilise different wastes and to characterise the products formed. It is also necessary that alternative encapsulant materials be considered for immobilising wastes not suited to the composite cements already being used. The techniques employed to do this include x-ray diffraction (XRD), to identify standard and non-standard hydration products, isothermal conduction calorimetry (ICC) and scanning electron microscopy (SEM).

INTRODUCTION

The disposal of radioactive waste is an integral part of the cycle for efficient use of nuclear energy and during the past twenty-five years, a great deal of research into suitable methods has resulted in the deployment of a variety of successful processes on an operational scale. These are dependent on the nature of the waste itself and in particular, its associated radioactive properties. Within the UK there are three categories, Low Level Waste (LLW), Intermediate Level Waste (ILW) and High Level Waste (HLW). The main distinctions between these is the nature and level of the radiation. HLW produces significant quantities of heat during storage whereas ILW and, more-so, LLW produce virtually no heat or only relatively low levels of heat evolution. It should be noted, however, that in the USA only two categories, HLW and LLW, are adopted, the UK’s ILW being essentially incorporated into the latter. The amount of heat evolution, amongst other factors, is important


when designing disposal methodologies which are demonstrably safe and stable over long time periods. The utilisation of cementitious materials has proved to be a satisfactory medium for the encapsulation and immobilisation of both LLW and ILW. Cements have several attributes that make them suitable for consideration as an encapsulation medium for radioactive waste. Thus, they • are readily available • are relatively inexpensive • can be formed into a fluid grout • are durable • can act as a barrier to diffusion of ions • can incorporate many ions into solid solution • maintain a high pH, which decreases radionuclide

solubility • provide radiation shielding that is not degraded by the

radiation • can have extremely low liquid and gas permeabilities in the

hardened state • can be modified to suit a particular purpose through

selection of appropriate constituents • are tolerant to a wide variety of wasteforms • can be relatively easily processed in a remotely operated

plant

NOMENCLATURE

Cement chemists have developed their own shorthand nomenclature in which C = CaO, S = SiO2, A = Al2O3, F = Fe2O3, H = H2O, S = SO3 so that the principal anhydrous cement phases are often referred to as C3S, � -C2S, C3A and C4AF, respectively

2. CHARACTERISTICS OF CEMENTITIOUS

SYSTEMS 2.1 Portland cement Although there are many cement systems, far and away the most common and extensively used for construction is Portland cement, patented in 1824 by Joseph Aspdin. Portland cement is made by the high temperature calcination of limestone (or chalk) and clay (or shale) in a rotary kiln. The maximum kiln temperature reaches ca.1450oC at which about 30% of the raw materials are liquid. Rapid cooling, of the cement clinker provides alite, Ca3SiO5, belite, � -Ca2SiO4, tricalcium aluminate, Ca3Al2O6, and a ferrite phase, Ca2AlFeO5. Two points should be made. The first is that because the Portland cement is made in large quantities from natural raw materials, it is inevitable that additional elements are introduced during manufacture, e.g. alkalis, notably sodium and potassium, and also magnesium, titanium, manganese and fluorine. Additional impurities are introduced by the fuel used, frequently pulverised coal, but sometimes involving the

combustion of waste materials such as used tyres and waste organic solvents. These, along with the iron and other elements in the starting materials enter into solid solution, so that none of the four phases mentioned above has its ideal composition. The actual compositions vary according to the composition of the raw materials and the fuel used, but typically the following substitutions occur [1]: alite: 3-4% of other oxides, including Mg, Al, Fe belite: 4-6% of other oxides, notably Al, Fe, K, Mg, SO3 aluminate: can be up to 20%, notably Fe, Si, Mg, Na, K, Ti ferrite: part of a series varying between Ca2Fe2O5 and

Ca2Al1.4Fe0.6O5. The typical composition in Portland cement has a lower iron content than that of C4AF. It approximates to the replacement of Fe(III) by approximately equal amounts of Mg and Si (and Ti) to maintain charge neutrality

