Barry E. Scheetz

Ettringite and CSH Portland cement phases for waste ion immobilization: A review

Abstract The formation, structure and chemistry of the ettringite and CSH phases of Portland cement have been reviewed as they relate to waste ion immobilization. The purpose of this review was to investigate the use of Portland cement as a host for priority metallic pollutants (Cr, Pb, Ba, Se, Zn, Ag, Hg, As and Cd) as identified by the Environmental Protection Agency and as a host for radioactive waste ions (various isotopes of Am, C, Cs, I, Np, Pu, Ra, Sr, Tc, Th, Sn, U) as identified in 40 CFR 191. Ettringite acts as host to a number of these ions in both the columnar and channel sections of the crystal structure. Substitutions have been made at the calcium, aluminum, hydroxide and sulfate sites. CSH also hosts a number of the waste species in both ionic and salt form. Immobilization mechanisms for CSH include sorption, phase mixing and substitution. The following ions have not apparently been reported as specifically immobilized by one of these phases: Ag, Am, Np, Pu, Ra, Tc, Th and Sn; however, some of these ions are immobilized by Portland cement.

Sodium zirconium phosphate (NZP) as a host structure for nuclear waste immobilization: A review

Sodium zirconium phosphate [NZP] structural family, of which NaZr[sub 2]P[sub 3]O[sub 12] is the parent composition, has been reviewed as a host ceramic waste form for nuclear waste immobilization. NZP compounds are characterized for their ionic conductivity, low thermal expansion and structural flexibility to accommodate a large number of multivalent ions. This latter property of the [NZP] structure allows the incorporation of almost all 42 nuclides present in a typical commercial nuclear waste. The leach studies of simulated waste forms based on NZP have shown reasonable resistance for the release of its constituents. The calculation of dissolution rates of NZP structure has demonstrated that it would take 20,000 times longer to dissolved NZP than quartz.

Utilization of fly ash

Abstract Fly ash is a waste material predominantly generated in the production of electricity. As a gigascale material, it represents a potential major environmental liability; however, because of its bulk mineralogy and chemistry, it can serve as a source of materials for large-volume, low tech applications.

Hydration of fly ash-portland cements

Abstract Hydration products of fly ash-portland cements were studied with x-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM) as part of a continuing research effort to understand the pozzolanic activity of fly ashes. It was found that the amount of calcium hydroxide crystals in the cement pastes is diminished due to the addition of fly ash to the cement. Ettringite was produced in the early age, and the consumption of sulfate by the formation of ettringite was accelerated by the addition of fly ash. A partial conversion of ettringite to monosulfate within the first 7 days of hydration in the fly ash-portland cement pastes, but the formation of ettringite continued to form up to at least 28 days of hydration in the pastes without fly ash. Examination of the fly ash bearing pastes showed, in all cases, varying amounts of calcium hydroxide and unreacted portland cement, with minor quartz and gehlenite hydrate. It appears that hydration reactions actually occur in the fly ash cement pastes more or less on a particle-by-particle basis.

Characterization of silicate sealers on concrete

Sealing concrete with soluble sodium silicate may improve surface properties such as hardness, permeability, chemical durability, and abrasion resistance. Previously, such treated surfaces had not been characterized sufficiently to provide a complete understanding of how silicate improves concrete properties. Therefore, it is the object of this work to contribute to the understanding of how the application of soluble silicate alters the surface of concrete by verifying its effects on absorption, abrasion resistance, chloride permeability, chloride penetration, and surface composition.

The Role of Boron in Monitoring the Leaching of Borosilicate Glass Waste Forms

In the absence of any identified solid phase host (other than the original glass), boron has been assumed to accumulate in the fluid during the reaction of borosilicate glass waste forms with aqueous fluids. Using this assumption, it is possible to define a boron index which can be used to monitor the amount of glass that has been dissolved and to provide a worst-case measure of the degradation of the primary glass waste form. Several boron-containing silicate phases have been identified thus invalidating the assumption that boron does not precipitate. The effect is apparently small and the assumption that boron release is a direct measure of degree of alteration of borosilicate glass is still probably a good one.

