What Is the Chemical Name for Lime? All 3 Types

Lime refers to three related calcium-based chemicals, each with a distinct chemical name: calcium carbonate (CaCO₃), calcium oxide (CaO), and calcium hydroxide (Ca(OH)₂). These are commonly known as limestone (or agricultural lime), quicklime (or burnt lime), and slaked lime (or hydrated lime), respectively. Despite sharing the word “lime,” these three substances differ in composition, behavior, and how they are used, and the relationships among them form one of the oldest chemical cycles in human industry.

Calcium Carbonate, the Starting Material

Calcium carbonate is the chemical name for what most people encounter as limestone, chalk, or marble. It is the raw material from which the other two types of lime are derived. In nature, calcium carbonate is extraordinarily common. It makes up the bulk of limestone rock formations, coral reefs, seashells, and eggshells. When someone refers to “agricultural lime” or “aglime,” they almost always mean ground-up calcium carbonate spread onto fields to reduce soil acidity. In the United Kingdom, almost 70% of the liming material used on farmland is ground limestone.¹

Calcium carbonate is the mildest of the three limes. It has a roughly neutral pH and does not burn skin or generate heat. Its role in raising soil pH works gradually: the calcium carbonate dissolves slowly in the mildly acidic soil water, releasing calcium ions and neutralizing acidity over weeks or months. A closely related variant, dolomitic limestone, contains both calcium carbonate and magnesium carbonate (CaMg(CO₃)₂), which means it supplies magnesium alongside calcium. Chalk is chemically the same as limestone but is softer and breaks down more readily in soil.¹

Calcium Oxide, or Quicklime

When calcium carbonate is heated to very high temperatures, it breaks down into calcium oxide and carbon dioxide gas. This process, called calcination, typically requires temperatures between 900 °C and 1100 °C.² The calcium oxide left behind is known as quicklime, burnt lime, or simply “lime” in many industrial contexts. It is a white, caustic solid that reacts aggressively with water and can cause serious chemical burns on contact with moist skin.

Quicklime has been produced for thousands of years. Romans built specialized kilns to burn limestone and used the resulting calcium oxide in construction, agriculture, tanning, and medicine.² The basic chemistry has not changed, though modern kilns are far more efficient. An interesting modern twist on this ancient process involves using chicken eggshell waste as a renewable source of calcium carbonate. Research on thermal decomposition of hatchery waste in a rotary kiln has produced bio-calcium oxide at purities of 97 to 98%, with production yields in the range of 49 to 56% by weight.³ That approach turns a waste stream into a useful industrial product while producing essentially the same calcium oxide you would get from a traditional limestone quarry.

In limestone samples that contain dolomite (the calcium-magnesium carbonate mineral), the dolomite fraction breaks down first during heating, followed by the pure calcite.⁴ This matters in practice because many natural limestones are not pure calcite; they contain some magnesium, and the decomposition behavior of the mixed material affects the quality of the quicklime produced.

Calcium Hydroxide, or Slaked Lime

Add water to quicklime and you get calcium hydroxide, the third type. This reaction, called slaking, is dramatically exothermic: the calcium oxide absorbs water, releases substantial heat, and expands in volume. During slaking, water molecules penetrate into the crystal structure of calcium oxide, and the resulting conversion creates expansion stresses that crack the newly formed calcium hydroxide particles.⁵ The product is a white powder or putty, depending on how much water is used, and it is strongly alkaline with a pH of about 12.4 at room temperature.⁶

Calcium hydroxide goes by several everyday names: slaked lime, hydrated lime, and pickling lime. In construction, it is a key ingredient in traditional lime mortars and plasters. In food processing, it is used in nixtamalization, the process of soaking maize in an alkaline solution that loosens the hulls and improves the nutritional value of corn tortillas. In water treatment, it is used to soften hard water and adjust pH. Its versatility comes from the combination of strong alkalinity and relative safety compared to quicklime, since calcium hydroxide does not react violently with water the way calcium oxide does.

