CaO
Calcium oxide · quicklime, burnt lime, unslaked lime, calcia
Calcium oxide (CaO), commonly known as quicklime, is a binary alkaline-earth oxide with the cubic rock-salt (halite) crystal structure. It is a thermodynamically stable, wide-band-gap insulator made on a very large scale by calcining calcium carbonate (limestone).
About Calcium oxide
What is CaO and what structure does it adopt?
Calcium oxide is a binary ionic oxide made of calcium cations (Ca2+) and oxide anions (O2-) in a one-to-one ratio. At ambient conditions it crystallizes in the cubic rock-salt (halite) structure, space group Fm-3m. Each calcium ion sits at the centre of an octahedron of six oxide ions, and each oxide ion is surrounded by six calcium ions. It is a standard reference material for studying ionic bonding, defect states, and surface basicity in alkaline-earth oxides.
What is calcium oxide used for?
Calcium oxide is a major industrial chemical. Its largest use is in steelmaking, especially the basic oxygen process, where it acts as a basic flux that removes acidic impurities such as silica, phosphorus, and sulfur into the slag. It is also used for building lime and mortar, in water treatment, in flue-gas desulfurization, and as a sorbent in calcium-looping cycles for carbon capture. It reacts with water to give calcium hydroxide (slaked lime).
How stable is CaO and how is it synthesized?
Calcium oxide is thermodynamically stable. Industry makes it by heating limestone (calcium carbonate) in a process called calcination, which drives off carbon dioxide. CaO reacts readily with water in a strongly exothermic slaking reaction that produces calcium hydroxide. It also absorbs carbon dioxide from the air and turns back into calcium carbonate, so it has to be stored sealed and dry.
Is CaO a conductor, semiconductor, or insulator?
Calcium oxide is an electronic insulator with a wide band gap. Its bonding is mainly ionic: the valence band is formed mostly from oxygen states, and the conduction band is empty under ordinary conditions. Because the gap is so wide, its electronic conductivity at room temperature is negligible. Pure CaO crystals are colourless and transparent to visible light.
How does CaO compare to other alkaline-earth oxides?
CaO belongs to the alkaline-earth oxide family together with magnesium oxide (MgO), strontium oxide (SrO), and barium oxide (BaO). All four have the rock-salt structure at room conditions and are wide-band-gap insulators. Going down the group from magnesium to barium, the cation gets larger, so the unit cell grows and the lattice energy falls. CaO is more basic than MgO and reacts with moisture much faster, since MgO slakes far more slowly. It is less basic than the heavier SrO and BaO.
Key Properties
Cross-validated computational properties for Calcium oxide, aggregated across 5 databases.
Band GapEnergy needed to move an electron from the valence band to the conduction band. Lower or zero values tend to behave more metallic; larger gaps are more insulating or semiconducting.
Energy Above HullThermodynamic distance from the most stable set of competing phases. 0 eV/atom is on the convex hull; small positive values may still be experimentally accessible.
StabilityA plain-language summary of the best reported energy-above-hull result. It reflects whether the lowest-energy structure is on, near, or far from the stability hull.
StructuresCount of reported calculated crystal structures for this formula, including alternate polymorphs, source databases, and observed space groups.
Cross-Source DFT Agreement
How well independent DFT databases agree on the thermodynamics of CaO. Tight agreement means computed properties can be trusted without re-running calculations.
Agreement ScoreA normalized confidence score summarizing how closely independent DFT databases agree. Higher scores mean tighter cross-source agreement.
Hull SpreadDifference between the highest and lowest energy-above-hull values reported by comparable sources. Smaller spread means less thermodynamic disagreement.
Sources ComparedNumber and names of computational sources with comparable entries for this formula.
Space Group ConsensusWhether independent sources predict the same crystal symmetry for the lowest-energy structure.