Lack of data relating to how such solid solutions affect hydration and product formation is inhibiting modelling studies of the hydration of Portland cement, and provides results that are prone to uncertainty. Minor elements can have a major effect on the performance and durability of Portland cement. The second point is that in addition to the few major phases present, several other minor phases are also present and play an important role. Free lime (CaO) is always present (often > 1%) because some decomposed limestone may pass through the kiln without reacting and especially because alite decomposes during cooling into belite and CaO. Some magnesia is also not in solid solution but present as periclase. Gypsum (often ~ 5%) is added to the cement clinker before it is ball milled, which results in some of the gypsum being dehydrated into hemihydrate or even anhydrite. Portland cement is, therefore, a complex mixture of several phases that can vary appreciably in composition and hence reactivity. Indeed, advantage is taken of this in the production of modified Portland cements, such as sulfate-resisting Portland cement, by deliberately modifying the relative amounts and composition of these phases. In the UK, ordinary Portland cements (OPC) contain as the major phase, alite (~60%) along with belite (~15%), C3A(~8%), ferrite (~8%) and gypsum (~5%). The composition of OPC has changed over the last two centuries, particularly so over the past forty years to meet ever increasing demands from the construction industry for rapid strength development, with a notable increase in the amount of alite present at the expense of belite. This change has been deliberate, because alite contributes greatly to the development of compressive strength, especially during the first 28 days curing. It has, however, one major disadvantage for the encapsulation of radioactive waste and that is the extent and rate of heat evolution. The hydration of alite to form calcium silicate hydrate gel and up to 20% w/w portlandite, Ca(OH)2, is a highly exothermic reaction. In concrete, this is normally not a problem except in very thick pours (such as in the construction of dams or in mass foundations and similar structures) because


the cement is effectively diluted by the presence of fine aggregate (usually sand) and coarse aggregate (e.g. gravel). The main hydration product known simply as calcium silicate hydrate, C-S-H, is an amorphous calcium silicate hydrate of variable composition and can have many other elements substituted into its structure in minor amounts. The key factor in determining the long term quality of a cement paste is the development of microstructure and associated porosity. This is best characterised by scanning electron microscopy (SEM) and back scattered electron images (BEI) are shown in Fig. 1 for OPC cured at 20oC and at 90oC. Because of the increased rate of reaction, less porosity was evident in these young pastes at the higher curing temperature, but after longer curing times the reverse may be true in mature pastes [2]. Figure 1. BEI of OPC hydrated at 20°C for 3 days (top) and 90°C for 24 hours (bottom) 2.2 Composite cements For most civil engineering applications, the compressive strength of concrete, notably at early ages, is the over-riding physical property. In general, if the compressive strength is high, it is assumed that the tensile strength will be high, the permeability and the porosity are low, hence the durability will be good. For the encapsulation of radioactive waste, however, the compressive strength is of less importance than ensuring the absence of thermal gradients that might lead to structural

instability and hence crack formation which could possibly compromise product quality. The exotherm associated with Portland cement hydration reaches a maximum after a few hours, following shortly after the setting of the cement (Fig. 2, curve A). There are three ways to reduce the amount of heat evolution: (a) by incorporation of an inert filler (b) by addition of a pozzolanic material (c) by addition of a latent hydraulic material. Figure 2. Isothermal conduction calorimetry curves for (A) OPC, (B) 3:1BFS:OPC, (C) 9:1 BFS:OPC and (D) 3:1 PFA:OPC composite cements cured at 35oC with a w:s ratio of 0.35. The advantage of using routes (b) and (c) over (a) is that the replacement material also contributes to the ultimate strength of the cement, giving rise to either blended or composite cements. Further, these routes can make a significant improvement to the ultimate microstructure of the resulting cement product. During the initial hydration of the alite in Portland cement, Ca(OH)2 is released in the setting reaction. Pozzolanic material reacts with water and Ca(OH)2 to produce calcium silicate hydrate, C-S-H. The pozzolanic reaction is strongly dependent on temperature, proceeding slowly at 20oC for several months or even years, but more rapidly at higher curing temperatures. Pulverised fuel ash (PFA) from coal burning power stations, natural volcanic ash, rice husk ash, metakaolin and silica fume are examples of pozzolanic materials. They all have a low CaO content so that the Ca/Si ratio of the C-S-H eventually formed is lower than that found in Portland cement pastes. As it is the Ca(OH)2 which largely determines the high alkalinity of the cement paste, composites containing pozzolanic materials have a lower internal pH. Latent hydraulic cements have a higher CaO content than pozzolanic materials, and will act as hydraulic cements if mixed with water and activated by the presence of an alkaline phase. The most important hydraulic material is ground, granulated blastfurnace slag (BFS), which is readily activated by the products of Portland cement hydration.

