Immobilisation of Radioactive Waste in Cement

CHAPTER 17

Immobilisation of Radioactive Waste in Cement Contents 17.1 17.2 17.3 17.4 17.5 17.6 17.7 17.8

Wasteforms Cementitious Wasteforms Hydraulic Cements Cement Hydration Phase Composition of Hydrated Cements Cementation of Radioactive Wastes Modified and Composite Cement Systems Alternative Cementitious Systems 17.8.1 Calcium Aluminate Cements 17.8.2 Calcium Sulphoaluminate Cements 17.8.3 Phosphate Cements 17.8.4 Geopolymers 17.8.5 Hydroceramics 17.8.6 Application of Alternative Cementitious Systems in Waste Immobilisation 17.9 Cementation Technology 17.10 Acceptance Criteria References

271 272 273 276 282 283 284 287 289 290 291 292 292 293 296 299 302

17.1 WASTEFORMS Waste immobilisation converts raw waste, usually containing mobile contaminants, into a solid and stable form termed a wasteform. The properties of the wasteform enable it to be handled, stored and disposed of safely and conveniently, significantly reducing potential release of radionuclides into the environment. For long-term storage and disposal waste immobilisation should be an irreversible process, which avoids release of contaminants from the matrix during storage and disposal. Choosing a suitable wasteform for nuclear waste immobilisation is difficult and durability is not the sole criterion. In any immobilisation process where radioactive materials are used, the process and operational conditions can become complicated, An Introduction to Nuclear Waste Immobilisation DOI: https://doi.org/10.1016/B978-0-08-102702-8.00017-0

© 2019 Elsevier Ltd. All rights reserved.

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particularly if operated remotely and equipment maintenance is required. Therefore priority is given to reliable, simple, rugged technologies and equipment, which may have advantages over complex or sensitive equipment. A variety of matrix materials and techniques is available for immobilisation (IAEA, 2017; NRC, 2011). The choice of the immobilisation technology depends on the physical and chemical nature of the waste and the acceptance criteria for the long-term storage and disposal facility to which the waste will be consigned. A host of regulatory, process and product requirements has led to the investigation and adoption of a variety of matrices and technologies for waste immobilisation. The main immobilisation technologies that are available commercially and have been demonstrated to be viable are cementation, bituminisation and vitrification. These will be described in some detail in Chapters 17
19. Immobilisation technologies can be separated into thermal methods which require significant heat input, such as vitrification and non-thermal methods done at or close to room temperature such as cementation (Jantzen, Ojovan, & Lee, 2013). A number of new immobilisation technologies are under development (Lee, Gilbert, Murphy, & Grimes, 2013) including non-thermal methods based on novel cements and geopolymers which will be described in this chapter and thermal methods based on cold crucible melting which will be described in Chapter 19, Immobilisation of Radioactive Wastes in Glass. Other new techniques are discussed further in Chapter 20, Ceramics and Novel Technologies.

17.2 CEMENTITIOUS WASTEFORMS This chapter considers waste cementation, which is based on the use of hydraulic cements. Hydraulic cements are inorganic materials that have the ability to react with water under ambient conditions to form a hardened and water-resistant product. The most common cements are those based on calcium silicates, such as the Portland cements. Cementation of radioactive waste has been practised for many years basically for immobilisation of low and intermediate level waste. The prominent advantages of immobilisation by cementation are that (Abdel Rahman, Rahimov, Rahimova, & Ojovan, 2015; Bart, Cau-di-Coumes, Frizon, & Lorente, 2012; Glasser, 2011; Spence & Shi, 2004): • •

Cements are inexpensive and readily available. It is simple and low cost processing at ambient temperature.

Immobilisation of Radioactive Waste in Cement

• • • • • • • • •

273

The cement matrix acts as a diffusion barrier and provides sorption and reaction sites. It is suitable for sludges, liquors, emulsified organic liquids and dry solids. The wasteforms have good thermal, chemical and physical stability. The alkaline chemistry ensures low solubility for many key radionuclides. The wasteform is non-flammable. The wasteform is not degraded by radiation and provides good selfshielding. The wasteform has good compressive strength which facilitates handling. It is easily processed remotely. It is flexible and can be modified for a particular wastestream.

Ordinary Portland cement (OPC) is the most common type of cement used for immobilising liquid and wet solid wastes worldwide. Several OPC-based mixtures are currently used to improve the characteristics of wasteforms and overcome the incompatibility problems associated with the chemical composition of certain types of radioactive waste. Composite cement systems (Section 17.7) may use additional powders such as blast furnace slag (BFS) and pulverised fuel ash (PFA). These offer cost reduction, energy saving and potentially superior long-term performance. As well as the wasteform matrix, OPCs will be used in structural components of any repository (such as walls and floors) and are potential backfill materials so an understanding of their durability in an underground environment even without waste is important.

17.3 HYDRAULIC CEMENTS Hydraulic cements set and harden when mixed with water. Portland cement is an example of a hydraulic cement which sets normally in air as well as when emplaced underwater. This property is used in entombment, for example, in situ decommissioning operations (IAEA, 2013; Laraia, 2012). It is produced by pulverising clinkers consisting essentially of hydraulic calcium silicates with calcium sulphate (gypsum) as an interground addition. Clinkers are produced by heating clay materials with lime at high temperatures ( . 1500°C) to form nodules (diameters 5
25 mm). The low cost and wide availability of limestone and naturally occurring silica sources make Portland cement one of the lowest cost materials used worldwide. The manufacture and composition of Portland

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cements, hydration processes, and cement chemical and physical properties have been extensively studied (e.g., Taylor, 1997). Portland cement comprises on oxide basis lime (63
66 wt.% CaO), silica (21
24 wt.% SiO2), alumina (4
8 wt.% Al2O3), and ferric oxide (1
6 wt.% Fe2O3), but also contains small quantities of magnesia (0.5
5 wt.% MgO), sulphur trioxide (1
3 wt.% SO3) and other oxides introduced as impurities from the raw materials used in its manufacture. Major phases present in unhydrated Portland cement powder are: • • • • • •

alite (Ca3SiO5
tricalcium silicate, ye’elimite); belite (Ca2SiO4
β-dicalcium silicate, larnite); aluminate [(Ca3Al2O6)
tricalcium aluminate (C3A)]; ferrite [Ca4(Al, Fe)2O7
tetracalcium aluminoferrite (C4AF), brownmillerite]; lime (CaO
lime); and calcium sulphate (CaSO4
gypsum, anhydrite).

