9309528 Brown Evidence continues to emerge that cements over a time-frame of years to produce compounds compositionally distinct from those which form during initial hardening. These reactions have two effects on the ability of cement-based systems to immobilize hazardous wastes. First, the capacities of the secondary compounds to bind hazardous materials are likely to be different from those of the primary compounds. Second, formation of secondary compounds often result in cracking and disruption of monolithic forms. Cracking results in an increase in the surface area of a monolith and may heighten the susceptibility to leaching. Three chemical processes are of particular importance in this regard. These are carbonation, the alkali-silica reaction and sulfate attack. Carbonation results from interaction of the hydration products of cement with atmospheric CO2 or bicarbonate in water. This results in the depression of pH. If the sequestration of a heavy metal or an oxyanion is pH-dependent, increased solubility may occur. However, the occurrence of the phenomena depends on the exposure of the mass to carbonate and the effects carbonation seem to have been ignored because, it is regarded as a surface phenomenon. The alkali-silica reaction in concrete is conventionally associated with the reaction between siliceous aggregate and the alkalis in cement. However, the reaction resulting in the conversion of the calcium silicate hydrate binder phase to a calcium-alkali silicate gel is thermodynamically favorable regardless of the presence of aggregate. Sulfate attack occurs when hydrated calcium silicate hydrate binder phase to a calcium-alkali silicate gel is thermodynamically favorable regardless of the presence of aggregate. Sulfate attack occurs when hydrated calcium aluminates react with sulfate to form a compound similar to the natural mineral ettringite. Both alkali-silica reaction and sulfate attack form expansive products which eventually dest roy the structural integrity of a monolith. As a result of the cracks formed, a significant increase in the effective surface area available for carbonation occurs. Thus, the synergy between various chemical reactions can result in the premature liberation of incorporated hazardous species. Although the alkali-silica reaction is recognized as a major cause of deterioration in structures, the conditions under which this deleterious reaction occurs are not well defined. Based on the generally accepted view of the sequence of reactions which lead to strength development in cement, sulfate attack should not occur in the absence of sulfate from an external source. However, internal sulfate attack is now being observed as well. The mechanism causing this does not seem to be understood. It is not generally recognized that onset of internal sulfate attack is linked synergistically with the alkali-silica reaction. Both reactions are able to occur in the absence of exposure to external sources of ions. This research program will establish the stability ranges of the compounds slowly formed in portland cement-based systems which are likely to compromise the integrity of waste forms. These include the alkali-silica reaction, sulfate attack, and carbonation. Because these are chemical processes, the progression of these reactions exhibit compositional dependencies. The products of these reactions are ettringite, calcium substituted potassium silicate hydrate and various carbonates. The conditions favoring the formation of these compounds will be established as functions of composition and temperature. This is a necessary step in the eventual determination of the effects of hazardous materials on the long- term stabilities of cements. Establishment of the stability ranges for these compounds in turn provide the link between conditions required for their formation and the bulk compositions of cement. Therefore, limiting these deleterious reactions, can, pot entially, be accomplished by controlling the compositions of the cement used when specifying a waste form. Pragmatically, this will make it possible to develop a rational, prescriptive basis to facilitate the long-term performance of cement-based systems used in waste disposal. ***