0

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0 5 10 15 20 25

Time (hrs)

Ra

te o

f h

ea

t o

utp

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In the construction industry, use of these replacement materials has become commonplace during the last twenty years, in part because it disposes of waste materials and provides a reduction in energy consumption. Additionally, these materials can be cost effective and also offer benefits in long-term durability, specifically in terms of chemical resistance in areas such as sulphate and acid attack. Typical replacement levels are up to 60% for BFS and 30% for PFA. To bring about dramatic reductions in the rate and amount of heat evolution, thus contributing to long-term integrity, much higher levels of replacement are used in composite cements for the encapsulation of radioactive waste. These can be as high as 95% for BFS and 80% for PFA. Isothermal conduction calorimetry curves (ICC) for some such cements are compared with that for ordinary Portland cement in Fig. 2, from which it can be seen that the maximum rate of heat evolution, Qm, is much reduced at all curing temperatures, whereas changes to tm, the time associated with this maximum, are significantly longer but not unacceptable in terms of the setting of the composite cements, which usually takes place prior to tm. Further details at a range of curing temperatures are provided in Table 1 for BFS:OPC composite cements. In all cases, there is a substantial reduction in the maximum rate of heat evolution and in all cases, except one, tm was less than 24 hours. Indeed, at higher curing temperatures, the values of tm in the range from 5 to 11 hours for the 9:1 BFS:OPC composite cement indicate a very acceptable setting time, whereas the setting of neat OPC may be undesirably rapid. When waste materials are incorporated, even in low concentrations, the setting time and rate of heat evolution of the cement is changed. Two examples of the effect of zinc salts on the ICC curve for neat OPC at 35oC are shown in Fig. 3. The addition of 0.1% zinc nitrate to the curing water caused classic retardation, i.e. the magnitude of Qm was reduced and tm was increased. An even greater effect was observed with addition of 0.1% zinc chloride. It has to be remembered that the effect of the anion present can be as important as that of the cation. Table 1. Values of Qm and tm from some composite OPC-BFS cements at various temperatures.

Temp (oC) Cement Tm (hours) Qm (Wkg-1)

20

40

60

80

100% OPC 1:1 BFS:OPC 9:1 BFS:OPC




10.8 12.6 29.5

4.7 6.2

10.7

1.9 3.5 6.5

1.8 3.5 5.6

3.7 2.0 0.7

9.8 5.3 1.5

26.3 10.7 3.1

44.8 20.7 6.3

At these very high levels of replacement, the distinction between a latent hydraulic material and a pozzolanic material becomes blurred. For example, Hill and Sharp [3] recently showed that in a 9:1 BFS:OPC composite cement, where the Ca(OH)2 is required for the formation of C-S-H, the loss of portlandite, Ca(OH)2, is almost as rapid as in a 3:1 PFA:OPC cement. Figure 3. Effect of incorporation of wastes on isothermal conduction calorimetry curves of (A) neat OPC, (B) A with 0.1% ZnCl2 and (C) A with 0.1% Zn(NO3)2 As might be expected, the composition and the microstructural features of the gel formed by these composite systems are inevitably different in the detail from those of the gel formed with ordinary Portland cement. The long term stability and behaviour in the presence of added waste materials of such gels is of major interest to the radioactive waste disposal industry and we are examining this aspect. At present, it is evident that these composite cements have an important role to play in the encapsulation of toxic and radioactive wastes. It is, however, impossible to state with