Table 17.1 shows the compositions and abbreviations for these compounds. Early hydration of cement is principally controlled by the amount and activity of C3A, balanced by the amount and type of sulphate interground with the cement. C3A hydrates rapidly and influences the early bonding characteristics. Abnormal hydration of C3A and poor control of its hydration by sulphate can lead to problems such as flash set, slump loss and cement
admixture incompatibility. Based on this information a number of cements were designed with different durabilities or high-early strengths. American Society for Testing and Materials (ASTM) specifications are broadly accepted as the basis for most national specifications. The ASTM recognises five cement types shown in Table 17.2. Type I, termed normal Portland cement or OPC, is the most commonly used when the special properties of the other types are not Table 17.1 Principal clinker minerals of cement Compound

Oxide composition

Abbreviation

Tricalcium silicate Dicalcium silicate Tricalcium aluminate Tetracalcium aluminoferrite Lime Calcium sulphate

3CaO  SiO2 2CaO  SiO2 3CaO  Al2O3 4CaO  Al2O3.Fe2O3 CaO CaO  SO3

C3S C2S C3A C4AF C C$

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Table 17.2 American Society for Testing and Materials (ASTM) C150 classes of Portland cements, their mineral composition (wt.%) and use Type C3S C2S C3A C4AF Others Use

I

50 24 11

8

7

II

42 33

5

13

7

III IV

60 13 26 50

9 5

8 12

10 7

V

40 40

4

7

7

General purpose cement, when there are no extenuating conditions Aids in providing moderate resistance to sulphate attack When high-early strength is required When a low heat of hydration is desired (in massive structures) When high sulphate resistance is required

required; for example, when it is not subject to sulphate attack from waste, or where the heat generated by the hydration of the cement will not cause an unacceptable rise in temperature. Type I cements typically have compressive (crushing) strength after 7 days . 19 MPa measured on 50-mm mortar cubes. Type II, modified Portland cement with reduced levels of C3S and C3A, has a lower rate of hydration than type I and generates heat at a slower rate. It also has improved resistance to sulphate attack and is intended for use where added precautions against moderate sulphate attack are important. Type III, high-early strength cement with high C3S and lower C2S levels, develops strength rapidly as a result of its high C3A and tricalcium silicate content. This rapid strength development, however, is accompanied by a high rate of heat production, which may preclude the use of type III cement for massive waste/cement monoliths. Type IV, low heat cement with low levels of C3S and C3A and hence high level of C2S, can be used primarily for massive waste/cement monoliths. The low rate of heat production in this cement type is attributable to its high dicalcium silicate content and corresponding low tricalcium silicate and C3A contents. Type V is sulphate-resistant cement due to its low C3A content. It is a special cement intended for use in monoliths exposed to severe sulphate action. It has a slower rate of strength gain than OPC. Portland cement types I, II and III are normally used in the immobilisation of radioactive waste. While type II has enhanced resistance to sulphate attack, sodium sulphate solutions have been successfully solidified, with all three types having roughly the same loadings. Aqueous waste

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containing boric acid can be solidified if an alkaline material (e.g., slaked lime or NaOH) or sodium silicate is added to the cement as well as when the alkalinity of the solution is increased to pH 8 to 12. Types I, II and III have been shown to work with such additives. Type III is preferred for boric acid type liquid waste because of the rapid curing characteristics of this cement (Section 17.6), which in many cases counteracts the retarding effects in hydration induced by boric acid (Section 17.8). The European Community classification of cements includes both cements and blended cements totalling 17 distinct classes whereas the British standard on cements BS EN 197
1:2000 specifies 27 products in the family of common cements including Portland-composite, blast furnace, pozzolanic and composite cements. In the United Kingdom the only cements made are type I and V. Type III is available as finely ground type I. In fact modern cements have gradually increased in C3S at the expense of C2S, so that OPC often corresponds better to the analysis shown in Table 17.2 for type III rather than type I.

17.4 CEMENT HYDRATION When the cement powder is mixed with water, the hydratable phases undergo a series of chemical reactions that eventually lead to hardening. Clinker solids react rapidly with water with evolution of heat so that within seconds of mixing the water becomes strongly alkaline. Metaloxygen bonds in anhydrous clinker minerals are disrupted because of the high content of hydroxide ions. The same processes occur in glassy supplementary materials added to cement such as fly ash or slag although at a slower rate. As a result, hydration progresses by attack of water on the surfaces of grains. The most soluble components are calcium, liberated from clinker, alkali present in soluble form and sulphate, from calcium sulphate. However, the solubilities of the main cement compounds are incongruent; therefore, the ions present in the solid do not dissolve at the same rate to maintain the stoichiometry of the dissolving solids. Thus the ratio of Ca/Si in solution at all stages of hydration is much greater than the ratio in C3S and C2S. As a result, dissolution inevitably leaves a solid silica
rich product which tends in part to accumulate as a coating on the surface of reactant grains. The initial rapid heat evolution caused by wetting and initial dissolution is followed by a dormant period, which can last from minutes to several hours, during which heat evolution is slow. Its duration is controlled by formation of surface films on clinker grains and

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277

its eventual disruption. Water diffuses through this film of hydrates and reaction continues by disruption of the protective layer and direct attack of water on mineral grains. The duration of the dormant period is normally a few hours during which the mix remains fluid, allowing it to be transported and emplaced. Hydration then occurs intensively and the mix loses fluidity. Setting occurs because of accumulation of solid hydrate products which fill the space between grains. The reasons of hydration resumption are not fully understood although it is believed that the initial insoluble products of dissolution form a semi-permeable envelope on cement grains, which is then disrupted by osmotic pressure upon exposure of fresh surfaces to water. As solid hydration products accumulate, they increasingly fill space between and around grains of solid reactants. Liquid water is increasingly bound into solids while the remaining aqueous phase is increasingly subdivided by the growth of solid hydration products. Eventually the remaining aqueous phase becomes trapped in isolated pores. This pore water is referred to as cement water and is characterised by high pH due to its intimate contact with cement solids. The evolution of the microstructure of a cement
water mixture is shown schematically in Fig. 17.1 and a summary of the early stages of cement hydration kinetics is shown in Fig. 17.2. Hydration reactions are conventionally divided into three periods: 1. A dormant period that usually lasts for minutes to hours, 2. Setting that occurs over hours, and 3. Hardening that takes many days or longer to complete.

Figure 17.1 Schematic of cement hydration with setting of a solid cementitious material.

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Figure 17.2 Schematic of cement hydration kinetics.

