Immobilization Science of Cement Systems

Abstract

Portland cement and Ca(OH)2, “slaked lime,” are representative of a family of materials that have conceptual and practical advantages as matrices for the immobilization of wastes. These are relatively proven construction materials with an extensive history of use in various ground-water regimes, and in various climates. The raw materials are widely available and they exhibit a reasonable amount of resistance to physical damage and attrition. They are also durable. Although modern Portland cements are only about 150 years old, numerous examples of Roman concrete made from slaked lime and volcanic pozzolanic ash survive; the Pantheon in Rome (circa A.D. 300) continues in regular use.Satisfactory immobilization matrices can also be formed by incorporating other artificial waste materials, e.g. coal combustion fly ash, blast furnace slag, and natural materials such as calcined kaolin or siliceous volcanic ashes containing glass and zeolites. These materials react with Ca(OH)2 and cement to become an integral part of a cement matrix. Thus one waste (ash, slag) can be used to help immobilize another, and may actually improve the retentive capacity of a cementitious matrix.Since cements require water to harden, they are tolerant of wet wastes and are especially useful for many effluents. In addition, they are low-cost, nonflammable materials with the potential to provide chemical and physical immobilization of wastes, but their set characteristics can be affected by certain material additions. Table I balances some advantages against disadvantages of the use of cement systems for waste immobilization.