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certainty that such materials will remain unchanged over centuries in a storage facility. Unfortunately, modelling studies are in their infancy and still subject to major uncertainties. There is a wide range of historic or legacy wastes dating from the period some forty or fifty years ago when atomic weapons and nuclear power stations were first developed. Many of these wastes are now in the form of poorly characterised slurries that may require individual consideration. It is, therefore, necessary to consider alternative composite cement systems and indeed alternative cements which could be used to immobilise such wastes. These are discussed in the following sections. 3. REPLACEMENT MATERIALS 3.1 Blastfurnace slag (BFS) As discussed above, the principal latent hydraulic replacement material used today is ground, granulated blastfurnace slag, produced as a by-product during the extraction of iron from its ore. The slag is cooled rapidly below 800oC in a large excess of water to produce a mainly glassy structure with a typical composition as shown in Table 2. This material is then ground to an almost white powder with a fineness similar to OPC. Large quantities of BFS, of the order of 600,000 tonnes/year, are used in the UK construction industry. Although the abbreviation GGBS is widely used by civil engineers, the abbreviation BFS is preferred in the nuclear industry and is adopted here. Although BFS has a significant content of CaO, it is not as high as that of OPC (Table 2). It contains more Si, Al and Mg than OPC, but less Fe. The metallurgical industry does an excellent job in retaining nearly all the iron in the liquid metal rather than the slag, and also deliberately transfers most of the sulfur impurities (from the fuel) to the slag. This is present in the form of sulfide ions, which during the initial hydration can give the slag cement a duck egg blue colour, although these are quickly oxidised when the cement fraction reverts to its normal, light grey colour. This composite cement, therefore, provides a reducing environment, which may offer advantages (or disadvantages) for the treatment of specific waste streams. BFS is normally activated by Portland cement but alkali silicates or alkali hydroxides can also be used to activate the hydration. Initial strengths are lower than those of equivalent OPC cements, markedly so at higher BFS replacement fractions, although ultimately the strength can exceed that for OPC alone.

Figure 4. BEI of BFS hydrated at 20°C for 5 days (top) and of 9:1 BFS:OPC hydrated at 90°C for 22 hours (bottom) SEM reveals the BFS grains to possess angular morphology, easily distinguished from the more rounded grains of alite. When BFS is hydrated at 20oC without addition of Portland cement, the BEI in Fig. 4 shows the presence of significant porosity, but incorporation of even small amounts of OPC brings about a big improvement in the microstructure. This is especially so when elevated curing temperatures are used, such as 90oC as for the BEI shown in Fig. 4. A high quality, dense microstructure has resulted within the first day of curing. Table 2. Typical Chemical Analyses of Various Cements and Replacement Materials

CaO SiO2 Al2O3 Fe2O3 MgO

OPC BFS GS PFA (low lime) SF MK Ciment Fondu Refractory CAC CSA

65 43 23 2

0.1 0.1 38 28 41

20 34 36 50 97 56 4

0.4 8

6 16 27 28 0.2 43 39 71 33

3 0.5 4

10 0.1 0.8 16 0.2 2

1 5 3 2

0.6 0.3 0.1 0.2 12*

* SO3 3.2 Gasifer slag The Lurgi slagging gasification process has been developed to convert coal into substitute natural gas for use when deposits of natural gas become scarce. It can be expected that gasifier slag (GS) produced as a by-product of this process will eventually become available in massive quantitites. It can be seen from data shown in Table 2, that the chemical composition of GS is intermediate between that of GGBFS and PFA, with a CaO content of around 23%. The hydration of GS-containing composite cements represents both pozzolanic activity (due to the high SiO2 and Al2O3 contents) and latent hydraulic activity (due to the CaO content). Both of these mechanisms require the interaction of gasifier slag and cement, and both result in