Reported Structures
Lowest-energy structures reported for CaO, ranked by energy above hull.
| Space GroupSymmetry classification of the crystal arrangement. The number is the international space-group index. | Crystal SystemBroad lattice family, such as cubic, tetragonal, monoclinic, or triclinic, derived from unit-cell symmetry. | Band Gap (eV)Electronic gap calculated for this specific reported structure, measured in electronvolts. | E above hull (eV/atom)Thermodynamic distance from the convex hull for this structure, normalized per atom. Lower is generally more stable. | E/atom (eV)Computed total energy normalized per atom. Use energy above hull, not this value alone, when comparing stability. | Density (g/cm³)Mass per relaxed crystal volume, reported in grams per cubic centimeter. |
|---|---|---|---|---|---|
| Fm-3m (No. 225) | cubic | 3.74 | 0.0000 | -3.273 | — |
| Fm-3m (No. 225) | cubic | 3.64 | 0.0000 | -6.783 | 3.35 |
| — | — | 3.74 | 0.0003 | -3.196 | — |
| — | — | 3.79 | 0.0006 | -3.195 | — |
| — | — | 3.74 | 0.0007 | -3.195 | — |
| — | — | 3.67 | 0.0008 | -3.195 | — |
| — | — | 3.74 | 0.0010 | -3.195 | — |
| — | — | 3.83 | 0.0010 | -3.195 | — |
| — | — | 3.86 | 0.0011 | -3.195 | — |
| — | — | 3.69 | 0.0022 | -3.194 | — |
| — | — | 3.87 | 0.0023 | -3.194 | — |
| — | — | 3.87 | 0.0023 | -3.194 | — |
Synthesis Routes
Literature-extracted synthesis procedures targeting CaO.
Applications
Where Calcium oxide is used.
Patent Landscape
10 patents reference CaO or close compositional variants.
| Patent | Title | Assignee | Granted |
|---|---|---|---|
| 5110772 | Fabrication of dense SI.sub.3 N.sub.4 ceramics using CaO-TiO.sub.2 SiO.sub. 2 | — | — |
| 3965237 | Dissolution process for ZrO.sub.2 -UO.sub.2 -CaO fuels | — | — |
| 3966421 | Ultra-high strength steel containing CaO, MgO, BaO or SrO having improved resistance to environmental stress c | — | — |
| 7087526 | Method of fabricating a p-type CaO-doped SrCu2O2 thin film | — | — |
| 4795725 | Method for the manufacture of refractory bricks containing CaO | — | — |
| 4486545 | SIO.sub.2 -CaO Based low cubically expansive flame-spraying material | — | — |
| 7595037 | Method and apparatus for hydration of a particulate or pulverulent material containing CaO, hydrated product, | — | — |
| 4710481 | Method for melting Ti or a high-Ti alloy in CaO refractories | — | — |
| 4605443 | CaO-Al.sub.2 O.sub.3 -SiO.sub.2 glass hydraulic cements | — | — |
| 5077260 | Compositions involving V.sub.2 O.sub.3 -CaO | — | — |
Frequently Asked Questions
Common questions about Calcium oxide, answered from cross-validated data and the published literature.
What is the difference between quicklime and slaked lime?
Quicklime is calcium oxide (CaO), and slaked lime is calcium hydroxide (Ca(OH)2). Slaked lime is made by reacting quicklime with water, a strongly exothermic reaction.
What crystal structure does CaO adopt?
Is calcium oxide hazardous to handle?
How is calcium oxide produced commercially?
Does CaO react with atmospheric carbon dioxide?
What is CaO?
What is CaO used for?
What is the band gap of CaO?
Is CaO a metal, semiconductor, or insulator?
Is CaO thermodynamically stable?
What is the crystal structure of CaO?
How many polymorphs of CaO are known?
How is CaO synthesized?
What elements does CaO contain?
Where does the data for CaO come from?
Data sources & attribution
- jarvis — Data from JARVIS (jarvis.nist.gov), NIST. Cite: Choudhary et al., npj Comp. Mater. 6, 173 (2020). (LicenseRef-US-Gov-PD)
- materials_project — Data from the Materials Project (materialsproject.org). Cite: Jain et al., APL Materials 1, 011002 (2013). (CC-BY-4.0)
- oqmd — Data from the OQMD (oqmd.org). Cite: Saal et al., JOM 65, 1501 (2013); Kirklin et al., npj Comp. Mater. 1, 15010 (2015). (CC-BY-4.0)
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