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Cement chemistry in nuclear waste disposal

Cement chemistry in nuclear waste disposal. Lian Wang and D. Jacques. Euridice exchange meeting. January 29, 2009, Mol, Belgium. The use of cement materials in deep disposal of radwaste. 55,000 tons cement materials. Contents. What is cement? Cement manufacture Chemical composition

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Cement chemistry in nuclear waste disposal

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  1. Cement chemistry in nuclear waste disposal Lian Wang and D. Jacques Euridice exchange meeting January 29, 2009, Mol, Belgium

  2. The use of cement materials in deep disposal of radwaste 55,000 tons cement materials

  3. Contents • What is cement? • Cement manufacture • Chemical composition • Cement pore water chemistry • Cement chemistry in nuclear waste disposal

  4. What is cement? Cement A binder that binds other materials together thorough hydration with water Concrete A mixture of cement, water, coarse and fine aggregates Lothenbach, 2003 Mortar A mixture of cement, water, and fine aggregate that is workable to fill gaps between other materials

  5. What is cement (cont.) ? • OPC (Ordinary Portland Cement) named from Isle of Portland (England) • >90% Portland cement clinker + CaSO4 + <5% others • 2/3 CS + AF with C/S > 2.0 • Normal Portland Cement (USA) • Type I ASTM general purpose Portland cement

  6. limestone/chalk clay/shale ball mill slurry to conveyor belt rotary kiln resultant clinker ground to a fine powder Portland cement Cement manufacture Lime stone + clay Cement clinker 1450 °C

  7. Chemical composition of cement • chemical composition • 65-70% CaO, 18-24% SiO2, 3-8% Fe2O3, 3-8% Al2O3 and other oxides • Clinker composition • Alite (50%) C3S, 3CaO SiO2 (CaO = C; SiO2 = S) • Belite (25%) C2S, 2CaO SiO2 • Aluminate (10%) C3A, 3CaO Al2O3 (Al2O3 = A) • Ferrite (10%) C4AF, 4CaO Al2O3 Fe2O3 (Fe2O3 = F) • Gypsum (5%, CaSO4·2H2O)

  8. Cement hydration: exothermic reaction between clinkers and water • Hydration of C3S • C3S + H2O = C-S-H (calcium silicate hydrate) + Ca(OH)2 (portlandite) • Hydration of C2S • C2S + H2O = C-S-H (calcium silicate hydrate) + Ca(OH)2 (portlandite) • Hydration of C3A • C3A + H2O = AFt (e.g. ettringite) = AFm (calcium monosulphoalumiate • Hydration of C4AF • Similar products as C3A with Fe3+ substitution for Al3+

  9. Cement hydration products (1) • Calcium silicate hydrate • C-S-H: the “binder” • C/S mole ratio 1.5-2.0 • High specific surface: 100-700 m2/g • Volume: 50-60% Poorly crystalline gel material

  10. Cement hydration products (2) • Calcium hydroxide (portlandite) • Ca(OH)2 well crystalline • Major pH buffer at 12.5 (25 °C) Ca(OH)2 = Ca2+ + 2OH- • Volume: 20-50%

  11. Cement hydration products (3) • Calcium sufoaluminate hydrates • Volume: 15-20% • First: ettringite (AFt) • After: monosulfate hydrate (AFm) ettringite

  12. Cement hydration evolution and end products Cement phases C-S-H Portlandite Ca(OH)2 Ettringite (AFt) Monosulfate (AFm) Hydrogarnet (C3A6H6) Hydrotalcite (Mg-Al-containing minerals) Other minor phases Calcite (as aggregate)

  13. Cement pore water chemistry • Pore water chemistry is controlled by equilibrium between hydration phases and pore water: solubility of cement hydration phases, solid solutions, chemical speciation • Staged pore water chemistry evolution with time • pH decreases with time

  14. Cement pore water chemistry:staged pH evolution • State I: pH > 13 (25 °C) • K2SO4 and Na2SO4 exist in clinkers and readily dissolve • SO4 are removed by low solubility solids, e.g., ettringite or uptake by C-S-H leaving pore water as KOH/NaOH • State II: pH 12.5 (25 °C) • Controlled by solubility of portlandite • Ca(OH)2 = Ca2+ 2 OH- logK = -5.43

  15. State III pH evolution • pH decreases with C/S in C-S-H • C/S evolves continuously- incongruent • dissolution of C-S-H • solid solution formation of C-S-H • concentration of Ca and Si evolves with • the C/S in C-S-H Chen et al., 2004

  16. Staged pH evolution

  17. Cement chemistry in nuclear waste disposal • Favorable condition • the corrosion of overpack is at a low rate (passivation film stable under high-pH) • Increase the retention of most of RN by low solubility and high sorption • Perturbation • Cement is thermodynamically unstable so tends to interact with clay

  18. Why cement chemistry matters in disposal? • Near-field chemistry • Corrosion studies • Source term of RN retention • Far-field chemistry • Negative effect: high pH plume tends to dissolve clays and organic matters • Positive effect: precipitation of zeolite, C-H-S, and calcite enhances RN retention

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