the formation of C-S-H gel [4]. This is presently not used to a wide extent in the construction or encapsulation industries but could become important. 3.3 Pulverised fuel ash (PFA) Pulverised fuel ash or fly ash is produced in enormous quantities from the burning of pulverised coal in power stations. The raw ash generally requires further processing by physical classification in order to comply with relevant construction standards for use in structural concrete. The main oxides present in PFA are SiO2 and Al2O3, with a much lower level of lime than in the slags discussed above (Table 2). The PFA used in the UK is siliceous with a CaO content less than 5%. In other parts of the world, some PFA materials are calcareous and can have CaO contents of 10% or more. These are less commonly used as replacement materials in composite cements but are receiving increased attention. The proportion of PFA used in a composite cement is variable, but is usually in the range from 5 to 35%. Because of its low lime content, PFA is not latently hydraulic, but contributes to the final CSH product via the pozzolanic reaction. In this, the silica present in the PFA (the pozzolanic material) reacts with Ca(OH)2 produced during the hydration of alite to form calcium silicate hydrate gel. There has been a long dispute about the nature and composition of this gel phase, but it is now believed to be closely similar to that formed during the hydration of ordinary Portland cement but with a slightly lower Ca/Si ratio [5]. PFA is now used in large quantities in the construction industry. Not surprisingly, the early strength development of Portland-fly ash cement blends is slower than in OPC, but the ultimate strength attained can be similar. OPC/PFA systems can be more durable than OPC systems when initial curing regimes are at higher temperatures. The porosity and permeability of the cementitious product can be low, leading to good durability. The consumption of calcium hydroxide leads to a product that is less susceptible to carbonation. An added advantage in the use of PFA in composite cements is that many of the particles exist as cenospheres. The spherical nature of the PFA particles enables them to act as tiny ball bearings and they can impart favourable rheological properties into grouts made from this material. Significant improvements in fluidity and fresh grout stability compared with other cementitious materials, produced with similar water contents, have been found. Moreover, PFA particles can replace some of the fine aggregate, which allows ready movement of larger particles giving a mix with improved flow characteristics. This can allow a reduced water:solids ratio and hence an improvement in strength [6]. 3.4 Silica fume (SF) Silica fume, or microsilica, is a by-product from the production of silicon or silicon-containing alloys. SF is an amorphous solid with a high silica content (Table 2). It consists of very small particles, normally less than 1 micron in size. Because of these small particles, it displays very high reactivity at early

ages and is found to accelerate the hydration of Portland cement [7]. Addition rates of SF are normally low, up to 10% w/w of the main cementitious material. While PFA affects the rheology of the cement paste in a positive way, SF can have an adverse effect due to its extremely small particle size and as a result, is generally used with the addition of organic superplasticising water-reducing additives which unfortunately have limitations in the nuclear industry. Attention is now being given to possible combined incorporation of SF and PFA, the former to affect the early strength development and the latter to contribute to the ultimate strength. Such systems might require a lower water to solids ratio to give the same workability, but provide increased strength and durability. 3.5 Metakaolin When kaolinite, or china clay, is heated, it loses its structural water (dehydroxylation) between 450 and 800oC, crystallising as mullite, quartz and cristobalite above 980oC. The product formed at intermediate temperatures is known as metakaolin (MK), its properties varying with the temperature and duration of heating. MK calcined at 730oC for 6 hours is an amorphous product that acts as an excellent pozzolanic material when incorporated in a composite cement [8]. Because of its relatively high Al2O3 content (Table 2), a new type of cementing phase, stratlingite, C2ASH8, is formed as a hydration product. Again, addition rates are normally low, up to 10% w/w of the main cementitious material 3.6 Limestone dust International cement standards, such as the European standard ENV 197-1, now permit the incorporation of up to 5% addition of a secondary cement or filler to Portland cement clinker [9]. The most common such filler is fine limestone, readily available at cement manufacturing works. Acceleration of setting can occur and a reduction in bleeding has been found when 5% limestone is incorporated [7]. This shows that limestone acts as more than a mere inert filler, making a positive contribution to the overall strength of the hydrated cement. As limestone is softer than clinker, it grinds preferentially. A most important consequence of this is that it is now possible to grind the modern highly reactive Portland cement clinker to a coarser powder, thereby reducing the rate of hydration and early strength development, without causing increased bleeding and segregation of the concrete mixes, since the limestone filler provides the fine material necessary for this desirable outcome. The incorporation of 5% limestone filler enables overall cement fineness to be maintained and 28 day strengths to be controlled at the required levels. 4. ALTERNATIVE CEMENTS 4.1 Calcium aluminate cements (CAC) CAC was first developed in France in 1914 as an alternative construction material to Portland cement for use where acidic


