How to Make Calcium Hydroxide: The Two-Step Process

Calcium hydroxide is made by first burning limestone (calcium carbonate) at high heat to drive off carbon dioxide, producing quicklime (calcium oxide), and then adding water to that quicklime in a reaction called slaking. These two steps, calcination followed by hydration, have been used for thousands of years and remain the standard industrial method today. The chemistry is straightforward, but the details of temperature, water ratio, and timing make the difference between a fine, reactive product and a lumpy, half-converted mess.

Step One: Turning Limestone Into Quicklime

The first step is calcination, a process where you heat calcium carbonate (CaCO₃) until it thermally decomposes into calcium oxide (CaO) and carbon dioxide gas. In industrial lime kilns, this reaction begins around 820 °C, but complete conversion requires temperatures near 900 °C.¹ The reaction is highly endothermic, demanding about 1,784 kJ of heat energy for every kilogram of calcium carbonate processed.¹ That is a lot of energy, which is why lime production has historically been tied to abundant fuel sources like wood, coal, or natural gas.

The quality of the starting limestone matters. Pure calcite or chalk gives the cleanest quicklime, while impurities like magnesium carbonate, silica, or clay minerals change the product’s behavior. Dolomitic limestone, which contains significant magnesium, produces a mixed oxide that slakes more slowly and has different properties than high-calcium quicklime. For anyone sourcing limestone for a small project, look for high-purity calcium carbonate. Marble chips, chalk, or clean limestone nodules are all viable starting materials.

Temperature control during calcination is critical. If you heat the stone too gently, you get underburned lime with an unreacted core of calcium carbonate still inside each piece. If you overshoot the temperature substantially, especially above about 1,100 °C, you risk “dead burning” the lime, creating dense, sintered calcium oxide that reacts sluggishly with water. The sweet spot for most lime production sits between 900 °C and 1,100 °C, held long enough for the heat to penetrate all the way through each piece of stone. Smaller pieces calcine faster because the heat has less distance to travel.

An interesting finding from laboratory research is that the presence of steam in the kiln atmosphere can lower the energy barrier for calcination. One study measured an activation energy of about 162 kJ/mol for calcination in the presence of steam, compared with roughly 194 kJ/mol in a pure nitrogen atmosphere, suggesting that steam has a catalytic effect on the decomposition reaction.² In practice, this means that kilns with some moisture in their gas stream may achieve calcination more efficiently.

Step Two: Slaking Quicklime With Water

Once you have quicklime, the second step is hydration, traditionally called slaking. You add water to the calcium oxide, and it reacts vigorously to form calcium hydroxide (Ca(OH)₂). The reaction releases a substantial amount of heat. If you have ever seen a lump of quicklime dropped into water, the water can boil and steam within seconds. This exothermic burst is why quicklime was historically used as a weapon and why safety precautions matter even at small scales.

The amount of water you add relative to the quicklime determines what kind of product you get. There are essentially two approaches:

Dry hydration: You add just enough water (roughly the stoichiometric amount, plus a small excess to account for evaporation from the heat generated) to convert the oxide to hydroxide. The result is a dry powder, sometimes called hydrated lime or slaked lime powder. This is the form most commonly sold commercially.
Wet slaking (putty): You add a large excess of water, producing a thick paste or putty. This form has been prized for millennia in lime plastering and mortar work. The excess water helps keep the reaction temperature controlled and produces a finer particle structure.

Research confirms that the dissolution speed of lime slaked as a suspension (wet method) is about eight times higher than that of lime hydrated to a dry powder.³ The hydration process itself starts as a surface-driven chemical reaction and then transitions to a slower, diffusion-controlled process as the outer layer of hydroxide forms a shell around the remaining oxide core.³ This is why thorough mixing or extended soaking helps ensure complete conversion.

How Water Temperature Changes the Product

One detail that surprises people is how much the temperature and form of the water affect the resulting calcium hydroxide. Research on historic lime mortars has examined three approaches: slaking with water at room temperature (around 20 °C), slaking with hot water (75 °C), and slaking with steam alone. The differences in the final product are dramatic.

