What Is \"Free Lime\" in Clinker?
Free lime is uncombined CaO left in clinker after burning. Typical target 0.5-1.5%, and the primary burnability and kiln-control signal.
Free lime is the uncombined calcium oxide (CaO) that remains in cement clinker after burning, reported as a weight percent of the clinker. It is the lime that did not react into the clinker minerals in the kiln, and its value is the single most-watched readout of how completely the clinker was burned. The practical target leaving the burning zone is roughly 0.5 to 1.5% [1].
A quick disambiguation. Free lime here means residual uncombined CaO inside Portland clinker, not quicklime (the bulk CaO product of a dedicated lime kiln) and not the "free lime" of soil or water chemistry. Those are different materials with different specifications.
Free lime (free CaO): the calcium oxide in clinker that did not combine into the clinker minerals during burning. It is the unreacted remainder of the lime fed to the kiln, measured as a weight percent of clinker, and used as the primary indicator of burning completeness.
Why does any lime stay free? It is practically impossible to drive the clinkering reaction to 100% completion in a rotary kiln, so some unreacted CaO always remains [1]. The aim is not zero. It is a controlled, low value that confirms the alite (C3S) reaction went far enough without wasting fuel to chase the last fraction.
Free lime target ranges
The table below gives the operating ranges cited across kiln-control practice. Values are weight percent of clinker.
| Condition | Free lime (% by weight) | Source |
|---|---|---|
| Leaving the burning zone (typical target) | 0.5-1.5 | INFINITY for Cement Equipment [1] |
| Tight plant control band | 0.6-1.2 | IJAAR free-lime/LSF study [2] |
| Broad Portland clinker range observed | 0.5-3.0 | ScienceDirect [3] |
If you run a cement, lime, or mineral-processing kiln, the same heat balance that drives your free lime target also drives your fuel bill, and false air across worn kiln seals quietly pushes both the wrong way. Our engineering team maps seal condition to fuel and draft losses case by case across cement kiln sealing and integrated false air control. Contact us to walk through your kiln's configuration.
Sources
- INFINITY FOR CEMENT EQUIPMENT, *Kiln Control and Operation* (free lime leaving the burning zone 0.5-1.5%; swing-arm control)
- IJAAR, *Effect and Control of Free Lime and Lime Saturation Factor on Clinker Quality*
- ScienceDirect Topics, *Free Lime: an overview* (broad clinker range; XRD for weathered samples)
- JoVE Science Education, *Soundness of Cement* (free lime delayed hydration, expansion, autoclave >0.8% unsound)
- ScienceDirect Topics, *Lime Saturation Factor: an overview* (LSF near unity hard to burn, high free lime; LSF 92-98)
- INFINITY FOR CEMENT EQUIPMENT, *Everything You Need to Know About the Chemistry of Kiln Feed and Clinker* (burning-zone temperature 1,400-1,450 °C)
- C. L. Ford, *Determination of Uncombined Lime in Portland Cement: The Ethylene Glycol Method*, Analytical Chemistry (ethylene-glycol extraction, ~70 °C, 15 min, acid back-titration)
- iFactory, *Free Lime Soft Sensor AI: Predict 15-30 Min Ahead of Lab* (2-4 h lab lag; 30-50 kcal/kg over-burn penalty; predictive soft sensors)
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