An investigation of the structure and phase relations of C-S-H gels. Final report, August 15, 1991--September 14, 1997

Solid state NMR was used to obtain data on the atomic level structure of calcium silicate hydrate (C-S-H) that formed from a variety of starting materials under a variety of conditions. Because C-S-H is the major component of hydrated portland cement, a knowledge of its structure and of its structural evolution will ultimately allow users of cement containing materials such as mortars and concrete to more accurately predict the physical and mechanical behavior of these materials once they are placed into service. From the outset, the goal of the work was to observe the hydration process on the atomic level, integrate the findings with existing data in the literature, and refine hydration models as necessary to accommodate the newly acquired data

Hydroceramic Binders

This presentation was given at the DOE Office of Science-Environmental Management Science Program (EMSP) High-Level Waste Workshop held on January 19-20, 2005 at the Savannah River Site

Preparation and Properties of Hydroceramic Waste Forms Made with Simulated Hanford Low-Activity Waste

Approximately 85 million gallons of high-level waste (HLW) is currently stored in underground tanks at the Hanford Reservation and the Savannah River Site (SRS). The waste consists of a hydroxide-rich precipitate (sludge) and a sodium-rich supernate. The supernate is a NaOH rich solution containing lesser amounts of NaNO 3 and NaNO 2 and small amounts of soluble fission products, cladding materials, and organics (volatile organics and semi-volatile organics known as VOCs and SVOCs). The Department of Energy (DOE) has chosen glass as its waste form for both sludge and sodium-rich supernate. However, because of the volume of the supernate, alternatives to vitrification are being sought for some of this waste. One alternative is to remove 137 Cs and 90 Sr from the supernate. Decontaminating the waste in this way allows the waste to be designated as low-activity waste (LAW) and as such the waste now becomes eligible for solidification and disposal on site. SRS is solidifying its LAW with a blended Portland cement forming Saltstone. Hanford has been considering a bulk vitrification process in which the LAW will be mixed with Hanford soil and vitrified in place in a disposable carbon-arc powered glass melter/waste container. Both waste forms can then be buried on site in appropriate vaults or low-level waste land fills. A hydroceramic is an alternative waste form designed to solidify and stabilize LAW that is made from metakaolin plus NaOH and/or NaOH rich LAW supernate. In addition to NaOH, LAW can contain a wide range of sodium nitrate and sodium nitrite concentrations. Although a hydroceramic waste form can be made directly from some types of decontaminated waste, e.g., those that are highly alkaline (8-12M NaOH) and contain less than 25 mol% of NO x (NO x is used as the short-note for nitrates and nitrites in this article.) relative to the total Na in the waste, by simply mixing the LAW with metakaolin and curing the resulting paste at 901C, the remaining LAW, especially that stored at Hanford must be pretreated in some way before it can be similarly solidified; the relative molar proportion of NO x /Na must be reduced to 25% or less. In this paper calcination is evaluated as a potential pretreatment method for Hanford AN-107 (AN-107 is a waste storage tank on Hanford site) LAW, but in choosing this method it is necessary to divide the preparation of the hydroceramic waste form into two steps: denitration/denitrition of the liquid waste stream to produce a granular calcine followed by solidification using a metakaolin plus 4M NaOH binder. A simulated Hanford AN-107 LAW was calcined at 3751, 4501, 5251, 6001, and 6751C in the presence of sucrose and metakaolin added as a calcination aid. It was shown that the leachability of the calcines decreased as calcination temperature increased, i.e., the waste form became more crystalline. In the second step, each of the granular calcines was mixed with additional metakaolin and just enough 4M NaOH to form a thick paste. The paste was precured at 401C and then autoclaved at 901C to form a monolith. X-ray diffraction and scanning electron microscopy characterization showed that the calcines themselves contained an amorphous phase and crystalline hydroxysodalite, and that the hydroceramics made from these calcines plus additional metakaolin/NaOH binder consisted predominantly of zeolite A and hydroxysodalite. The temperature used to prepare the calcines not only affected the properties of the calcines, but those of the monolithic hydroceramics as well. Experimental results demonstrated that 5251C represented the optimal temperature for producing the most suitable calcine for subsequent solidification with metakaolin and 4M NaOH. The resulting hydroceramic nuclear waste form was strong and had the lowest overall leachability. The leachability of the hydroceramic is normally lower than that of the corresponding calcines up to B6001C. The product consistency test (PCT) determined normalized release rate NR Na for the hydroceramic (0.14 g/m 2 . day) was comparable to similar leach rates determined for Hanford's low-activity waste reference material glass (0.08 g/m 2 . day) and a steam reformed calcine made with Hanford's AN-107 tank waste (0.25 g/m 2 . day)

Selection and durability of seal materials for a bedded salt repository: preliminary studies

This report details preliminary results of both experimental and theoretical studies of cementitious seal materials for use in a proposed nuclear waste repository in bedded salt. Effects of changes in bulk composition and environment upon phase stability and physical/mechanical properties have been evaluated for more than 25 formulations. Bonding and interfacial characteristics of the region between host rock and seal material or concrete aggregate and cementitious matrix for selected formulations have been studied. Compatibilities of clays and zeolites in brines typical of the SE New Mexico region have been investigated, and their stabilities reviewed. Results of these studies have led to the conclusion that cementitious materials can be formulated which are compatible with the major rock types in a bedded salt repository environment. Strengths are more than adequate, permeabilities are consistently very low, and elastic moduli generally increase only very slightly with time. Seal formulation guidelines and recommendations for present and future work are presented. 73 references, 25 figures, 61 tables

New high temperature cementing materials for geothermal wells: stability and properties. Final report

Potential high-temperature cements have been formulated and evaluated in terms of their properties for geothermal well cementing. Phase formation and compatibility in two major compositional regions were investigated in the temperature region between 200 and 400/sup 0/C and pressures up to 69 MPa (10,000 psi). These were followed by an evaluation of properties of the cements formed. One area in the system Ca0-Mg0-Si0/sub 2/-H/sub 2/0 centered around the xonotlite-chrysotile join while the other area of interest centered around the anorthite composition in the system Ca0-Al/sub 2/O/sub 3/-SiO/sub 2/-H/sub 2/O. After numerous exploratory studies, the magnesia-containing mixtures were prepared by mixing a Class J cement with various sources of magnesia such as calcined chrysotile, or magnesium oxide. Calcium oxide and silica fine quartz powder were also added to adjust the compositions. The aluminous system cements were formulated from high-alumina cements which were mixed with various silica sources

Chemical stability of cementitious materials based on metakaolin

[EN] The alkali activation of metakaolin is a way of producing high strength cementitious materials. The processing of these materials has been the subject of numerous investigations. The present paper describes the results of a research project initiated to study the stability of these materials when exposed to aggressive solutions. Prisms of mortar made of sand and alkali-activated metakaolin were immersed in deionized water, ASTM sea water, sodium sulfate solution (4.4% wt), and sulfuric acid solution (0.001 M). The prisms were removed from the solutions at 7, 28, 56, 90, 180, and 270 days. Their microstructure was characterized and their physical, mechanical, and microstructural properties were measured. It was observed that the nature of the aggressive solution had little negative effect on the evolution of microstructure and the strength of these materials. It was also found that the 90-day and older samples experienced a slight increase in their flexural strengths with time. This tendency was most pronounced in those samples cured in sodium sulfate solutions. This behavior may be related to the change in microstructure of the cementitious matrix of the mortars cured longer than 90 days. Some of the amorphous material present had crystallized to a zeolite-like material belonging to the faujasite family of zeolites.Peer reviewe