Patent Number: 051695669
Section: summary

2. The Field of the Invention The present invention relates to engineered cementitious contaminant barriers. More particularly, the present invention is directed to contaminant barriers prepared from cementitious materials capable of isolating contaminants, including toxic and radioactive waste materials, from a substantially uncontaminated environment. The cementitious barriers include compounds capable of adsorbing, absorbing, chemically reacting with, bonding with, or otherwise trapping contaminants in the form of liquids, dissolved ions, and gases which might otherwise penetrate or leach through the barrier. 3. Technology Review In recent years, the public has become more sensitive to the environment and the effect of hazardous and toxic waste materials on the environmental ecosystem. In particular, the public has recognized the need and desirability of being free from exposure to toxic wastes and other hazardous chemicals and chemical by-products. One of the most serious exposures to toxic waste materials occurs when the ground water of a community becomes contaminated. Ground water contamination not only effects the health and safety of humans, but also other forms of plant and animal life Ground water contamination can result from direct introduction of harmful chemicals into the water source. In such cases, the problem is usually remedied by identifying the source of contamination and prohibiting future disposal of the waste without adequate waste treatment. A more difficult problem occurs when the water supply becomes contaminated through harmful chemicals which enter and migrate through the soil into the water supply. Often this happens when barriers built to contain the hazardous waste, such as hazardous waste containers and in situ barriers, allow waste chemicals to leach through the barrier into the uncontaminated environment. There is, therefore, a serious need for effective barriers used in the storage and containment of hazardous and toxic waste materials until they decompose into harmless compounds. Of the various toxic and hazardous waste materials which need to be contained, nuclear waste materials are some of the most dangerous because their damage is permanent and they can remain radioactive for extremely long periods of time. Much of the radioactive waste materials which needs to be disposed of includes refuse from nuclear weapons plants, civilian nuclear power plants, and medical industry sources. Unlike spent fuel rods which decay by emitting high level gamma radiation, the plutonium waste from weapons plants decays by emitting alpha particles. Alpha particles do not even penetrate paper. As a results the plutonium waste materials from weapons plants may be handled without protective clothing and pose no danger, as long as they remain sealed Nevertheless, plutonium is extremely toxic and very long-lived. In addition, it is estimated that sixty percent (60%) of the plutonium-contaminated waste from weapons plants is "mixed" waste. Mixed waste is particularly difficult to handle be cause it contains both radioactive waste and hazardous chemicals such as industrial solvents. Containing and disposing of mixed waste is difficult because disposal techniques used for radioactive waste are not suitable for the organic hazardous waste and disposal techniques for hazardous waste are ineffective for the radioactive waste. Gloves, shoes, uniforms, tools, floor sweepings, and sludge contaminated with transuranicradioactive materials while manufacturing nuclear warheads are typically contained in 55 gallon steel drums. The Waste Isolation Pilot Project ("WIPP") site near Carlsbad, N.M. is one possible radioactive waste disposal site. The WIPP site was excavated in a massive underground salt formation. Underground salt formations, such as the WIPP site, are considered as possible permanent radioactive waste disposal sites because of the long-term stability of the underground formation and because the salt strata has a low water permeability. In one possible disposal plan using underground disposal sites for low-level radioactive waste materials, the underground rooms are filled with the waste containers and back-filled with a grout material to fill as much empty space as possible. During the first 100 years, the underground storage rooms would collapse and crush the waste containers. One problem with conventional 55 gallon steel drums is that they often contain a lot of empty space inside the drums. Eventually, the drums will be crushed when the storage room collapses; however, the presence of empty spaces permits ground water to seep into the cavities which can cause corrosion of the steel drum, gas generation, and decomposition of organic waste materials. Since the disposal site is not completely sealed until the underground storage room collapses and fills all void spaces, rapid collapse of the storage room is desirable so that the disposal site is sealed and stabilized as quickly as possible. Another disadvantage of conventional 55 gallon steel drums is that they are potentially capable of undergoing corrosion which would produce gases, especially hydrogen gas (H.sub.2), and which may lead to high pressure bubbles. Corrosion and its related gas evolution are considered long term liabilities. Corrosion is caused by groundwater, usually containing high concentrations of dissolved ions (i.e., 1 to 2 molar). If the hazardous waste includes organic materials, such as contaminated rubber and certain waste solvents, carbon dioxide gas (CO.sub.2) may be produced which may also lead to high pressure bubbles. Only recently has the need to avoid formation of the so-called high pressure