Quicklime slaked in room-temperature water produces portlandite crystals with a range of recognizable shapes, including hexagonal platelets and short prisms, typically up to about 1.5 micrometers in size, embedded in a matrix of much smaller crystals.⁴ Slaking with hot water at 75 °C produced crystals with less regular facets and a wider size distribution.⁴ The crystallite sizes measured by X-ray diffraction tell the story even more starkly: room-temperature water produced crystallites around 394 angstroms, hot water produced crystallites near 900 angstroms (though with a very wide distribution), and steam-slaked lime had crystallites of only about 180 angstroms.⁵

These microstructural differences matter for anyone using calcium hydroxide in mortars, plasters, or conservation work. Smaller crystallites generally mean higher surface area, which affects how the lime sets and how it interacts with carbon dioxide during hardening. The steam-slaked lime’s much finer crystallites result from a fundamentally different reaction pathway at the gas-solid interface, where the absence of liquid water slows ionic diffusion and produces a distinct particle structure.⁵

Making Calcium Hydroxide on a Small Scale

If you want to produce calcium hydroxide at home or in a small workshop, the basic sequence is the same as the industrial process, just scaled down. The hardest part is achieving and sustaining the temperatures needed for calcination. A wood-fired kiln, a charcoal forge, or even a well-built campfire pit can reach the necessary temperatures if designed to concentrate heat around the stone. Historically, communities built field kilns from local stone, packed them with alternating layers of limestone and fuel, and fired them for days.

Start with pieces of clean limestone or marble roughly the size of a fist or smaller. Smaller pieces calcine faster and more completely. Fire the kiln for several hours at the highest temperature you can maintain. When the stone has been thoroughly calcined, the resulting quicklime will be lighter in weight (it has lost all its CO₂), will look chalky white, and will feel noticeably lighter when you pick it up. An incompletely calcined piece will have a harder, denser core when you break it open.

To slake the quicklime, place it in a metal or stone container and gradually add water. For a dry powder product, add water slowly, in small increments, stirring or turning the lime as the exothermic reaction proceeds. The lumps will crack, steam, and crumble into powder as they hydrate. For lime putty, submerge the quicklime in a generous excess of water and let it sit, stirring occasionally. Traditional lime putty was sometimes aged for months or even years, with the understanding that prolonged soaking further improves the fineness and workability of the paste.

A few practical warnings: the slaking reaction is violent enough to spit boiling water and steam. Wear eye protection and heavy gloves. Work outdoors or in a very well-ventilated space. Never add water to a large amount of quicklime all at once in a sealed container, as the steam pressure can cause an eruption. Add water incrementally and stand back.

Safety Hazards Worth Taking Seriously

Both quicklime and calcium hydroxide are strongly alkaline. Quicklime is the more dangerous of the two because it reacts with moisture on contact, including the moisture in your skin, eyes, and respiratory tract. The heat generated from even small amounts contacting damp skin can cause thermal and chemical burns simultaneously. Calcium hydroxide itself, while not exothermic in the same way, is still caustic enough to damage tissue.

Alkaline burns are generally more dangerous than acid burns of equivalent concentration because alkalis penetrate tissue more deeply. A review of chemical burn literature notes that alkaline burns cause deeper injury than acid burns, which tend to form a coagulated protein barrier that limits further penetration.⁶ This is relevant because quicklime and calcium hydroxide are both alkaline. If either gets on your skin, flush with copious water immediately and keep flushing for at least 15 to 20 minutes. If it contacts your eyes, treat it as a medical emergency.

Dust inhalation is the other major risk. Fine calcium hydroxide powder suspended in air irritates the lungs and mucous membranes. In any production or handling scenario, a dust mask rated for fine particles is essential. During calcination, carbon dioxide is released in large volumes, which can displace oxygen in enclosed spaces. Never calcine limestone in a poorly ventilated indoor space.

How Calcium Hydroxide Hardens and Sets

Once calcium hydroxide is placed as a mortar, plaster, or coating, it gradually recombines with carbon dioxide from the atmosphere and converts back to calcium carbonate. This process, called carbonation, is essentially a slow reversal of the original calcination step, completing what is known as the lime cycle. The reaction proceeds from the surface inward, which is why thick lime plaster can take months or years to fully harden.

