Geopolymers - Alkali Activated Composites for Encapsulation of Intermediate Level Wastes

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Geopolymers - Alkali Activated Composites for
Encapsulation of Intermediate Level Wastes

• Neil Milestone
• Immobilisation Science Laboratory
• University of Sheffield

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Presentation outline

• Current cements for encapsulation
• Geopolymers
• What are they?
• History
• Mechanism of formation
• Different types
• Current status of ISL research
• Potential for immobilisation and issues

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Current composite cements

• In UK, ILW is converted to a monolithic waste
  form through encapsulation with cement in steel
  drums.
• Formulations based around use of Portland cement
  (OPC) with large replacements of supplementary
  cementing materials (SCMs) such as blast furnace
  slag (BFS) or pulverised fuel ash (PFA)

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Advantages of cement
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Encapsulation of metals
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Encapsulation of slurries
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Mixing

• Mixing of encapsulating cement carried out in two
  ways
• Internal mixing, usually for sludges or resins.
  Liquid or flowable waste added to drum and then
  cement powders stirred in and mixed with internal
  paddle which is left in drum (lost paddle)
• External mixing, usually for metals. Cement
  formulation mixed outside drum and fluid grout is
  pumped into drum which contains solid material.
  This needs a highly fluid grout to ensure
  complete encapsulation

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Current BFS formulations
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Grout Properties

• High fluidity
• Long working life before stiffening
• Bleed free
• Low water content
• Controlled hydration temperatures
• Initial low temperature
• Known thermal properties
• Radiation stability
• Known mechanical and physical properties
• Acceptable chemical/physical compatibility with
  waste form

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Issues with OPC composites

• High replacement level of OPC means SCMs are not
  completely reacted. This leaves large amounts of
  unreacted, mobile, and highly alkaline pore water
  in a porous matrix.
• These conditions can give rise to ongoing
  reactions such as metal corrosion, attack on
  zeolites and desiccants as well as ready leaching
• Many salts such as PO43-, BO33-, Zn, Sn, affect
  setting
• Hence the programme at ISL to look at properties
  of alternative cement systems

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Alternative cement systems

• Calcium aluminate cements
• Calcium sulfoaluminate cements
• Activated slags
• Phosphate cements
• Mg phosphates
• Calcium phosphate and aluminophosphate
• Geopolymers

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Geopolymers or Inorganic Polymers

• New cementing system based on alkali
  aluminosilicates, not calcium silicate hydrates,
  pioneered by Joseph Davidovits
• Variously called geopolymers, inorganic polymers,
  geocements, hydroceramics,

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Geopolymers

• Binder is not a hydrate but relies on the
  formation of an amorphous 3-D alkali
  aluminosilicate network
• Chemical structure is similar to that of a glass
  no long range order
• Definition
• A family of composite (ceramic) matrices that can
  be fabricated at low temperature from a reactive
  alumino-silicate oxide precursor (Si2O5Al2O2)n
  with alkali polysilicates to form a
  three-dimensional macromolecular structure with
  polymeric Si-O-Al bonds

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Geopolymers

• Made from highly alkaline alkali silicate
  solution, usually with Na or K hydroxide.
• Typical reactive aluminosilicate precursors are
• Metakaolinite
• PFA
• Calcined clays
• Reactive glasses
• BFS
• Recent work has concentrated on alkali hydroxide
  activated PFA

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Geopolymer setting reaction

• Attack by alkali on reactive aluminosilicate to
  release Al(OH)4- ions which polymerise the
  soluble silicate into a network
• Setting is dictated by several factors
• Composition of precursor
• rate of release of soluble Al and amount of Si in
  solution,
• degree of polymerisation of alkali silicate
• calcium ion concentration in solution
• temperature

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Model of mechanism
J Mat Science 2006
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Properties of hardened matrix(after curing
60-80oC

• Good compressive strength 40MPa
• Resistant to acid
• Stable to 800oC
• Compatible with many wastes
• Impermeable

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Structure of geopolymer

• Poly(sialate) structure
• Silica tetrahedra share oxygen atoms
• Al3 exchanges for Si4 in tetrahedra
• Alkali metal acts as a charge balancer
• E.g. Na, K, Cs or Ca2
• Structure still not fully resolved

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Suggested geopolymer structure

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Analogous materials

• Low temperature glass
• Short range ordering
• Over long range are amorphous
• An amorphous equivalent to zeolites
• Crystalline aluminosilicate ion exchange
  materials with a cage like structure
• Naturally occurring
• Generally form over long periods under controlled
  conditions.