The principal hydration products formed are calcium hydroxide (CH), ettringite (a hydrous calcium aluminium sulphate) and the calcium silicate hydrate gel also termed tobermorite gel (CSH). CH and ettringite are intimately mixed with the poorly defined CSH gel, which forms the major constituent of the matrix phase. The formation of CH and ettringite begins early in the course of hydration and electron micrographs disclose the characteristic needles of ettringite and the plate-like morphology of CH develops within minutes of adding water. The bulk of the CSH only forms after the end of the dormant period. Because the early-formed phases can grow into water-filled space, they often have well-developed crystal morphologies. CSH mostly develops later and morphological features reported are fibrillar and crumpled foils and mainly result from CSH growing without restraint into liquid-filled pores. It may appear to be massive and featureless in electron micrographs and morphological features do not appear until resolution is increased to the nanometre range where it can be seen that it consists of strongly adherent platelet shaped 5
20 nm particles. The exceedingly fine scale implies a high surface area (B50
200 m2/g) with high capacity for sorption of charged species. CSH is the major contributor to the properties of the final hardened product, defining permeability, diffusivity and thermodynamic stability. Hydration reactions are complex and not stoichiometrically rigorous because of variations both in the products formed and their compositions. The mechanism for hydration is the conversion of isolated orthosilicate

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units SiO442 within the anhydrous cement clinker to disilicate units Si2O762. As hydration proceeds the silicate ions polymerise to form chains of higher polymeric form. Polymerisation corresponds to the setting and curing of the paste. Basically, the two calcium silicates (C3S and C2S) that constitute about three quarters of Portland cement by weight react with water to produce two new compounds: CH and CSH, often called tobermorite gel. The hydration reactions are as follows: ðC3 SÞ: 6CaOU2SiO2 1 6H2 O 5 3CaOU2SiO2 U3H2 O ðCSH Þ 1 3CaðOHÞ2 1 114 kJ=mol ðC2 SÞ: 4CaOU2SiO2 1 4H2 O 5 3CaOU2SiO2 U3H2 O ðCSH Þ 1 CaðOHÞ2 1 43 kJ=mol

(17.1)

(17.2)

The C3A and C4AF combine with considerably more water on a molar basis than do the calcium silicate compounds. Hydration reaction are as follows: ðC3 AÞ: 3CaOUAl2 O3 1 30H2 O 1 3CaSO4 U2H2 O 5 3CaOUAl2 O3 U3CaSO4 U32H2 O ðettringiteÞ 1 200 kJ=mol (17.3) ðC4 AFÞ: 4CaOUAl2 O3 UFe2 O3 1 10H2 O 1 2CaðOHÞ2 5 3CaOUFe2 O3 U6H2 O 1 3CaOUAl2 O3 U6H2 O 1 100 kJ=mol (17.4) Modern Portland cements are characterised by B80% or more of the total heat liberation during the first 28 days of curing with 20% heat released slowly over months or years. A summary of the heat evolution during early stages of cement hydration is shown in Fig. 17.3. Portland cements typically reach 70%
80% hydration by 28 days and .95% hydration by 1 year at 20°C. C3S and C2S have the most influence on long-term structure development. Aluminates may contribute to formation of compounds such as ettringite, which can lead to fracture of the concrete. Cements high in C3S hydrate more rapidly and lead to higher early strength. However, the hydration products formed make it more difficult for late hydration to proceed resulting in an ultimate lower strength. Cements high in C2S hydrate much more slowly, at 20°C taking approximate 1 year to reach a good ultimate strength, leading to a denser

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100 C3A C3A+CSH2

Degree of hydration (%)

80

C3H 60

40 C2H 20

0

20

40

60 Time (days)

80

100

Figure 17.3 Schematic of heat evolution during cement hydration.

ultimate structure and a higher long-term strength. Increased curing temperature effects are important and need to be accounted for in the hydration progress of real systems. Normal cements and concretes gradually stiffen with time and no longer flow within 2
6 hours after mixing. This point is known as initial set, which is characterised using a weighted Vicat needle to measure depth of penetration of the needle at a fixed load. The Vicat needle is used to define both the initial and the final set, which corresponds approximately to the onset of strength gain. Final strengths of cementitious materials vary with cement type, water-to-cement ratio, cement content and time. Cement composition and fineness play a major role in controlling concrete properties. The average finenesses of cement range from 3000 to 5000 cm2/g. Greater fineness increases the surface available for hydration, causing greater early strength, but also more rapid heat generation. Coarse cement produces pastes with higher porosity than those produced by finer cement. Unconfined compressive strengths for commercially available ready mix concretes with low cement content B275
325 kg/m3 are typically in the range 32.5
50 MPa after 28 days. Higher compressive strengths in the range 60
120 MPa are readily achievable at higher cement content and with high-strength aggregates. Note that compressive strength in nuclear waste immobilisation is of secondary importance compared to leaching rates, water conductivity and porosity. Development of the microstructure of hydrated cement occurs after the concrete has set and continues for months (and even years) after

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281

placement. OPC is typically 95%
98% hydrated after 12 months and comprises an aqueous phase, which is largely confined to filling pores less than 1 μm in radius (pore water), and a heterogeneous paste matrix (Bensted & Barnes, 2001; Taylor, 1997; Winter, 2012). The porosity ϕ of a porous material such as hydrated cement paste is defined as the ratio of the non-solid volume such as pores filled by water or gas Vp (m3) to the total volume of material Vt (m3): φ5

Vp Vt

(17.5)

The porosity of a cement paste is a function of time (see Fig. 17.2) with a typical ultimate porosity from 30% to 40%. An important physical property of a porous medium is its conductivity K (m/s), which determines the rate of flow of a fluid (water) through it. The rate of flow of water Q (m3/s) through a bed of surface S (m2) made of a porous medium is described by Darcy’s law: Q 5 KS

ðH 1 hÞ H

(17.6)

where H is the thickness of the bed (m) and h is the height of the water on the top of the bed (m). As the conductivity of hydrated cement pastes depends on porosity the conductivity is a function of time. Typical data on conductivity of hydrated OPC as a function of curing time are given in Table 17.3. The strength and chemical resistance of a cement paste is a strong function of the water/cement (w/c) ratio: w=c 5

MH2 O Mcement

(17.7)

where MH2 O is the mass of water used for hydration (kg) and Mcement is the mass of cement powder (kg) used to produce the cement paste. In making concrete, more water has to be used than required for hydration to make a workable and flowable mixture. High w/c ratios result in large void volume which affects mechanical and chemical durability of the concrete. For waste encapsulation cement paste permeability should be as low Table 17.3 Conductivity of cement paste K (m/s)

Age (days) K (m/s), 10214

Fresh 2 3 107

5 4 3 103

6 1 3 103

8 4 3 102

13 50

24 10

Ultimate 6

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140

Conductivity,·10–14 (m/s)

