Characterization of pozzolanic cement for radioactive waste disposal
Material characterization studies were conducted to analyze the chemical and physical properties of pozzolanic cement. The principle objective of this research was to determine whether pozzolanic cement is a suitable material for permanently encasing high-level radioactive waste (HLW) and spent-nuclear fuel (SNF) in disposal canisters. This particular cement was produced by mixing lime or portland cement with volcanic tuff, a natural pozzolanic material extracted from Yucca Mountain, Nevada; Portland-pozzolan and lime-pozzolan mixtures were created, analyzed and tested for compressive strength. In addition, the pH of several mixtures was monitored and the values recorded periodically over the first eight days of curing. Particle size (coarse vs fine), crystal structure (welded vs nonwelded tuff) and composition were all varied to study the interactive effects on the pozzolanic reaction between the pozzolan and lime. The pozzolanic material varied from approximately 90% crystalline silica with the welded tuff to over 95% crystalline silica with the unwelded tuff from Yucca Mountain; The results indicated that the particular volcanic tuff from Yucca Mountain, Nevada, is not a good pozzolan because it has poor reactivity with lime. The compressive strength of portland-pozzolan and lime-pozzolan mixtures were, on average, significantly lower than that of ordinary portland cement. The data showed a strong correlation between the particle size of the pozzolan and the reactivity of the cement paste. In most of the coarse grained mixtures, for example, the cement never solidified. Others partially solidified, but crumbled with less than 100 psi of compressive stress. With the fine-grain mixtures, however, both the welded and nonwelded lime-pozzolan mixtures produced a solid cement matrix.