ground waters, especially those containing sulfate ions, might lead to durability problems. Because this cement has a higher aluminia content than that of Portland cement (Table 2), it is often called high alumina cement, but it has a much lower alumina content than the more refractory CACs discussed below, hence the term calcium aluminate cement is to be preferred. The modern form of the original cement is known as Ciment Fondu, indicating its mode of production, by melting the raw materials together. The raw materials are in fact limestone and bauxite, which usually incorporates a significant amount of iron. Although Ciment Fondu was developed because of its resistance to sulfate attack, its rapid hardening and refractory properties were soon discovered and exploited. The setting time of Ciment Fondu is very dependent on temperature [10], but is generally comparable with (sometimes longer than) that of Portland cement. Subsequently it gains strength rapidly, so much so that its compressive strength after 24 hours of curing is comparable with that of OPC after about 28 days. Ciment Fondu has significantly better heat resisting properties than those of Portland cement, but its temperature range is restricted by liquid formation associated with the presence of iron impurities. Improved refractory cements were, therefore, developed to allow concretes to be submitted to higher temperatures, by using purer starting materials, often Al(OH)3 from the Bayer process. The alumina content of the cement was increased from about 39% in Ciment Fondu to 70-80% in various commercial products sold as refractory cements. The use of for encapsulating radioactive wastes has been examined previously [11,12]. CACs have potential advantages when used to encapsulate certain toxic and radioactive wastes, because they do not liberate portlandite, Ca(OH)2, and hence the pore solution has a lower pH, although it is still alkaline. There are, however, two problems that must be overcome. The first is to avoid (or design for) the volume changes that accompany the conversion of the metastable hydrates formed initially (CAH10 and C2AH8) into the thermodynamically stable hydrogarnet, C3AH6. The second is that the rapid development of strength is accompanied by considerable evolution of heat. One interesting way to reduce both of these properties is to replace some of the CAC by BFS. Majumdar et al. [13] showed that with concrete based on equal amounts of Ciment Fondu and BFS cured at either 20oC or 38oC, the compressive strength increased continuously over a period of one year, in contrast to control samples made without any BFS content, which showed decreases in strength associated with conversion. Stratlingite, C2ASH8, was formed in the mixes incorporating BFS. It would be of interest to extend Majumdar’s studies to include much higher contents of BFS and to determine the heat evolution, hydrate formation and microstructure development of such composite cements, with a view to assessing them for possible use to immobilize radioactive wastes at a lower pH. 4.2 Calcium Sulfoaluminate (CSA)

