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What will actually form when CKD is exposed to the atmosphere is a cohesive mass that looks <br />like rock but which is soft, easily broken, and which can be pulverized with gentle abrasion even <br />in the hands. In this memo, this type of CKD is called "weathered" CKD. "Weathered"CKD is <br />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 great affinity for water. When CKD is exposed to atmospheric moisture or meteoric water <br />it begins to hydrate or slake. To fully slake CKD, it would have to be thoroughly mixed with <br />two to three times it weight of water. This is unlikely to occur in most CKD disposal areas, but <br />some portion of disposed CKD will slake. More so if the CKD is disposed of in a water filled <br />pit. When the lime in CKD combines with water calcium hydroxide is formed in an exothermic <br />reaction. The resulting product is a finely divided calcium hydroxide which, upon cooling, <br />stiffens to a putty and wilt eventually season and cure to weathered CKD. Incomplete slaking of <br />CKD, which is the likely situation at a quarry disposal site, will likely result in a variable mass of <br />weathered and fresh CKD and hydrated CKD present as a fine powder. Unless there is <br />significant dilution, CKD in water will increase pH. Paste pH measurements can range up to 12. <br />Source of CKD Pollutants. Fully "weathered" CKD, which is actually limestone, tends strongly <br />not to release metals when placed in water. However, if pulverized, metals can be released, but <br />pH does not increase. Release of metals due to pulverization is a surface area effect, alone; any <br />rock, if crushed or ground will release more metal to solution in a fixed period of time than that <br />same rock will release if left uncrvshed. Because pulverization generates a high surface area, <br />some "weathered" CKD can release high metals. <br />Elements that were present in the clay or limestone prior to kilning for cement manufacture may <br />dissolve when "fresh" CKD is placed in water. The major element and trace element <br />compositions of the clay and limestone prior to calcining determines what elements may appear <br />dissolved in the water to which "fresh" CKD is added. Most of the dissolved metals come from <br />clay, not limestone. As discussed previously, volatile metals tend to concentrate in CKD at kilns <br />that employ continuous CKD recycling. <br />Limestone (CaC03) typically does not contain significant concentrations of trace elements other <br />than strontium because the calcite structure accommodates elements only of specific charge and <br />size. Locally, Zn and/or Ba can be high in some limestones, but most other trace elements appear <br />at low levels in limestone. Moreover, limestones that are selected for cement manufacture are <br />chosen for their low percentage of impurities (clay and other silicates) so tend toward more pure, <br />or high CaC03 rocks. Thus, the limestone raw material component cannot contribute much <br />dissolved metal when lime or "fresh" CKD is exposed to water. <br />Clay minerals, however, are noted for containing high concentrations of many trace elements. <br />Informally, clay minerals aze called "garbage" minerals because their structure accommodates <br />elements covering the complete range of size and charge. While these elements are tightly <br />bound and not significantly released during weathering, the process of calcining breaks bonds <br />and disrupts the mineral structure, freeing up both the major elements and associated trace <br />elements from the clay minerals. Thus, elements that may be released when CKD is exposed to <br />water come dominantly from the calcined clay minerals. It follows that the pollutants which may <br />7 <br />