bubbles been recognized. Current government regulations of long-term hazardous waste storage sites assume that at some time over the storage lifetime, the storage medium will be breached by underground drilling devices. If high pressure bubbles exist at the location where the storage medium is breached, then it is possible that contaminated materials may be inadvertently released under pressure. Processing and reprocessing of radioactive materials, including spent fuel, produces large quantities of liquid waste materials. It is a common practice, when dealing with low level liquid waste, to concentrate the radioactive waste's values in the liquid before disposal. The following concentrating methods have been suggested: evaporation of the liquids, fixation of radioisotopic elements by solids, precipitation of radioisotopic elements by solids, precipitation of radioisotopic elements from the waste liquids, and calcination of the waste liquids. One of the most practical approaches to the disposal of waste liquids is fixation of the radioactive elements on a solid, as by adsorption or ion-exchange, in which radioactive ions in the waste are exchanged with nonradioactive ions in a solid ion-exchanger. The liquid, free from radioactive ions, may then be safely released for further purification processing. The spent ion-exchanger is then typically vitrified to form a leach-resistant and corrosion-resistant glass which can be stored in geologically stable sites. Although vitrification of spent ion-exchangers is a common industry practice, there remain several troublesome problems. One such problem is the volatilization of radioactive cesium and strontium during the melting operation. Moreover, the extremely high temperatures required for melting the glass matrix renders disposal of the large volumes of waste uneconomical. Therefore, there is a significant need in the art for low temperature fixation techniques of hazardous waste materials. One low temperature technique for solidifying non heat-generating radioactive wastes in the form of liquids, sludges, or solids is by mixing the waste with cement and casting the mixture into drums. This low-cost cementation process is attractive for encapsulating relatively large volumes of intermediate level waste. However, it is possible for radionuclides to be released from the final waste form by leaching if there is contact with water. Thus, there is a need in the art for storage systems which do permit hazardous compounds to leaching into the uncontaminated environment. An ideal waste containment system should satisfy some of the following characteristics: (1) the contaminant barrier should be made of a nonmetal or other material which intrinsically does not corrode and produce gases; PA0 (2) the contaminant barrier should be inexpensive; PA0 (3) the contaminant barrier should prevent contaminant liquids, dissolved ions, or gases from escaping or permeating the barrier; PA0 (4) the contaminant barrier should be of a material which "self-heals" upon contact with an aqueous solution; and PA0 (5) for long term storage, the contaminant barrier should be made of a material having a record of long term geologic stability. The term "self-healing" means that if a crack forms in the contaminant barrier, the presence of water will cause a hydration reaction forming an integral bond between the cracked surfaces. From the foregoing, it will be appreciated that what is needed in the art are contaminant barriers which are constructed of materials which do not intrinsically corrode to produce gases. Additionally, it would be a significant advancement in the art to provide contaminant barriers which include liquid, ion, and gas getters. It would be a further advancement in the art to provide contaminant barriers constructed of materials which are self-healing upon contact with aqueous solution. It would be another advancement in the art to provide contaminant barriers which do not require high temperature or vitrification processes. Finally, it would be an important advancement in the art to provide contaminant barriers which are inexpensive. Such contaminant barriers are disclosed and claimed herein. BRIEF SUMMARY AND OBJECTS OF THE INVENTION The present invention is directed to engineered cementitious contaminant barriers. The cementitious contaminant barriers are formed by positioning a hydraulic cement composition and one or more liquid, ion, or gas getters into a predetermined configuration and then hydrating the cement composition without substantial mechanical mixing of the cement and water. The contaminant barriers of the present invention are capable of isolating contaminants, including toxic and radioactive waste materials, from a substantially uncontaminated environment. As such, the barriers may be prepared in a variety of configurations including, but not limited to, waste storage and disposal containers, in situ barrier walls, pipes, tanks, wells, or envelopes. The use of getters in combination with powdered hydraulic cement processing techniques, described in greater detail below, enable contaminant barriers to be engineered for effective isolation of a wide variety of different contaminants, including materials such as highly toxic and radioactive waste materials. As used herein, the term "getter" includes materials which adsorb, absorb, chemically react, ionically bond, trap, attract, or otherwise bind to selected liquids, gases, or ions. Zeolites and layered clays are examples of typical getters which might be used in the present invention. The contaminant barrier is preferably engineered or designed such that a sufficient quantity and type of getters are added to account for the anticipated