Composite cements incorporating BFS or PFA are often referred to as low energy cements. Although energy saving is desirable, an even more important consideration is the reduction in CO2 emissions associated with Portland cement manufacture. For every ton of OPC produced, approximately 1 ton of carbon dioxide is released to the atmosphere, partly from the burning of fossil fuels and partly from the decomposition of limestone. An alternative approach is to use such waste materials in the kiln together with limestone, leading to cements of radically different chemical compositions, notably containing little or no alite, C3S, content. Such a product is manufactured on a huge scale (>1Mtons p.a.) in China and is known as CSA or SAB (sulphoaluminate-belite) cement [14,15]. A typical chemical analysis is shown in Table 2. To make such cements, a mixture containing limestone, PFA and gypsum (or anhydrite) is heated in a rotary kiln to about 1300oC. The product contains � ’C2S and SAC 34 , often referred to as Klein’s compound. In the presence of anhydrite or gypsum, this phase hydrates rapidly to form ettringite, Ca6[Al(OH)6.6H2O]2(SO4)3.14H2O. At their best, these cements have outstandingly good properties, including high early and ultimate compressive strength, good durability, and setting times and expansion that can be controlled by adjustment of the amount of added gypsum. These volume changes can be controlled to some extent to give a range of shrinkage-compensated or expansive cements. While isothermal conduction calorimetry curves for such cements have not been published, the rate of heat evolution of the neat cements is likely to be rapid, although some moderation with replacement materials such as BFS may be possible. The pH of the internal pore solution is likely to be lower than that of Portland cement as these cements do not produce Ca(OH)2 . The use of these cements and the formation of ettringite to immobilise metal wastes was reviewed by Klemm[16] and their potential for binders in waste encapsulation discussed by Odler [17] 4.3 Magnesia-based Cements There are a wide range of inorganic cements based on the reaction of magnesia, MgO, available. None is currently produced in large quantities, but some do have important roles in niche markets. These cements should be considered for the processing of special radioactive waste streams that are not conducive to treatment in more conventional cement systems as their hydration chemistry is significantly different. The oldest such system is know as Sorel cement and dating back to 1867. It is based on the reaction between magnesium oxide powder and a solution of magnesium chloride to form hydrated magnesium oxychlorides [18]. This is a hard material, but is susceptible to moisture attack and dimensional instability. It can, however, be protected in a paraffin wax and is used as a flooring material. An alternative system based on the use of magnesium sulfate instead of magnesium chloride is also known and used in the food processing industry.


A related but somewhat different class of inorganic cements form the basis of glass ionomer cements used in dental and medical applications, as well as being suitable for rapid repairs to damaged concrete, for example to concrete pavements and airport runways. They rely on acid-base reactions, such as that which occurs between dead burnt magnesium oxide and an acid phosphate source, e.g., ammonium dihydrogen phosphate, NH4H2PO4 [19, 20], or aluminium orthophosphate, Al(H2PO4)3 [21]. The products are fast setting with high early strength development, but the reactions can be controlled to some extent by the use of retarders and various grades of filler, e.g. sand. The product is effectively a composite material with grains of unreacted oxide particles surrounded by a matrix of hydration products. The magnesia can be replaced, partially or fully, by other basic and amphoteric oxides. These materials are sometimes known as chemically bonded ceramics [22,23]. Considerable work has been conducted at Argonne National Laboratory in developing these systems for encapsulating radioactive wastes and the technology is being commercialised. Because of the ionic nature of the setting mechanism, which gives rise to a rapid reaction with considerable heat evolution, this system is unlikely to be extensively applicable to radioactive waste disposal. It may, however, be useful for the treatment of specific waste streams that require immobilisation in a product with a very much lower pH than that associated with calcareous cementing systems. The form of the magnesia is very important for all these cements. As the acid-base cements form very rapidly, the magnesia used is fired at a high temperature, or even fused, prior to use to retard the rate of reaction. In Sorel cement, the magnesia is formed at a relatively low temperature from the decomposition of magnesium hydroxide or magnesium carbonate to yield the oxide in a much more reactive state. This is also the case for a new cement that is currently being evaluated in many parts of the world. It originates from Australia, where it is described as an environmentally friendly cement to support sustainable construction [24]. Magnesia is formed at about 650oC with a high surface area, rendering it very reactive. It can then be used on its own or blended with Portland cement and a pozzolanic material such as PFA, microsilica or rice husk ash. It is claimed that the system can accommodate high levels of waste materials, including organic wastes such as sawdust, rubber and plastics. By varying the proportions of OPC and MgO used, the pH of the cement matrix can be adjusted. These properties would be very desirable in the treatment of toxic wastes and should also be evaluated for application to nuclear waste streams. Caution must be expressed at this stage, however, because there are large volume changes associated with the reaction of magnesia to form brucite, Mg(OH)2, so dimensional stability may be a serious problem. 4.4 Geopolymers Alternative cementing systems based on aluminosilicate polymers (inorganic polymers sometimes known as Geopolymers) were developed by Davidovits [26-28] and are