20 00 80 60 40 20 0 0.2

0.3

0.4

0.5 W/C

0.6

0.7

0.8

Figure 17.4 Conductivity of hydrated cement paste as a function of w/c ratio. Table 17.4 Phase composition of hydrated cement paste Phase

Description

Notation

Ettringite Monosulphate Hydrogarnet Portlandite Tobermorite gel

3CaO  Al2O3  3CaSO4  32H2O 3CaO  Al2O3  CaSO4  12H2O Ca3Al2(OH)12
Ca3Al2Si(OH)8 Ca(OH)2 Amorphous, typical Ca/Si molar ratio 1.6
1.9

AFt AFm C3AH6
C3ASH4 CH CSH

CH, Calcium hydroxide; CSH, calcium silicate hydrate gel.

as possible. The relationship between permeability and the w/c ratio of mature Portland cement pastes is shown in Fig. 17.4, illustrating the drawback of higher w/c ratio. High conductivity (K) results in increased leachability (e.g., NRi) and deterioration of the concrete when exposed to aggressive groundwaters.

17.5 PHASE COMPOSITION OF HYDRATED CEMENTS The phase content and microstructure of the cement hydrates determine the mechanical behaviour and durability of the resulting concrete. Table 17.4 lists the solid phases in hydrated cement.

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Derivative wt.%(%/min)

0.2 0 –0.2 –0.4 3

–0.6

Monosulphate

–0.8 –1

1 2

–1.2 –1.4

4 Ca(OH)2 CSH Water loss

0

100

200

300

400 500 600 Temperature (ºC)

5 CaCO3 OPC OPC+12 wt.% BaSO4 OPC+36 wt.% BaSO4 OPC+60 wt.% BaSO4 BaSO4

700

800

900

1000

Figure 17.5 TGA curves of an OPC
BaSO4 system (w/s 5 0.53) cured at 40°C for 28 days. OPC, Ordinary Portland cement; TGA, thermal gravimetric analysis. Courtesy Oday Hussein, University of Sheffield, United Kingdom.

Table 17.5 Temperature ranges of dehydration/decomposition of cement phases Phase

Temperature range (°C)

Tobermorite gel (CSH gel) Ettringite
3CaO  Al2O3  3CaSO4  32H2O Monosulphate
3CaO  Al2O3  CaSO4  12H2O Portlandite
Ca(OH)2 Calcite
CaCO3

110
120 130
140 180
210 450
500 600
800

CSH, Calcium silicate hydrate gel.

Each of the phases formed during cement hydration influences its structure and properties. By far the most important is the tobermorite gel, which is the main cementing component of concrete. Setting and hardening behaviour, strength and dimensional stability depend primarily on the CSH. The remaining three phases are crystalline and correspond more closely to defined stoichiometry. Portlandite typically comprises B20%
25% of the fully hydrated paste whereas AFm and AFt phases comprise B5%
10% of the paste. Thermal gravimetric analysis detects dehydration and decomposition of cement phases (Fig. 17.5), which occur within distinct temperature ranges (Table 17.5).

17.6 CEMENTATION OF RADIOACTIVE WASTES The practice of encapsulating radioactive waste in OPC began during the early years of the nuclear industry (IAEA, 1993). This was primarily due

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Table 17.6 Reactions occurring between cement and waste components Waste component

Reaction

Soluble borates, Pb, Zn

Precipitated salts coat cement grains or amorphous precipitates inhibit hydration Complexing agents: EDTA, Interfere with Ca availability by complexation. sugar, citric acid Retard hydration Flocs Uncertain action. Retard hydration Electropositive metals Evolve H. Reaction accelerated by OH2. Solid reaction products, oxides/hydroxides are expansive Organic ion exchangers Take up water in high pH matrices and expand EDTA, Ethylenediaminetetraacetic acid.

to its low cost, availability and compatibility with aqueous waste. It was soon realised, however, that some wastes interact with the cement and retard the hydration reactions (Table 17.6). The impact of cations on cement hydration reactions follows the series: Ca21 . Ni21 . Ba21 . Mg21 . Fe31 . Cr31 . Co21 . La31 . NH4 1 . K1 . Li1 . Cs1 . Na1 . Cu21 . Zn21 . Pb21 (17.8) whereas anions follow the series: OH2 . Cl2 . Br2 . NO3 2 . SO4 22 cCH3 COO2

(17.9)

To overcome deleterious cement
waste interaction effects, one or more additives may be used and such mixtures are termed modified cements. Several of the more successful modified Portland cements have been commercialised.

17.7 MODIFIED AND COMPOSITE CEMENT SYSTEMS OPC cements can be modified by using a range of additives. These may be defined as additives (at a level of B5%) such as gypsum which acts to retard hydration, replacements (at high levels up to 90%) such as BFS or PFA in blended or composite cements or as admixtures (at levels of B1%) such as superplasticisers. The main cement modifiers (Tables 17.7 and 17.8) include slaked lime, sodium silicate, natural pozzolans and BFS.

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285

Table 17.7 Cement modifiers Nature of modifier

Purpose

Polar, high molecular weight, water soluble organics (superplasticisers) Soluble organics

Reduce grouta viscosity

Slag, fly ash, silica fume, natural pozzolans

Accelerate or retard set, antifreeze, corrosion inhibitors Reduce heat evolution, improve fluidity and decrease permeability; may increase strength after long times

a Grout is a paste mixture of cement powder and water that solidifies. Mortar is grout containing sand. Concrete is mortar with added aggregate.

Table 17.8 Modified Portland cements and their use in waste immobilisation Type

Additive

Waste stream

Additive function

Masonry cement Portland sodium silicate cement Portland pozzolanic cement Portland blast furnace slag cement

Lime Boric acid Adjusts pH Sodium silicate Organic liquids Accelerates set, reduces porosity Reactive silica Sulphate Reacts with Ca(OH)2, reduces porosity Slag Sulphate Reacts with Ca(OH)2 latently hydraulic

Modified or composite cements are used to immobilise waste containing specific components and contaminants. Table 17.8 gives data on modified Portland cements, additives and waste streams for which they are used. Masonry cement is a mixture of Portland cement and slaked lime Ca (OH)2. When used for radioactive waste encapsulation, Portland cement and slaked lime are typically combined in equal proportions. In the presence of water, the extremely high alkalinity induced by the slaked lime induces a rapid set. Masonry cement is particularly useful for solidifying wastes such as boric acid and borate salts, bead resins and filter sludges, which tend to inhibit or retard hydration of other cements. Masonry cement also hydrates more quickly due to the alkalinity of the slaked lime. The bulk density of masonry cement is about 35% less than that of Portland cement so it can incorporate more waste per unit volume although the low density leads to low strength. Sodium silicate cement uses either sodium silicate (2Na2O  SiO2, water glass) or sodium metasilicate (Na2O  SiO2) as an additive to Portland

