Cements in Radioactive Waste Disposal
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MRS BULLETIN/DECEMBER 1994
ure 1). Immobilization timescales for radioactive wastes are long, of the order of 106 years for some radionuclides. Prediction of repository evolution over such time periods is subject to considerable uncertainty. Proof of the long-term performance of cements has placed new demands on materials scientists. Portland Cement The term cement is applied to many different materials; in engineering contexts,
the most common is ordinary Portland cement (OPC). OPC is formed by heating geologic materials, usually limestone and shale, until partial fusion occurs (~1450°C). The resulting product, called clinker, is cooled and ground to a high specific surface area, usually 3000-5000 cmVg. Table I gives the range of compositions associated with Portland cement. Mineralogically, OPC consists of four phases: two calcium silicates, Ca3SiO5 and Ca2SiO4, an aluminate, Ca3Al2O6, and an aluminoferrite solid solution, Ca2(Fe^\l)2Os. The two calcium silicates comprise 70-80% of the total clinker mass. Setting and strength gain are initiated by mixing clinker with water. The minimum theoretical water content to achieve full hydration corresponds to a water/solid (w/s) ratio of —0.25. For good plasticity and workability, however, this ratio is increased to 0.4-0.6. So, even after complete hydration, excess water is present, partially filling micropores; this is called "pore water." The hydration reactions are characterized by mechanical, thermal, and microstructural changes; the different stages are shown in Figure 2. An initial burst of heat evolution (Region I) is attributed to heat resulting from wetting and reaction
Biosphere
Encapsulated Waste in Drum
Drums in Overpack
GeoSphere
I
% Overpacks in Vault, Backfilled and Sealed
Figure 1. Conceptual design for a deep repository, for low and intermediate-level radioactive waste.
33
Cements in Radioactive Waste Disposal
Table I. Composition of Cement and Supplementary Materials (in Weight %). Component Oxide
Portland Cement
CaO
60-65
AI 2 O 3
3-8 1-5
FeO/Fe 2 O 3 SiO 2 NasO
KaO MgO S*
20-25 0.1-1.0 0.1-1.5 1-4 1-4
Blast Furnace Slag
37-44 12-16 0-1
41-45 0.1-1.0 0.1-1.0 7-12 0.2-1.2
Coal Combustion Fly Ash 2-40 5-42 2-10 20-60 0.1-2 0.1-2 1-5 0-4
*Present in Portland cement and fly ash as SO4, and in slag as sulfide, S 2 \ The intrinsic S content of Portland cement is frequently supplemented by grinding with CaSO4 (anhydrite or gypsum).
between surface oxide ions and H2O, which liberates OH ~ ions into solution. This initial exothermic burst is quickly followed by a relatively dormant period (Region II) during which the clinker grain surfaces become covered with a passivating gellike layer. During this stage, lasting 1-4 hours, the mixture remains plastic. The onset of a secondary burst of heat evolution (Region HI in Figure 2) causes subsequent stiffening, due to renewed hydration activity. As a continuous solid microstructure emerges, the amount of free water decreases with strength increasing thereafter. Hydration of OPC may take a y
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