liquid, gas, or ion contaminant generation by the waste material over the life of the waste. In some cases, the getters may be mixed with the hydraulic cement composition prior to forming the contaminant barrier. In other cases, the contaminant barrier may contain one or more carefully positioned getter layers combined with one or more layers of cement. The contaminant barriers within the scope of the present invention are intended to provide a boundary or barrier which separates the contaminated environment from the uncontaminated environment. On a large scale, the present invention includes in situ barriers for isolating hazardous waste materials from the environment. On a smaller scale, the present invention also includes waste containers in which the contaminant barriers form the container walls. Such containers may be divided into two general categories: (1) empty containers into which contaminants are added after the container is formed; and (2) containers which are prepared by surrounding contaminants with one or more getters and powdered hydraulic cement, compressing the cement and getter around the contaminants, and allowing at least a portion of the hydraulic cement to hydrate. The cementitious barriers preferably undergo some hydration to close the cement pore structure and to provide mechanical strength. The amount of hydration may vary from a very nominal amount to extensive hydration depending upon the desired properties and characteristics of the final contaminant barrier. In some cases, the cementitious barrier may even be hydrated from exposure to ambient water in the environment such as water vapor in the air or from ground water in an underground storage facility. It is also within the scope of the present invention to provide the water necessary for hydration from gypsum (a hydrated calcium sulphate, CaSO.sub.4.2H.sub.2 O), ettringite (a calcium sulphoaluminate, 3CaO.Al.sub.2 O.multidot.3CaSO.sub.4 .multidot.31H.sub.2 O), zeolites containing water (such as zeolite X and zeolite Y), or other compounds containing water in a crystalline form. The compounds are combined with the powdered hydraulic cement prior to forming the cementitious contaminant barrier. Subjecting the zeolites or gypsum in the waste container to mild heating (&lt;100.degree. C.) causes the crystals to release water which is capable of reacting with the cement. High green strengths may also be obtained using this technique. In addition to water-containing compounds, it is also within the scope of the present invention to use clays, zeolites, or zeolite-like compounds to selectively release beneficial chemicals into the cement matrix upon mild heating or pressure. For instance, cancrinite, a zeolite, and hydrotalcite, a clay, are capable of releasing carbonates into the cement matrix upon mild heating. Carbonates can enhance the strength, chemical stability, and durability of the final cementitious contaminant barrier. A wide variety of other compounds can be selectively released using clays, zeolites, or zeolite-like compounds. Lowering the pH by adding acid may selectively release basic species. Acids have also been used to release organic anions and carbonates. Likewise, raising the pH with a base releases acid components. In some cases, mild heating, at temperatures less than about 250.degree. C., may selectively release the compounds of interest. The term "contaminant," as used herein, includes liquids, ions, or gases for which isolation by the cementitious barrier is desired. Contaminants include solid, substantially solid, semisolid, liquid, and gaseous waste as well as liquids, ions, or gases generated by the waste. Contaminants include toxic and radioactive waste, as well as nonhazardous materials for which isolation by the barrier is desired. Hydraulic cements used within the scope of the present invention are inexpensive, geologically and environmentally stable, and do not produce gases. In some cases, more than one layer of powdered hydraulic cement may be used. For instance, an outer layer of Portland cement may surround an inner layer of expansive and fast reacting high alumina cement. Suitable getters may be mixed with the powdered hydraulic cement. Alternatively, one or more getter layers may be used with one or more cement layers. Pressure compaction processes, including isostatic compression, are preferably used to position the hydraulic cement and getters into the desired cementitious barrier configuration. Thereafter, the hydraulic cement is preferably hydrated. Various techniques may be used to hydrate the compressed hydraulic cement. For instance, if the barrier is part of a waste container, the container may be hydrated by soaking it in an aqueous solution The aqueous solution would diffuse into the container and hydrate the cement. In some cases, sufficient hydration may be obtained by exposure in a high relative humidity. It is, therefore, an object of the present invention to provide novel contaminant barriers which are constructed of materials which do not intrinsically corrode to produce gases. Another important object of the present invention is to provide contaminant barriers which include liquid, ion, or gas getters. Yet another important object of the present invention is to provide novel contaminant barriers constructed of materials which are self-healing upon contact with aqueous solution. A further significant object of the present invention is to provide contaminant barriers which do not require high temperature vitrification processes. An additional object of the present invention is to provide novel contaminant barriers which are inexpensive. These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention.