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Recalling the stages of cement clinkering in the kiln, i[ can be seen that the CKI) example above is made up of the raw materials for clinker manufacture, which are limestone (CaCOa) and calcined limestone or lime {Ca0). It makes sense that the kiln exhaust would conta.n dust made up of pre-clinker raw materials, because by the time clinker is formed the material i:> molten and would not produce much dust. Most cement plants recycle their kiln dust into the kiln. However, complete recycling is not possible due to a build up of undesirable elements as discussed below, CKD is not Portland cement and does no[ exhibit the complex cernen[itious properties of Portland cement. For most CKD, the chemical and physical behavior is dominated by the lime (Ca0) component. CKD contains little calcium silicate, which is the basic ingredient of cement. Continuous recycling of CKD concentrates alkalis (potassium and sodium), chloride, and sulfate, which adversely affect the clinkering process and force the kiln operato!- to shunt a certain percentage of CKD. Recycled CKD also concentrates metals making recycl:d CKD disposal potentially likely to release metals to the environment. This memo distinguishes between what is termed "fresh" CKD and "weathered" CKD. These materials differ in their physical properties and in their response to leach testing, metals mobility, and effects on pH. These differences are discussed. Some CKD may cone ain large quantities of calcined feed materials, alkalis and sulfur compounds, or both, while others may be primazily composed of uncalcined raw feed. Many CKDs are easily compacted and reactive with small quantities of water to form a low strength cementitious mass; some may show only mild reactivity and be non-consolidating. The following discussion of "fresh" and `'weathered" CKD assumes a limey CKD. "Weathered" CKD. As discussed above, most CKD is largely composed of lime (Ca0). Lime is a simple cementing material produced by driving free water and carbon dioxide from limestone (calcining). Its cementing propenies arise from the reabsorption of the liquid and g~~s that has been expelled and the formation of chemical compounds similaz to the original limestone raw material. [..eft exposed to the atmosphere, CKD will react, cure and harden. Its ultimate composition will become limestone. Thus, in that regard, "wentl)ered " CKD is not CKD at all. It is limestone. What will actually form when CKD is exposed to the atmosphere is a cohesive mass that looks like rock but which is soft, easily broken, and which can be pulverized with g~:ntle abrasion even in the hands. In this memo, this type of CKD is called "weathered" CKD. "Weathered" CKD is distinguished from "fresh" CKD, described following. "Fresh" CKD. The lime in "fresh" CKD -CKD that has not been exposed to the atmosphere - has a strong affinity for water. Exposed to atmospheric moisture or meteoric water, fresh CKD begins to hydrate or slake. To fully slake CKD, it would have to be thoroughly mixed with two to three times it weight of water. This is unlikely to occur in most CKD disposal areas, in a short period, but some portion of disposed CKD will slake -more so if placed in a water felted pit. When lime in CKD combines with water, calcium hydroxide forms via an exotherrruc reaction. The product is a finely divided calcium hydroxide which, upon cooling, stiffens to n putty and will eventually season and cure to weathered CKD. Incomplete slaking of CKD, which is the likely situation at a quarry disposal site, will likely result in a variable mass of weathered and fresh CKD and hydrated CKD present as a fine powder. Unless there is significant dilution, CKD in water will increase pH. Paste pH measurements can range up to 12.