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cement. Sodium silicate is a liquid additive, while sodium metasilicate is a soluble granular solid. The action of both additives during solidification is similar. Multivalent cations in the waste as well as soluble multivalent cations in the cement react with the sodium silicate additive to form low solubility silicate compounds that precipitate as a gel. Because this precipitation reaction is rapid, the sodium silicate additive is normally added after the cement and the waste have been mixed. Mixing after sodium silicate addition is limited so as to minimise disruption of the precipitated gel network that forms. This produces a waste
cement mixture that achieves an apparent set in a short time (in minutes) owing to the precipitate gel. The sodium silicate also accelerates the cement setting owing to its high alkalinity, although the formation of stable cement mineral hydrates (hardening) requires additional time, similar to that required for hardening in unmodified Portland cements. Sodium silicate cements are useful for encapsulating boric acid, borate salts and organic liquid wastes because of their rapid set. A pozzolana is a material that is capable of reacting with lime in the presence of water at ordinary temperatures to produce cementitious compounds. Italian pozzolana, trass and Santorin earth are examples of naturally occurring pozzolans of volcanic origin. Artificial pozzolans are prepared by burning at suitable temperatures certain clays, shales and diatomaceous earths that contain clay. Diatomaceous silica and some natural amorphous silica deposits may also form pozzolans, either with or without a heat treatment. PFA (or fly ash), a waste product from coal burning power stations, is now used in many countries on a large scale as a pozzolana. Pozzolanic cements are produced by grinding together Portland cement clinker and a pozzolan, or by mixing together a hydrated lime and a pozzolan. Pozzolanic cements are particularly suitable for immobilisation since the permeability of the concrete is greatly reduced by the continuous filling of the pore volume during the hydration reaction. The absence of leachable free lime in the concrete also contributes to its low permeability and high resistance to sulphate attack from sulphate bearing waste streams or aggressive groundwaters. BFS is a by-product obtained in the manufacture of pig iron and is formed by the combination of the earthy constituents of the iron ore with limestone flux. Portland blast furnace cement or blended cement is a mixture of Portland cement and granulated slag containing anywhere from 20 to 95 wt.% slag, depending on standards established in different countries. Since some slags hydrate slowly on contact with water, its

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287

Figure 17.6 Container with cemented LLW at Drigg disposal site, United Kingdom. LLW, Low-level waste. Courtesy NNL, United Kingdom.

hydration is activated by the addition of other compounds, such as CH, calcium sulphate, sodium carbonate and sodium sulphate. Lime for activation is most conveniently supplied by the hydration of the Portland cement in the mixture. The rate of the hydration reaction is mainly a function of slag concentration in the cement mixture. In addition to a reduced heat of hydration, the setting rate of BFS cements is also reduced. This may be beneficial in processing systems where quick setting cement is not desirable such as those involving large volume containers (Fig. 17.6). BFS cements have a lower permeability than Portland cements, which contributes to the lower diffusion rate of ions through the hardened cement and improved durability in the presence of salts such as chloride and sulphate. The microstructure of a composite cement system is shown in Fig. 17.7 revealing light grey angular BFS grains, and white unhydrated cement in a dark grey CSH matrix.

17.8 ALTERNATIVE CEMENTITIOUS SYSTEMS The flexibility of cement systems means that a toolbox of systems made at close to room temperature is being developed (Fig. 17.8; Lee et al., 2013) applicable to the complex array of wastes which they must encapsulate. Composite cements were considered in Section 17.7, but the other novel

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Figure 17.7 Scanning electron microscopy image of 9:1 BFS:OPC cement, w:c 0.33 cured at 20°C for 90 days. OPC, Ordinary Portland cement; BFS, blast furnace slag. Courtesy Antony Setiadi, University of Sheffield, United Kingdom.

Non-thermal technologies

Geopolymers

Composite cements

Calcium sulphoaluminate cements

Hydroceramics

Phosphatebonded ceramics

Figure 17.8 Non-thermal technologies under development predominantly for ILW which is reactive in OPC systems. ILW, Intermediate level waste; OPC, ordinary Portland cement.

systems under development using non-thermal methods are considered here. Some concern has been expressed about the limitations of conventional cement systems, for example their high pH and intolerance to certain process chemicals present in waste streams. Thus considerable interest exists in using cementitious materials with lower pH pore water, perhaps even extending it into acid regimes, pH , 7. Additionally, alternative formulations may improve the matrix binding capacity for selected radionuclides and may also reduce the set retardation caused by inactive components like zinc salts and borates. Four types of alternative cementitious systems are of most interest (Table 17.9).

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Table 17.9 Alternative cement systems under consideration for waste encapsulation (IAEA, 2013) Cementitious Typical formation conditions material

Comments

CAC

Based on clinkers or fused Widely available as commercial products with dicalcium products with a long history silicate and CaAl2O4 of use in construction Has a history of use (B40 years) CSA or C$A, Available commercially or made by mixing commercial as a construction cement. where CAC with calcium sulphate Developed in China but now $ 5 SO3 widely available Magnesium Mixture of fine-grained MgO Many variants are known, phosphate (periclase) and a phosphate differing in pH and solubility. cements source such as phosphoric Not fully commercial except acid or monopotassium for small scale applications, phosphate such as refractory or dental cements Geopolymers Mixture of sodium silicate Geopolymer-type matrix which (hydrate) with metakaolin is characteristically X-ray amorphous CAC, Calcium aluminate cements; CSA or C$A, calcium sulphoaluminate cements.

17.8.1 Calcium Aluminate Cements Calcium aluminate cement (CAC) was first developed in France in 1914 as an alternative construction material to Portland cement for use where acidic groundwaters, especially those containing sulphate ions, might lead to durability problems. Because this cement has a higher alumina content than that of Portland cement, it is often called high alumina cement. 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 incorporate a significant amount of iron. Although Ciment Fondu was developed because of its resistance to sulphate attack, its rapid hardening and refractory properties were soon discovered and exploited. The setting time of Ciment Fondu is dependent on temperature, 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

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Figure 17.9 Cements and replacement material oxide compositions.

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 like high alumina cements (Fig. 17.9). 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 so that stratlingite, C2ASH8, is formed.