proving suitable for cements needed in chemically demanding situations. The ingredients are a soluble alkali silicate and a reactive aluminosilicate precursor such as metakaolinite. On-going work has shown that blastfurnace slag [28,29] or PFA [30] can also be used. Strength development is more rapid than in OPC systems and accompanied by lower heat output, (typically ~ 116-247 kJkg-1 depending on starting materials [27] giving a product with moderate strength, (15-20MPa at 6 hours and ~ 80MPa at 24 hours) [26]; low permeability (10-8mmsec-1) [25], moderate pH (9-10) and high thermal stability (samples have been tested to 800oC). The range of materials that can be used indicates flexibility to incorporate a range of waste materials in a medium which ultimately does not rely on a hydrate matrix for strength, as the binder is a 3-D amorphous aluminosilicate. Work by van Jaarsveld and van Deventer [31] and Davidovits [26] indicates the likelihood of immobilising radioactive waste with these materials. Recent work at ANSTO [32] also indicates that Cs and Sr are retained in the system. Eurotech Ltd are marketing a version of geopolymers, Ekor, to encapsulate waste at Chernobyl [33]. Further work is needed to understand the hardening mechanism as well as the long term durability of such a new system. The lack of Ca present means this system does not suffer from effects such as carbonation or acid leaching. Industrial trials show this material can be satisfactorily manufactured on a large scale. 5. LONG-TERM STABILITY OF GROUTS AND

CEMENTED PRODUCTS Some of the ILW materials will continue to react with any free water that is inevitably left in the pore structure of the cement grout. Notable among these are corrosive metals, such as aluminium and magnesium, which may release hydrogen gas associated with the formation of metal hydroxides. The hydroxides formed have a low density and occupy more volume than the metals, leading to an expansive corrosion product from the waste material, hence applying stress to the surrounding grout. To determine the optimum level of loading that is safe for long-term storage of the waste, it is necessary to be able to quantify this problem It is important, therefore, to obtain accurate knowledge of the amount of reactive water remaining in the system after the bulk of the hydration has been completed. The rate and extent of corrosion in such waste forms will be determined by the state in which the water exists. The corrosion reactions will slow down as free water is consumed and build up of protective layers of corrosion products induce passivation of the metal.

Since many hydration reactions, especially in composite cements, continue slowly for many months or even years, the gel, or pore, water within the system will be less readily available for continued corrosion. It is important, therefore, to determine the amounts of bound, gel and capillary (free) water that are present at various ages. Ultimately then, the total amount of corrosion that will take place should be calculable.


Such a study will provide realistic data to use in modelling the long-term product behaviour and in turn predicting the long-term product integrity during extended storage. 6. CONCLUDING REMARKS There are a range of cementing systems available that have been reported as suitable to encapsulate or immobilise wastes produced from use of nuclear materials. Each has particular advantages as the nuclear industry moves to treat all the wastes including those from earlier operations. While OPC based systems have been used successfully for current operating wastes, the legacy wastes offer a different challenge. As storage will be long term, the properties of the cemented waste forms, both new and existing, need to be studied further to ensure that the waste is effectively immobilised. This includes studies of the effects of water on metal corrosion. Development of suitable models which can predict these properties is of key importance to ensure that these systems will provide sufficient integrity for future generations. ACKNOWLEDGMENTS We are grateful to BNFL for financial support and to Wendy Dutton for helping in the preparation of the final manuscript.


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