How the Three Types Connect

The three types of lime are not unrelated chemicals that happen to share a name. They form a cycle. Start with calcium carbonate, heat it to drive off carbon dioxide, and you get calcium oxide. Add water to calcium oxide and you get calcium hydroxide. Expose calcium hydroxide to carbon dioxide in the air and, over time, it slowly reverts to calcium carbonate. That final step, called carbonation, is what makes lime mortar harden: atmospheric carbon dioxide reacts with the alkaline calcium hydroxide to produce calcium carbonate, essentially turning the mortar back into stone.⁷

This cycle means that a Roman building plastered with lime mortar 2,000 years ago has walls whose outer surface has chemically returned to the same calcium carbonate that was originally quarried. The lime cycle is one reason lime-based construction is considered more environmentally friendly than Portland cement in some applications: the carbonation process reabsorbs some of the carbon dioxide released during the initial calcination, though not all of it. The cycle also explains why the three types of lime tend to cause confusion. They are genuinely different stages of the same substance.

Why the Names Get Confusing

Part of the confusion is that “lime” without a qualifier can refer to any of the three, and the intended meaning shifts depending on the industry. A farmer saying “lime” almost certainly means ground calcium carbonate. A steelworker or a water treatment operator saying “lime” usually means calcium oxide. A plasterer or mason saying “lime” might mean calcium hydroxide (as a putty or powder) or might mean a lime mortar that is in the process of carbonating back to calcium carbonate.

Then there is the citrus fruit. The lime you squeeze into a drink has no chemical relationship to any of these calcium compounds, though the fruit does contain calcium among its minerals. Research on citrus mineral content has found that limes actually have the highest calcium concentration of the common citrus fruits, roughly two and a half times the calcium found in pomelo and nearly double that in lemons, mandarins, and grapefruit.⁸ That is a coincidence and a modest one, but it does occasionally trip people up when they search for “lime” and “calcium” together. The chemical limes and the citrus fruit share a name for purely historical and linguistic reasons, not chemical ones.

Agricultural Uses Across All Three Types

All three lime types see use in agriculture, but for different purposes and at different scales. Ground limestone (calcium carbonate) dominates the market because it is cheap, safe to handle, and effective for the gradual correction of acidic soil. Dolomitic limestone adds magnesium, which is beneficial in magnesium-deficient soils. Chalk works the same way but dissolves somewhat faster.¹

Quicklime and hydrated lime are more reactive and raise soil pH faster, but they are more expensive and harder to handle safely. They tend to be used in situations where speed matters or where a farmer needs a large pH adjustment in a short time. Newer research has explored alternative forms of calcium carbonate, including a mineral form called vaterite, that can adjust soil pH in as little as a day while using a fraction of the material needed with conventional ground limestone.⁹

Calcium hydroxide also plays a role in livestock farming as a disinfectant. Spread across farm entrances and around buildings, it creates an alkaline barrier that inactivates bacteria and viruses. Research has shown that this barrier needs about 15 to 20% moisture to work properly and that its disinfecting power fades after roughly two weeks to a month of outdoor exposure as the pH gradually drops.⁶ Farmers periodically replace or refresh the lime to maintain the barrier, especially during disease outbreaks.

Lime in Dentistry and Medicine

Calcium hydroxide has a long history in dental practice. Dentists use it as a disinfecting paste placed inside root canals between appointments, as a material for capping exposed tooth pulp, and as an ingredient in root canal sealers. Its strong alkalinity, around pH 12.5 to 12.8 in pure paste form, kills bacteria and encourages the deposition of hard tissue at the treatment site.¹⁰ The mechanism relies on the release of calcium and hydroxide ions, which together create an environment hostile to microbes while promoting mineralization.

In the treatment of chronic infections around tooth roots, calcium hydroxide has been considered a gold standard, particularly valued for its very alkaline pH.¹¹ Newer materials like mineral trioxide aggregate (MTA) and bioceramics have emerged as alternatives in some applications, but calcium hydroxide remains widely used because it is effective, well understood, and inexpensive.

Construction, From Ancient Mortars to Modern Plasters

The widespread use of lime in construction goes back at least to the Romans, who developed hydraulic lime mortars and concretes that could even set underwater. Roman engineers built lime kilns, and the historian Cato described their construction and operation in enough detail that modern researchers have been able to reconstruct the original process.² For most Roman farming purposes, though, lime was produced in simpler pits or above-ground stacks rather than permanent kilns.

In construction, the sequence typically works like this: calcium oxide is slaked with water to produce calcium hydroxide putty or powder, which is then mixed with sand and water to create mortar. Once applied, the mortar gradually absorbs carbon dioxide from the air and converts back to calcium carbonate, hardening as it goes. This carbonation process can take months or even years to complete fully, which is one reason lime-based mortars develop strength more slowly than cement-based ones. But the resulting material is more flexible, more breathable, and generally more compatible with older stone and brick than Portland cement, which is why conservation architects strongly prefer lime mortars for historic building repairs.

