INSPEC35716

• Permit No: M1977361
• IBM Index Class Name: Inspection
• Doc Date: 12/3/1999
• Doc Name: CEMENT KILN DUST ISSUES OF LONG TERM STORAGE AT LIMESTONE MINES
• From: DMG
• To: BRUCE HUMPHRIES
• Media Type: D
• Archive: No

•
Recalling the stages of cement clinkering in the kiln, it can be seen that [he CKD example
above is made up of the raw materials for clinker manufacture, which are limestone (CaCO~) and
calcined limestone or lime (Ca0). It makes sense that the kiln exhaust would contain dust made
up of pre-clinker raw materials, because by the time clinker is Formed the material is molten and
would not produce much dust. Nlost cement plants recycle their kiln dust into [he kiln.
However, complete recycling is not possible due to a build up of undesirable elements as
discussed below. CKD is not purtland cement and does not exhibit the complex cementitious
properties of pottland cement. For most CKD. [he 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 operator to shunt a
certain percentage of CKD. Recycled CKD also concentrates metals making recycled CKD
disposal potentially likely to release metals to the environment.
This memo distinguishes bettveen 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 contain large
quantities of calcined teed materials, alkalis and sulfur compounds, or both, while others may be
primarily 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 properties arise from the reabsorption of the liquid and gas that has
been expelled and the formation of chemical compounds similar to the original limestone raw
material. Left exposed to the atmosphere, CKD will react, cure and harden. Its ultimate
composition will become linresrone. Thus, in that regard, "rvearhered" CKD is not CKD at all.
II is limestone.
What will actually form when CKD is exposed [o the atmosphere is a cohesive mass that
looks like rock but which is soft, easily broken, and which can be pulverized with gentle 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 [o 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 tilled pit.
When lime in CKD combines with water, calcium hydroxide forms via an exothermic reaction.
The product is a finely divided calcium hydroxide which, upon cooling, stiffens to a 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.
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