Research on calcium hydroxide’s interaction with CO₂ shows that it absorbs carbon dioxide well at low temperatures, between about 20 °C and 150 °C, with finer particles performing better because of their greater exposed surface area.⁷ Moisture in the surrounding air also enhances the carbonation process.⁷ This is consistent with what plasterers and masons have known for centuries: lime plaster sets best in moderately humid conditions, and it can fail to carbonate properly in extremely dry environments.

The durability of carbonated lime can be remarkable. Studies of ancient lime mortars exposed to harsh coastal environments found that despite centuries of salt spray and biological growth, the mortars showed only superficial degradation like staining and biological colonization, with the bulk material remaining sound.⁸ That kind of longevity is why lime mortars are still preferred for conservation work on historic buildings, where Portland cement’s rigidity and high salt content can damage old masonry.

Calcium Hydroxide in Food Processing

If you have eaten a corn tortilla, you have eaten food processed with calcium hydroxide. Nixtamalization, the ancient Mesoamerican technique of soaking dried corn in an alkaline solution, relies on calcium hydroxide (called “cal” in Spanish) to soften the kernels, loosen the hulls, and improve the nutritional availability of niacin. The process transforms dry field corn into the pliable dough called masa that forms the basis of tortillas, tamales, and many other foods.

Beyond its traditional role, calcium hydroxide in nixtamalization also has a measurable effect on food safety. Research on tortilla chips made from nixtamalized corn found that adding calcium hydroxide during processing significantly reduced acrylamide levels in the fried product. At a concentration of 1.5 grams of calcium hydroxide per 100 grams of corn, acrylamide content dropped by over 50% in chips fried for 30 seconds.⁹ Acrylamide is a compound that forms during high-temperature cooking and has been flagged as a potential health concern, so this reduction is a genuine bonus of the traditional process.

Calcium hydroxide also shows up in other food applications. It is used in pickling to keep cucumbers firm, in sugar refining to clarify juice, and in some Asian cuisines to give certain noodles and desserts their characteristic texture. Food-grade calcium hydroxide is produced to higher purity standards than construction-grade material, so you should never substitute one for the other.

Uses in Dentistry

Calcium hydroxide has been a staple material in dental practice for decades, particularly in endodontics (root canal treatment). After the interior of a tooth is cleaned out during a root canal procedure, calcium hydroxide paste is often placed inside the canal as a temporary dressing between appointments. Its high pH, around 12.5, creates an environment hostile to the bacteria that cause dental infections.¹⁰

The antimicrobial action works through the release of hydroxide ions, which damage bacterial cell membranes, denature proteins, and disrupt DNA.¹¹ Calcium hydroxide also stimulates the formation of hard tissue, which is why it is used in procedures like direct pulp capping, where the goal is to encourage the tooth to lay down a protective bridge of new dentin over an exposed nerve. It has wide-spectrum activity against common bacteria found in infected root canals.¹¹

Environmental and Industrial Applications

Calcium hydroxide is one of the most widely used chemicals in water treatment and pollution control. Municipal water plants add it to adjust pH, soften hard water by precipitating out excess calcium and magnesium, and help remove impurities through flocculation. In wastewater from industrial processes, calcium hydroxide can precipitate out heavy metals and other contaminants that would otherwise be released into waterways.

A striking example comes from the treatment of flue gas desulfurization wastewater, the liquid waste produced when power plants scrub sulfur dioxide from their exhaust. In one integrated treatment process, calcium hydroxide precipitation removed essentially 100% of sulfate ions, over 97% of fluoride ions, and more than 99% of magnesium from the wastewater.¹² The process also cut total dissolved solids by about 78% and chemical oxygen demand by over 94%.¹² These are dramatic cleanup numbers, and they show why calcium hydroxide remains a workhorse chemical in environmental engineering despite the availability of more exotic treatment methods.