17.8.2 Calcium Sulphoaluminate Cements An alternative approach to produce cements is to use waste materials in the kiln together with limestone, leading to cements of radically different chemical compositions, notably containing little or no alite, C3S. Such a

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product is manufactured on a huge scale ( . 106 tonnes p.a.) in China and is known as calcium sulphoaluminate (CSA) or sulphoaluminate-belite cement. To make such cements, a mixture containing limestone, PFA (fly ash, which is produced in enormous quantities from the burning of pulverised coal in power stations; see Fig. 17.9) and gypsum (or anhydrite) is heated in a rotary kiln to about 1300°C. The product contains α0 C2S and C4A3S, often referred to as Klein’s compound. In the presence of anhydrite or gypsum, this phase hydrates rapidly to form ettringite. 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.

17.8.3 Phosphate Cements These are a very different class of inorganic cements and 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 magnesium oxide and an acid phosphate source such as ammonium dihydrogen phosphate, NH4H2PO4, or aluminium orthophosphate, Al(H2PO4)3. 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 such as 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. 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

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streams that require immobilisation in a product with a much lower pH than that associated with calcareous cementing systems.

17.8.4 Geopolymers Alternative cementing systems based on aluminosilicate polymers (inorganic polymers sometimes known as geopolymers) were first developed by Davidovits and are proving suitable for cements needed in chemically demanding situations (Davidovits, 2011; Provis & van Deventer, 2009). The ingredients are a soluble alkali silicate and a reactive aluminosilicate precursor such as metakaolinite. Strength development is more rapid than in OPC systems and is accompanied by lower heat output (typically B116
247 kJ/kg depending on starting materials), giving a product with moderate strength (B15
20 MPa at 6 hours and B80 MPa at 24 hours); low permeability (B1028 mm/s), moderate pH (9
10) and high thermal stability (samples have been tested to 800°C). 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 3D amorphous aluminosilicate. 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.

17.8.5 Hydroceramics Hydroceramics are another concrete-type material that is similar to zeolitised rock (Lee et al., 2013). It is made by curing a mixture of inorganic waste, calcined clay, vermiculite and Na2S, NaOH with water under hydrothermal conditions (60
200°C) to form a matrix containing crystalline zeolites embedded in a sodium aluminosilicate matrix. The solidification process occurs as a result of hydration reactions. The NaOH solution dissolves the metakaolin (Al2O3  2SiO2) much the same as in geopolymers, but abundant water or hydroxides enable the water to create crystalline silicates instead of an amorphous matrix. The hydroceramic process takes advantage of the sodalite and cancrinite structures in immobilising oxyanion salts such as nitrate, nitrite, chloride, fluoride and iodide within the physical cage like structures of the crystals created. Hydroceramic wasteforms have been shown to be effective on low-activity sodium-bearing

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waste. The technology is still under investigation with studies focused on the optimisation of waste pretreatment (calcination), waste stream
specific optimisation of the formulations and a study of scale-up factors to ensure viability for full-scale operation. In cases where the waste has a high nitrate
nitrite composition, the waste must first be denitrified in some manner, such as calcination, to remove the nitrates and nitrites from the waste. If sodium
nitrate-based waste is pretreated with metakaolin, sucrose, and then calcined, it can be used to make a hydroceramic wasteform. Successful wasteforms have been achieved with waste loadings of 40
60 wt.% waste.

17.8.6 Application of Alternative Cementitious Systems in Waste Immobilisation CAC and C$A are the most engineered materials with a long experience of use. Both types handle and mix like Portland cement and are physically compatible with hardened OPC, to which they bond strongly. Both CAC and C$A cement can be used with fine mineral aggregates to make free-flowing grouts or mortars, or with aggregate to make concrete. Both types set and gain strength more rapidly than OPC. Moreover C$A is often sold with a retarder incorporated to ensure a period of workability. On the other hand, both CAC and C$A compositions have relatively high heat of hydration which is liberated over a shorter period of time than comparable Portland cement mixes. The heat evolution in large size monoliths may have to be managed to avoid thermal cracking. There are, however, important differences between the two types. CAC cement is sulphate-free and hardens to give mainly hydrated calcium aluminates or carboaluminates with C
S
H: Ca(OH)2 is absent. Some of the matrixforming hydrates become unstable in warm conditions and on that account, CAC cements may lose strength in service above 20°C in moist environments. The sequence of phase changes in the hydrate phase assemblage is termed ‘conversion’. As a result, CAC cements should be used with caution where thermal excursions or prolonged warm service is anticipated. On the other hand, C$A cements avoid this problem: the hardened paste of C$A cement contains a high proportion of ettringite, AFt, together with smaller amounts of AFm and CSH gel. CH is again absent. In both CAC and C$A types, the absence of portlandite, Ca (OH)2, among the hydration products gives rise to a matrix with an internal pH about 1 unit less than of comparable Portland cement matrices. Nevertheless, both types have sufficiently high pH, ca. 11.5, to give

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adequate corrosion protection for embedded steel as well as good pH buffering capacity, so the pH is well-maintained in service environments. The water demand for hydration of C$A compositions is characteristically greater than that required for fluidity, particularly if a superplasticiser is included, with the result that it is possible to have a ‘dry’ internal environment. In this context ‘dry’ means that that liquid water is absent. This dry environment is especially favourable to slow reaction between cement and embedded electropositive metals such as Magnox, with the result that hydrogen evolution ceases to be a problem. Although C$A cements have a shorter history of use than OPC, it is apparent from older (B30-year) constructions that no major durability problems have been encountered. CAC cements have recently celebrated their 1st century of commercial use in engineered structures. Both CAC and C$A cements, although alkaline, exhibit moderate resistance to acid attack. Moreover both types, C $A and CAC, have excellent resistance to attack by sulphate and chloridecontaining groundwaters. Both types are amenable to chemical manipulation by means of additives and, on that account, are often used in special applications, for example, in self-levelling floor screeds, tile-setting cements and as repair materials for Portland cement. It is difficult to compare leach resistance values directly but, as reported here, the permeability and leachability of C$A cements is satisfactory to excellent. Phosphate cements have been known for many years but have had mainly niche applications; for example, zinc phosphate cements in dentistry. Most of the experience in the nuclear field is with magnesium potassium phosphate (MPP) cements. These consist of dead burned MgO plus a source of soluble phosphate, for example, monopotassium phosphate (MKP). The reaction between the MgO and acid phosphate is an acid
base reaction which, in the presence of water, results in precipitation of MPP salts. For example, when MgO and MKP are mixed with H2O in the proportions 1:1:5 moles, respectively, the resulting reaction product is struvite, MgKPO4  6H2O, a crystalline material that forms a dense cementitious ceramic (often termed a ‘cold ceramic’). MgO 1 KH2 PO4 1 5H2 O-MgKPO4  6H2 O

