Cement Stability

[9309528]布朗不断出现的证据表明，经过多年的时间框架，形成的化合物在成分上与最初硬化时形成的化合物不同。这些反应对水泥基系统固定危险废物的能力有两个影响。首先，次级化合物结合有害物质的能力可能与初级化合物不同。其次，次生化合物的形成常常导致整体结构的破裂和破坏。开裂导致整体表面积的增加，并可能增加浸出的敏感性。在这方面，有三个化学过程特别重要。它们是碳化作用、碱-硅反应和硫酸盐侵蚀。碳化是水泥的水化产物与大气中的二氧化碳或水中的碳酸氢盐相互作用的结果。这导致ph值降低。如果重金属或氧阴离子的固存依赖于ph值，则可能出现溶解度增加的情况。然而，这种现象的发生取决于物质与碳酸盐的接触，而碳酸化的影响似乎被忽视了，因为它被认为是一种表面现象。混凝土中的碱-硅反应通常与硅质骨料与水泥中的碱之间的反应有关。然而，无论聚合体是否存在，导致水合硅酸钙粘结剂相转化为硅酸钙碱凝胶的反应在热力学上都是有利的。当水合硅酸钙水合结合物相对钙碱硅酸凝胶在热力学上有利时，就会发生硫酸盐侵蚀，而不管骨料是否存在。当水合铝酸钙与硫酸盐反应形成类似天然矿物钙辉石的化合物时，就会发生硫酸盐侵蚀。碱-硅反应和硫酸盐侵蚀形成膨胀产物，最终破坏整体结构的完整性。由于形成了裂纹，可用于碳酸化的有效表面积显著增加。因此，各种化学反应之间的协同作用可导致合并的有害物种过早释放。虽然碱-硅反应被认为是结构恶化的主要原因，但这种有害反应发生的条件并没有很好地定义。根据普遍接受的导致水泥强度发展的反应顺序的观点，在没有来自外部来源的硫酸盐的情况下，不应发生硫酸盐侵蚀。然而，现在也观察到内部硫酸盐的攻击。造成这种现象的机制似乎还不清楚。目前还没有普遍认识到，内部硫酸盐攻击的发生与碱-硅反应有协同作用。这两种反应都能在不暴露于外部离子源的情况下发生。该研究计划将确定波特兰水泥基体系中缓慢形成的化合物的稳定范围，这些化合物可能会损害废物形式的完整性。这些反应包括碱-硅反应、硫酸盐侵蚀和碳酸化。因为这些都是化学过程，这些反应的进展表现出组分依赖性。这些反应的产物是钙矾石、钙取代水合硅酸钾和各种碳酸盐。有利于形成这些化合物的条件将被确定为组成和温度的函数。这是最终确定有害物质对水泥长期稳定性影响的必要步骤。这些化合物稳定范围的建立反过来又提供了它们形成所需条件与水泥体积成分之间的联系。因此，限制这些有害的反应，有可能通过在指定废物形式时控制所使用的水泥的组成来实现。实际上，这将有可能制定一个合理的、规范性的基础，以促进用于废物处理的水泥基系统的长期性能。＊＊＊

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. ***