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Studies in geopolymers

• work by several groups in Australia-NZ ANSTO-IRL,
  Uni Melbourne, CSIRO, Curtin have made major
  inroads into understanding the material
• Paloma in Spain has concentrated on utilising PFA
  activated with NaOH
• Kriven, University of Illinois
• ISL

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Matrix made with metakaolinite
J Mat Science 2006
P Duxson et al
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Matrix made with PFA
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NaOH activated PFA
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Research at ISL

• We have worked with the current BNFL PFA and been
  able to produce a hardened product both with
  sodium hydroxide/silicate and sodium hydroxide
  alone.
• We have also used sodium aluminate to reduce the
  pH of the resulting matrix

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Experimental

• Synthesis of geopolymer
• Mixed PFA, Na2O.SiO2 solution, NaOH and 3 wt
  metal salt
• Cured for 24 hours at 80oC
• Aged for 7, 28 days
• Tested with
• Leach tests
• X-ray Diffraction (XRD) - CuKa
• Scanning Electron Microscopy (SEM)

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Compositional Problems

• Current PFA is bimodal in particle size
  distribution
• Problems when used as is
• Segregation
• Lack of strength with one fraction
• Two matrices chosen with following compositions

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Compositions
Matrix F Matrix H
Fly ash 150.0 g 150.0 g
Na Sil soln. 88.0 g 88.0 g
NaOH 31.0 g 21.2 g
Metal ion 7.9 g 7.9 g
Extra water 0 g (control) 5g (other) 0 g (control) 5g (other)
Metal Ion Form of use Code
CsCl Caesium Chloride Cs
SnCl4.5H2O Tin (IV) Chloride Sn
Na2Cr2O7.2H2O Sodium Dichromate Cr
Na3PO4.12H2O Sodium orthohosphate P
Pb(NO3)2 Lead Nitrate Pb
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Leach tests

• A soxhlet was used
• Hot, distilled water passed over sample of
  geopolymer broken up into 2-4mm pieces
• Resulting solution was sent for ICP analysis and
  tested for
• Al, Si, Na, K and metal ion

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Results -XRD

• Geopolymer had same trace as PFA with an
  amorphous hump.
• Can see
• Quartz -Q
• Mullite - M
• Haematite - H

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Results XRD Zeolite formation

• Salt (NaCl) formation for Cl containing metal
  salts
• Zeolite (Faujasite) formation in geopolymers
• Formed in high NaOH geopolymers with Sn, Pb and
  Cs
• Metastable zeolite
• Potential problem as changes leach
  characteristics
• Only very small amount (5)

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XRD of zeolite
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Results - Leaching

• Problem with large amounts of Na being leached
• Due to mobility within the aqueous pores of
  geopolymer

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Leaching Toxic metals

• Cations have been immobilised (Sn, Pb and Cs).
• Anions (CrO42- and PO43-) not immobilised

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Why use sodium aluminate?

• High concentration of NaOH needed
• Heating required to develop strength when PFA
  used
• Na leaches
• Destruction of zeolite containing wastes
• Can we use an alternative?

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XRD of Na aluminate activated PFA
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SEM of Na aluminate activated PFA
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Advantages of Geopolymers for Waste Immobilisation

• Structure retains cations such as Cs, Sr
• Appears able to handle moderate salt
  concentrations
• Stable at elevated temperatures
• Utilises waste materials (ashes)
• Potential to initiate setting if PFA used after
  packing in drum by impregnation with NaOH
  solution and heating
• Stability, although Na leaches

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Issues with system

• Uses highly alkaline solutions 8M NaOH
• Usually requires heating for 24 hours
• Each precursor has its own characteristics which
  must be determined empirically
• Setting time
• Long term durability not known
• Still largely an unproved system
• Several groups are now making real concrete
  articles

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Future work

• Starting materials
• Research at ISL is investigating use of PFA in
  different forms with activators
• Could allow delayed setting as it is a two pot
  system
• Will need to be investigated if liquid phase can
  be added later.
• Durability testing

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Conclusions

• It is possible to produce geopolymer from local
  PFA
• Geopolymers can immobilise not just encapsulate
  cations which are difficult to retain in an OPC
  system.
• More work needs to be carried out.