(17.10)

Thus these phosphate cements bridge a gap between conventional ceramics and Portland cements. For this system, a cure temperature between 60°C and 65°C is required to produce a monolithic product. At lower temperatures, the reaction to struvite, MgKPO4  6H2O, is incomplete and the resulting reaction products are amorphous hydrated

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phosphate phase(s) which are less dense and not durable. The reactivity of MPP cement is controlled by the reactivity of the MgO; for example, the higher the temperature, the harder the product is to grind (i.e., larger particles and the slower the reaction rate). Set retarders such as boric acid are also used to achieve the desired working time. MPP cements do not have classical induction (dormant) periods typical of Portland cements: the use of set retarders can extend the final set for days to weeks at a more or less constant rate. This feature allows the heat production and dissipation rates to be balanced. These materials handle and mix like Portland cements and have been successfully formulated at full (200 L drum) scale. In packaged MPP products based on MgO and MKP, the stoichiometric ratio is often not used because a portion of the dead/hard burned MgO is not reactive. Consequently, the amount of MKP is reportedly reduced by up to 30 wt. % to eliminate potassium and phosphate leaching, which causes an increase in porosity and efflorescence. However, the rheology of MPP wasteforms and backfill is less sensitive to the amount of water in the mix and to temperature than that of Portland cements. Compressive strength is more sensitive to the amount of MPP in the mixture. Inert aggregates (powders, sand and gravel) can be used to reduce the heat generation and modify/enhance the rheology. MPP cements typically exhibit a slight expansion, 1
2 vol.% (measured on pastes: less for mortars and concrete), when prepared in stoichiometric proportions for commercial applications. Consequently, excellent bonding to substrates, including stainless steel and aluminium metal, is a characteristic of these materials. MPP-based cements have limited use in the construction industry for a variety of structural applications, including rapid-set road and runway repairs and rock anchor bolt cements, so they have some track record of persistence in the natural environment. Geopolymers are typically made from alkali silicate (sodium, potassium) and metakaolin. This type of cement is both new and old: old in the sense that it has a history of patenting and small-scale application extending over more than a century, but novel inasmuch as it has only recently been considered for use in structural and large-scale applications. Typically the reactants are kaolin, a naturally occurring clay mineral, calcined at B700
750°C so as to expel structural water but at low enough temperature partially to preserve the basic layer structure of the precursor mineral, and a concentrated aqueous solution of alkali silicate; the sodium silicate is sold as ‘water glass’. Waste is added at the fluid stage. Setting and hardening follow. Hardening is frequently accelerated by liberated heat or using warm

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(at B40°C) curing. The favourable experiences reported (Davidovits, 2011; Provis & van Deventer, 2009; Vance & Perera, 2011) suggest that more widespread application to waste conditioning is possible. Nevertheless standards for the precursors and experience of process optimisation are lacking. Also, and perhaps crucially, the long-term stability of these materials has not been demonstrated. The binding phase appears to be an amorphous but rigid alkali aluminosilicate gel about which little is known. However, research activity in non-nuclear applications of geopolymers is active and is likely to generate more knowledge of formulation and in particular, their long-term durability. These confidence-building exercises will influence the application potential for geopolymers in waste immobilisation.

17.9 CEMENTATION TECHNOLOGY Cements are used to immobilise both liquid (aqueous) and solid radioactive wastes. Solid radioactive waste is embedded into cementitious matrices by pouring grout or mortar into containers with solid waste. Fig. 17.10 shows examples of solid low-level waste (LLW) encapsulated via cementation at the waste encapsulation plant in Sellafield, United Kingdom. Liquid waste
cement mixtures may be prepared either directly in the container (in-drum mixing), which is the final product container, or prior to pouring into the container (in-line mixing). After in-drum mixing, the cement
waste mixture is allowed to set, the container is capped with a different composition cement to minimise void space and to avoid surface

Figure 17.10 Cement-encapsulated solid, liquid and slurry ILW in 500 L drums. ILW, Intermediate-level waste. Courtesy of NNL, United Kingdom.

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contamination, and a lid is fitted. A simplified process flow diagram for a cement in-drum system is shown in Fig. 17.11A and B shows an actual in-drum mixer. Mobile cementation facilities simple in design are often used for small volume aqueous LLW streams (Fig. 17.12). A reusable mixer such as in Fig. 17.11B may be used, which is removed before the container is capped and the mixture sets, or a disposable mixer may be used, which is left in the container. This is referred to as the lost paddle approach and involves the use of a paddle that is

Figure 17.11 Schematic (A) and an actual view (B) of an in-drum mixing cementation system.

Figure 17.12 Schematic of a mobile in-drum mixing cementation system mounted in an ISO-type container.

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Figure 17.13 Schematic of tumble mixing.

inexpensive to fabricate but capable of producing a homogeneous mix. A disadvantage of in-drum cementation is that the residue on the mixing paddle must be removed and the paddle washed to prevent area and container contamination. Tumble mixing is a cementation process without mixing paddles. Fig. 17.13 illustrates an in-drum mixing technique in which the drum and its contents are attached to a tumbling frame and rotated end-over-end to mix the contents thoroughly. In this process dry cement and a disposable mixing weight are placed in a 220 L drum, followed by the waste and any additional chemicals. The drum is then capped prior to end-over-end tumbling. For this system, cap removal, filling, cap replacement and mixing are done automatically. Homogeneous mixing however cannot always be assured by tumble mixing. In-line mixing processes combine the waste, any additives, water and cement before they are placed into a disposal container. A simplified schematic of in-line cementation is shown in Fig. 17.14. In this process the cement and the waste are separately metered into the mixer. The cement is fed by a screw feeder, while the waste is fed by a positive displacement pump. The cement/waste mix is released directly from the mixer into the container. The level of cement/waste in the container is monitored, possibly by ultrasonic or contact probes. The container is then sealed, decontaminated, monitored and sent for storage. The waste tank and mixer can be flushed through after each run. If desired, the rinsing water can be

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Figure 17.14 Schematic of in-line cementation.

stored and used to prepare the feed slurry for the next run. In-line type cementing facilities use different types of mixer such as mechanical, hydraulic and small volume vortex induction mixers. Operating cementation facilities are complex in design and operation as they comprise a number of additional important technological operations to ensure reliable immobilisation and final product quality. A flow sheet diagram of an industrial cementation facility is shown in Fig. 17.15. Fig. 17.16 shows an actual view of such a facility. Additional vibration of drums enables void filling and development of a dense cement paste. Various additives are used to enhance workability and increase waste loading. For example, vermiculite, bentonite, clinoptilolite and shales enhance radionuclide retention, enabling immobilisation of specific waste streams by cementation. For example, addition of 3 wt.% bentonite decreases the normalised leaching rates of cements (NRi) by an order of magnitude (see Section 14.5).