The Role of Resolution and Sample Preparation in Hydration Rim Measurement: Implications for Experimentally Determined Hydration Rates

One aspect of the obsidian hydration dating method that has received too little attention in the 25 years since its inception is the condition of resolution associated with the optical microscope, and its potential effect on age-date determinations. Archaeologists have long been aware of some of the potential errors associated with hydration rim measurement. Specifically, these concerns have addressed single-operator measurement variation (Friedman and Smith 1960; Michels 1965), interlaboratory measurement variation, differences in magnification, and standardized techniques of thin section preparation (Jackson 1984). Despite the number of potential errors associated with the dating process, precision estimates routinely have been reported as being the magnitude of .2 micrometers (uim) (Findlow and De Atley 1976), .1 Am (Friedman and Smith 1960), and .07 ,m (Michels 1967) for archaeological samples. Standard deviations typically associated with the measurement of artificially induced hydration rims used in the development of experimental hydration rates are reported to range between .05 jum and .39 Ajm (Michels, Tsong, and Smith 1983). It also has been assumed that measurement errors will tend to cancel one another out and approach an average value when multiple readings are taken (Clark 1984). In our consideration of potential sources of error associated with obsidian dating, it became apparent that the optical parameters associated with specific microscope systems and the optical characteristics of the sample never had been examined explicitly. In this paper we document the resolution limitations associated with the optical microscope and the error introduced by incorrect sample preparation. In addition, we show that measurement errors indeed are cumulative in ratedetermination procedures and are much higher than those traditionally reported in the literature.

Hopewell Obsidian Studies; Behavioral Implications of Recent Sourcing and Dating Research

Specific questions regarding the antiquity of major midwestern Hopewell culture sites and their role in regional exchange systems are addressed in this paper through the dating (obsidian hydration) and compositional characterization (neutron activation analysis [NAA] and atomic absorption spectroscopy [AAS]) of obsidian artifacts. The analysis of 34 specimens from the Seip, Mound City, and Hopewell sites, Ohio, and the Naples site, Illinois, increases fivefold the number of chronometric dates available from these sites and expands the sample of compositionally identified specimens beyond those resulting from Griffin et al. s (1969) pioneering work. The resulting hydration dates support earlier estimates of the age of these contexts based on 14C or artifact seriation alone. The range of dates (78 B.C.-A.D. 347) and the compositional variety within the sample favors an expanded view of the nature of obsidian trade in the Midwest to include additional western sources, a longer episode of importation, and possible changes in the sources used through time.

Strength development, hydration reaction and pore structure of autoclaved slag cement with added silica fume

Under continuous hydrothermal treatment the strength of portland cement paste decreases with curing time and the pore structure coarsens. It was found in this study that the compressive strength of slag cement paste containing 67.5 wt.% ggbfs also decreases with time after 24 hour hydrothermal processing, but with a small addition of silica fume to the slag cement, the cement strength increases and the pore structure densifies when processed under comparable conditions. Based on observations XRD and SEM, these changes are attributed to: (a), changes in the hydration reactions and products by highly reactive silica fume, such that amorphous products dominate and the strength reducing phase α-C2SH does not form; (b), slower hydration of slag, partially caused by the decreased pH of the pore solution, favors the formation of a dense pore structure; and (c), the space filling properties of the micro particles of silica fume.

Synthesis and optical absorption spectra of Cr2+-containing orthosilicates

By reacting chromium metal with appropriate oxides in a neutral atmosphere at high temperature it is possible to prepare Mg2SiO4 with the olivine structure containing Cr++ and also Cr2SiO4. X-ray powder diffraction data for chromous orthosilicate may be indexed on an orthorhombic cell with a=5.690, b=11.262, and c=9.584 Å. The compound is not isotructural with olivine. Optical absorption spectra of Cr2+ in both hosts have been obtained and also indicate large differences between the two structures.