Immobilization Science of Cement Systems
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mally needed for hydration persists in hardened cement paste, trapped in pores. Many of the concepts applicable to aqueous solutions can therefore be applied to cement. The trapped "water" in a hardened cement paste is not pure, but dissolves much of the sodium and potassium present in the cement. Since many potential anions—silicate, aluminate, sulfate—are removed as relatively insoluble cementing substances, the principal counter anion remaining is hydroxide, OH '; the high hydroxyl ion content ensures that cement pore fluids are alkaline; their pH typically lies in the range 12.5-13.5. The relatively high hydroxide content, supported by alkali, suppresses the solubility of Ca(OH)2. If, however, alkali is not present, or is eventually removed by leaching, Ca(OH)2 is itself somewhat soluble; a saturated solution at 18°C has a
pH —12.5. Thus, the large reserve of solid Ca(OH)2 present in cement, or added as slaked lime, acts as a buffer so that the internal pH tends not to decrease below this value. An important component of cement is a gel-like calcium-silicate-hydrate phase, usually designated as C-S-H. Its composition is variable, ranging from molar Ca:Si ~ 1.7 in contact with Ca(OH)2 to as low as =1.2 in some fly ash or slag blends. Its high specific surface area and strong unsatisfied surface charges lead to a sorption potential, although the mechanism of sorption is not always known. However, the most immediate impact of the cement arises from its conditioning action, in conjunction with Ca(OH)2, on the pH. The CS-H/Ca(OH)2 buffer has a pH close to 12.5.
Wastes in Cement Systems Waste Classification There are many ways of classifying wastes. Classification according to toxicity seems obvious, but toxicity is not capable of a single exact definition; for example, the toxic potential of a radioactive waste species is quite different from that of a heavy metal. From the engineering standpoint, classification schemes should take into account waste production rates; strategies devised for low-volume toxic streams may be inappropriate for large volumes of waste, e.g., municipal incinerator ash. Moreover, the efficiency of alternative toxicity abatement processes varies. Organics, which owe their toxic effect to molecular structure, can be permanently transmuted by combustion to relatively innocuous
Table I: Cement-Based Systems as Matrices for Waste Immobilization. Advantages: • Proven technology, including automated processing, if required. • Inexpensive raw materials. • Standards exist for production and quality checking. Codes of practice for construction applications can be adapted. High degree of acceptability as a proven matrix material. • Ability to incorporate solid matrix modifiers (slag, coal combustion fly ash, etc.) for improved immobilization of other wastes. • Good durability in nature. • Relatively nontoxic, nonflammable matrix. • Tolerant of wet wastes. • Provides chemical as well as physical immobilization; in addition, high alkalinity discourages microbiological activity. Disadvantages: • L
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