17.10 ACCEPTANCE CRITERIA A key property of any wasteform is its leach resistance, which determines how well the radionuclides of concern are retained within the wasteform in a wet environment. There are two mechanisms that influence leaching

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Figure 17.15 Schematic of a modular cementation plant. 1, cement and additives bunkers; 2, vibrator; 3, jet pump; 4, air valve; 5, cement service bunker with batcher; 6, additives service bunker with batcher; 7, mixer drive; 8, tank with mixer; 9, hose bolts; 10, vibrator; 11, vortex type mixer; 12, valves; 13, pump-batchers; 14, manometers; 15, cartridge filter for impregnation; 16, vibrating platform; 17, drum with solid waste; 18, crane; 19, vortex mixer service desk; 20, control desk.

Figure 17.16 Radioactive waste cementation plants: (A) modular radioactive waste cementation plant, courtesy Andrey Varlakov, FSUE RADON, Russia, and (B) mobile facility for cementation of liquid radioactive waste (sludge). Courtesy IAEA.

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behaviour. One is the creation of a physical (prophylactic) barrier between the radionuclide and the environment, which is how most polymer and in part bitumen systems work. The other mechanism involves chemical reaction between the radionuclides and the matrix, which occurs most often in cement-based systems. This behaviour is wasteform- and radionuclide-specific and can be altered by the waste chemistry, the formulation of the immobilisation matrix and the leaching water chemistry. Most transuranic elements are retained well by common cement phases owing to the high pH (basic) conditions and the chemical reactions that occur in the matrix. The choice of cement type and cementation technology depends on a number of factors, although waste acceptance criteria (WAC) are among the most important. WAC (e.g., Table 17.10 from the Russian state standard GOST R 51 993-2002) specify the required characteristics of matrix materials and waste packages. Cements are particularly suitable for immobilisation of low and intermediate level radioactive wastes. Cementitious wasteforms have demonstrated limited degradation with time and a high retention of radionuclides in disposal conditions. Fig. 17.17 shows two opened experimental repositories which demonstrated good performance of cementitious wasteforms. The normalised leaching rate for the main waste radionuclide 137Cs in conditions of near-surface disposal can be as low as 1.7 3 1026 g/cm2 day (Ojovan, Varlackova, Golubeva, & Burlaka, 2011), which is at the level of leaching rate of some glasses and ceramics in laboratory conditions (see Fig. 14.7). Table 17.10 Acceptance criteria for low- and intermediate-level radioactive waste cementitious wasteforms in the Russian Federation Criterion

Limit

Normalised leaching rate (by 137Cs) [g/(cm2 day)] Mechanical durability (compressive strength) (MPa) Radiation durability (Gy)

,1023 GOST 29114

Freeze durability (freeze
thaw cycles) Durability to long-term water immersion (days)

Testing method

.4.9

GOST 310.4

106

Change of mechanical durability GOST 10060.1 Change of mechanical durability

.30 90

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Figure 17.17 An experimental repository with cemented aqueous radioactive waste after 12 years in the natural environment (A) and view of cemented waste blocks after 40 years in the natural environment (B). Courtesy Natalia Ojovan, Moscow SIA ‘Radon’.

REFERENCES Abdel Rahman, R. O., Rahimov, R. Z., Rahimova, N. R., & Ojovan, M. I. (2015). Cementitious materials for nuclear waste immobilization. Chichester: Wiley. Bart, F., Cau-di-Coumes, C., Frizon, F., & Lorente, S. (2012). Cement-based materials for nuclear waste storage. New York: Springer. Bensted, J., & Barnes, P. (2001). Structure and performance of cements. London: Spon Press, 565 pp. Davidovits, J. (2011). Geopolymer chemistry and applications. Saint-Quentin: Institut Geopolymere, 632 pp. Glasser, F. P. (2011). Application of inorganic cements to the conditioning and immobilisation of radioactive wastes. In M. I. Ojovan (Ed.), Handbook of advanced radioactive waste conditioning technologies (pp. 67
135). Cambridge: Woodhead. IAEA. (1993). Improved cement solidification of low and intermediate level radioactive wastes. TRS350. Vienna: IAEA. IAEA. (2013). The behaviours of cementitious materials in long term storage and disposal of radioactive waste
results of a coordinated research project. TECDOC-1701. Vienna: IAEA. IAEA. (2017). Selection of technical solutions for the management of radioactive waste. IAEA TECDOC-1817. Vienna: IAEA. Jantzen, C. M., Ojovan, M. I., & Lee, W. E. (2013). Radioactive waste (RAW) conditioning, immobilisation, and encapsulation processes and technologies: overview and advances. Radioactive waste management and contaminated site clean-up: Processes, technologies and international experience (pp. 171
246). Cambridge, UK: Woodhead, Chapter 6. Laraia, M. (2012). Nuclear decommissioning: Planning, execution and international experience. Cambridge: Woodhead, 856 pp. Lee, W. E., Gilbert, M., Murphy, S., & Grimes, R. W. (2013). Opportunities for advanced ceramics and composites in the nuclear sector. Journal of the American Ceramic Society, 96(7), 2005
2030. NRC, National Research Council. (2011). Waste forms technology and performance: Final report. Washington, DC: The National Academies Press, 340 pp. Ojovan, M. I., Varlackova, G. A., Golubeva, Z. I., & Burlaka, O. N. (2011). Long-term field and laboratory leaching tests of cemented radioactive wastes. Journal of Hazardous Materials, 187, 296
302. Provis, J. L., & van Deventer, J. S. J. (2009). Geopolymers: Structure, processing, properties and industrial applications. Cambridge: Woodhead, 454 pp.

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Spence, R. D., & Shi, C. (2004). Stabilization and solidification of hazardous, radioactive, and mixed wastes. Boca Raton, FL: CRC Press, 392 pp. Taylor, H. F. W. (1997). Cement chemistry. London: Thomas Telford, 459 pp. Vance, E. R., & Perera, D. S. (2011). Development of geopolymers for nuclear waste immobilisation. In M. I. Ojovan (Ed.), Handbook of advanced radioactive waste conditioning technologies (pp. 207
229). Cambridge: Woodhead. Winter, N. B. (2012). Understanding cement. WHD Microanalysis Consultants Ltd., 206 pp. E-book available on ,www.understanding-cement.com. Accessed 12.02.13.