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<br />Recalling [he stages of cement clmkerinQ in the kiln, a can be seen that the CKD example <br />above is made up of the raw matertals For clinker manufacture, which are limestone (CaCO;) and <br />calcined limestone or lime (Ca0). It makes sense that the kiln exhaust would contain dust made <br />up of pre-choker raw materials, because by the time clinker is formed the material is molten and <br />would not produce much dust. Most cement plants rec}'cle their kiln dust into the kiln. <br />However, complete recycling is not possible due to a build up of undesirable elements as <br />discussed below. CKD is not por[land cement and does not exhibit the complex cementitious <br />properties of Portland cement. For most CKD, the chemical and physical behavior is dominated <br />by the lime (Ca0) component. CKD contains little calcium sihcare, which is the basic ingredient <br />of cement. Continuous recyclins of CKD concentrates alkalis (potassium and sodium), chloride. <br />and sulfate, whtch adversely affect the clinkering process and force the kiln operator to shunt a <br />certain percentage of CKD. Recycled CKD also concentrates metals making recycled CKD <br />disposal potentially likely to release metals to the environment. <br />This memo distinguishes between what is termed "fresh" CKD and "weathered" CKD. <br />These materials differ in their physical properties and in their response to leach testing, metals <br />mobility, and effects on pH. These differences are discussed. Some CKD may contain large <br />quantities of calcined teed materials, alkalis' and sulfur compounds, or both, while others may be <br />primarily composed of uncalcined raw feed. Many CKDs are easily compacted and reactive <br />with small quantities of water to form a low strength cementitious mass; some may show only <br />mild reactivity and be non-consolidating. The following discussion of "fresh" and "weathered" <br />CKD assumes a limey CKD. <br />"Weathered" CKD. As discussed above, most CKD is largely composed of lime (Ca0). Lime is <br />a simple cementing material produced by driving free water and carbon dioxide from limestone <br />(calcining). Its cementing properties arise from the reabsorption of the liquid and gas that has <br />been expelled and the formation of chemical compounds similar to the original limestone raw <br />material. Left exposed to the atmosphere. CKD will react, cure and harden. Its ultimate <br />composition will become limestone. Thus, in that regard, "weathered" CKD is not CKD at all. <br />It is limestone. <br />What will actually form when CKD is exposed to the atmosphere is a cohesive mass that <br />looks like rock but which is soft, easily broken, and which can be pulverized with gentle abrasion <br />even in the hands. In this memo, this type of CKD is called "weathered" CKD. "Weathered" <br />CKD is distinguished from "fresh" CKD, described following. <br />"Fresh" CKD. The lime in "fresh" CKD -CKD that has not been exposed to the atmosphere - <br />has a strong affinity for water. Exposed to atmospheric moisture or meteoric water, fresh CKD <br />begins [o hydrate or slake. To fully slake CKD, it would have ro be thoroughly mixed with nvo <br />to three times it weight of water. This is unlikely to occur in most CKD disposal areas, in a short <br />period, but some portion of disposed CKD will slake -more so if placed in a water filled pit. <br />When lime in CKD combines with water, calcium hydroxide forms via an exothermic reaction. <br />The product is a finely divided calcium hydroxide which, upon cooling, stiffens to a putty and <br />will eventually season and cure to weathered CKD. Incomplete slaking of CKD, which is the <br />likely situation at a quarry disposal site, will likely result in a variable mass of weathered and <br />fresh CKD and hydrated CKD present as a fine powder. Unless there is significant dilution, <br />CKD in water will increase pH. Paste pH measurements can range up to l2. <br />5 <br />