Potential cements for high water content sludges

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Potential cements for high water content sludges

• Neil Milestone
• Immobilisation Science Laboratory
• University of Sheffield

2
Staffing

• Dr Y Bai appointed 1 May 2006 but left March 2007
  to take up permanent appointment at Queens
  University, Belfast
• Post has been advertised and new appointment, Dr
  N Collier will start from September.
• Mr X Li has been working since October on
  developing current OPCBFS systems through
  additional activation with sulphates. He leaves
  in September.

3
Current cement formulations

• Formulations now used for immobilising ILW were
  developed about 18 years ago
• Compositions of slag, and OPC have changed
• High replacement levels of OPC are used to avoid
  temperature rise which could lead to thermal
  stresses. Typically 91 BFSOPC or 41 PFAOPC
  are used with w/s ratios of 0.37 and 0.42
  respectively
• These relatively high w/s ratios needed for
  workability but leads to very high w/c ratios.

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Current formulations

• Low amount of cement means low amount of
  activation of replacement materials. In 91
  BFSOPC no more than 45 of slag is hydrated
  after 2 years. Porosity is around 23
• At 60oC curing, more slag is hydrated, 60, but
  hydration product is denser and porosity about
  the same.

5
Water content (Vol.) in capillaries (black
circles) intermediate size pores (open circles)
and gel pores (grey small circles) as a function
of hydration time for (a) 31 BFSOPC blend
hydrated at 20ºC, (b) 31 BFSOPC blend hydrated
at 60ºC, (c) 91 BFSOPC blend hydrated at 20ºC
and (d) 91 BFSOPC blend hydrated at 60ºC.
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Hydration with an Fe floc

• Iron floc used in is 15 solids and a viscous
  colloidal suspension. It must be pretreated with
  Ca(OH)2 to allow it to be cemented.
• Treated floc mixed directly with 31 PFAOPC
  blend to give w/s ratio of 0.83
• Porosity of hardened matrix around 53.
  Permeability very high and strength low.

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Advantages of OPC composites

• Highly alkaline matrix ensures insolubility of
  many ionic species
• Ease of mixing and workability
• Compatible with many wastes
• Tried and true with properties well known

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Disadvantages with OPC composite cements

• Highly alkaline matrix attacks zeolites, resins,
  desiccants, reactive metals
• Lack of reaction means high free water content
  that is free to move and high porosity
• When used with flocs or sludges, high w/c matrix
  leads weak matrix with open structure.
• Increase in temperature changes known chemistry

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Alternative cements

• Calcium sulphoaluminate cement (CSA)
• Binder is ettringite, 3CaO.Al2O3.3CaSO4.32H2O
• Can be mixed at high w/c ratio
• Bound water per Ca 5 compared to C-S-H where it
  is 1
• Al and zeolites do not react as internal pH 11
• Compatible with many wastes as many ions can be
  substituted in ettringite structure.
• Long term durability largely unknown, although
  has been used as construction material in China
  for 30 years. Being used to encapsulate
  B-containing resins due to low pH which does not
  exchange B or destroy resin

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Alternative cements II

• Sulphate activated slags
• NaSO4 activation
• CaSO4 activation
• Work being undertaken in collaboration with
  Chinese researchers

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Sulphate activated slags

• Both CaSO4 (15) and Na2SO4 (4) will activate
  BFS
• Strength development is slow
• Both ettringite and C-S-H are formed as binder
• pH with Na2SO4 activation is higher than with
  CaSO4 activation and less ettringite is formed
• pH can be controlled by PFA addition

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Strength development
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Strength development
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CaSO4 activated BFS
15
Na2SO4 activated slag
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Leaching of Cs
Cs is one of the most difficult ions to immobilise
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Issues

• Ettringite is only stable to around 60oC when it
  starts to lose water
• CSA cement has a high Heat of Hydration but can
  be modified with PFA

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Future work

• Durability of system with ettringite as binder
• Compatibility with other wastes
• Leachability
• Development of suitable formulations for remote
  use workability
• Need to understand why Al does not corrode in
  sulfate systems

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Acknowledgements

• Y. Bai, now Queens University, Belfast
• C.H. Yang, Y.J. Shi, Department of Building
  Materials Engineering, Chongqing University,
  P.R.China
• X.C. Li ,Yunnan Construction Concrete Co. Ltd,
  P.R.China
• Q Zhou, UCL
• Martin Hayes, Nexia Solutions