Hot-mixed lime mortar, which uses quicklime mixed directly with wet sand rather than pre-slaked lime, has seen a revival in conservation work. The slaking reaction happens within the mortar mix itself, and the heat generated can affect the final texture and performance. Research into this process has revealed that the crystallographic expansion during slaking, where the material grows in volume as water molecules insert themselves into the crystal lattice, creates microscopic cracks that actually improve the mortar’s workability and bonding characteristics.⁵

Lime and Climate Change

Lime production is a significant source of carbon dioxide emissions. Calcining limestone releases both the CO₂ stored in the rock itself and the CO₂ from the fuel burned to reach the necessary temperatures. The global lime and cement industries together account for a substantial fraction of industrial carbon emissions. That creates an uncomfortable irony: a material whose hardening process naturally absorbs carbon dioxide from the air is produced in a way that emits more than it eventually reabsorbs.

One area of active research involves using alkaline byproducts of the lime and cement industries for ocean alkalinity enhancement, a proposed method of pulling carbon dioxide from the atmosphere. The idea is to add alkaline materials to ocean surface water, raising the water’s ability to absorb and store atmospheric CO₂. Cement kiln dust and lime kiln dust are alkaline side streams from these industries that could serve this purpose because they are widely available and already ground to a fine particle size.¹² Whether this approach can be scaled up enough to make a meaningful climate impact, and whether the ecological effects on marine ecosystems are acceptable, remains an open question. But the chemistry is straightforward: the alkalinity of lime-derived materials is the same property that makes them useful in soil, water treatment, and construction, just applied to a vastly larger system.

Safety Differences You Should Know

If you are handling any of these materials, the safety profiles are very different. Calcium carbonate is the gentlest of the three. Ground limestone dust can irritate your eyes and lungs, but it is not caustic and poses no burn risk. You can buy it at a garden center and spread it with your bare hands if you are not too worried about dust.

Calcium hydroxide is significantly more hazardous. Its high pH means it can cause chemical burns to skin, eyes, and mucous membranes. If you are working with hydrated lime powder, you need gloves, eye protection, and ideally a dust mask. The powder becomes much less dangerous once it is mixed into a wet mortar or paste, but the dry powder is a real irritant.

Calcium oxide is the most dangerous of the three. It reacts violently with water, including sweat and moisture in your eyes or throat, releasing intense heat in the process. A splash of water onto a pile of quicklime can produce enough heat to cause thermal burns on top of the chemical burns from the alkalinity. People who work with quicklime professionally treat it with serious respect, using full personal protective equipment and careful handling procedures. If you encounter quicklime outside an industrial setting, do not add water to it casually. The exothermic reaction can be intense enough to ignite nearby combustible materials in rare cases.

These differences explain why calcium carbonate dominates consumer and agricultural use, while calcium oxide and calcium hydroxide are mostly confined to industrial and professional applications where the handling risks can be managed. Understanding which type of “lime” someone is referring to is not just a chemistry question; it is a safety question.

1
PubMed Central. Soil acidification and the importance of liming agricultural soils with particular reference to the United Kingdom
2
Oxford Journal of Archaeology. THE MANUFACTURE of LIME and ITS USES IN the WESTERN ROMAN PROVINCES
3
PubMed Central. Scalable production of bio-calcium oxide via thermal decomposition of solid hatchery waste in a laboratory-scale rotary kiln
4
Journal of the American Ceramic Society. A Kinetic Study of Thermal Decomposition of Limestone Using In Situ High Temperature X‐Ray Powder Diffraction
5
Heritage Science. Towards a better understanding of hot-mixed mortars for the conservation of historic buildings: the role of water temperature and steam during lime slaking
6
Nature. Farm use of calcium hydroxide as an effective barrier against pathogens
7
Elsevier. Accelerating carbonation in lime-based mortar in high CO2 environments
8
PubMed Central. Mineral Content of the Pulp and Peel of Various Citrus Fruit Cultivars
9
Elsevier. Fast-dissolving calcium carbonate particles as a high-performance lime for amelioration of agricultural acidic soils
10
PubMed. Properties and applications of calcium hydroxide in endodontics and dental traumatology
11
Romanian Journal of Military Medicine. The role of calcium hydroxide in the treatment of chronic apical periodontitis in patients with type 2 diabetes – a case report
12
Biogeosciences. The carbon dioxide removal potential of cement and lime kiln dust via ocean alkalinity enhancement