In agriculture, calcium hydroxide is spread on acidic soils to raise pH and improve growing conditions. In soil stabilization for construction, hydrated lime is mixed into weak clay soils to improve their load-bearing capacity, making them suitable for roads and foundations.¹³

The Carbon Footprint Problem

There is an uncomfortable environmental reality embedded in the two-step process. Every kilogram of calcium carbonate you calcine releases about 440 grams of CO₂ just from the chemical decomposition itself, before you even count the fuel burned to reach those temperatures. The lime and cement industries together represent a significant source of industrial carbon emissions globally. The CO₂ released during calcination is inherent to the chemistry. You cannot make quicklime without it.

The industry is exploring several routes to reduce this footprint. These include switching to lower-carbon fuels, capturing the CO₂ as it is released and storing or utilizing it, and using renewable electricity to provide the heat instead of burning fossil fuels.¹⁴ Carbon capture is particularly well suited to lime production because the exhaust gas from a well-designed kiln has a relatively high CO₂ concentration, making capture more efficient than in many other industrial processes. Some researchers have also noted the irony that calcium hydroxide itself can be used as a CO₂ sorbent in carbon capture systems, creating a loop where the product helps address the emissions from its own manufacture.⁷

For small-scale producers, the carbon footprint per kilogram is likely higher than industrial operations, since backyard kilns waste more heat and burn fuel less efficiently. If you are making calcium hydroxide for a garden or building project and the environmental angle matters to you, sourcing commercially produced hydrated lime is almost certainly the lower-emission option compared to firing your own kiln.

Testing Whether Your Quicklime Is Any Good

If you have just pulled freshly calcined material from a kiln, how do you know it actually converted? The simplest field test is to drop a small piece into water. Good quicklime reacts vigorously, crumbling, hissing, and heating the water noticeably within seconds. Underburned material (still partly calcium carbonate) will react weakly or not at all. Dead-burned material that was overheated will also react sluggishly, though it eventually hydrates given enough time and water.

Industry uses more formal quality tests. Two standard approaches are an acid neutralization test, which measures the total available lime content, and a reactivity test that tracks how fast the quicklime reacts with water, producing a numerical reactivity value.¹⁵ The reactivity test is the more informative of the two for practical purposes, because two batches of quicklime with the same total CaO content can perform very differently depending on how aggressively they react with water. A high-reactivity lime slakes quickly and completely; a low-reactivity lime may leave unreacted lumps that expand and crack later if the hydroxide is used in a plaster or mortar.

For home producers without laboratory equipment, the water test remains your best diagnostic tool. If the quicklime heats water dramatically and crumbles to a fine powder with no hard cores left over, you have a good product. If pieces remain stubbornly intact, either calcine them longer at higher heat or break them into smaller pieces and try again.

1
ScienceDirect. Energy and exergy assessments of a lime shaft kiln
2
ScienceDirect. Effects of steam on the kinetics of calcium carbonate calcination
3
The Canadian Journal of Chemical Engineering. A Trend to the Production of Calcium Hydroxide and Precipitated Calcium Carbonate with Defined Properties
4
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
5
npj 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
PubMed Central. Rare chemical burns: Review of the Literature
7
PubMed Central. Post combustion CO2 capture with calcium and lithium hydroxide
8
Construction and Building Materials. Durability of ancient lime mortars in humid environment
9
LWT – Food Science and Technology. Effect of added calcium hydroxide during corn nixtamalization on acrylamide content in tortilla chips
10
PubMed Central. Antimicrobial effect of calcium hydroxide as an intracanal medicament in root canal treatment: a literature review – Part I. In vitro studies
11
PubMed. Properties and applications of calcium hydroxide in endodontics and dental traumatology
12
Journal of Cleaner Production. An integrated process of calcium hydroxide precipitation and air stripping for pretreatment of flue gas desulfurization wastewater towards zero liquid discharge
13
ScienceDirect. Modern Earth Buildings – Soil stabilisation and earth construction: materials, properties and techniques
14
Renewable and Sustainable Energy Reviews. Decarbonising the lime industry: State-of-the-art
15
ResearchGate. The standard reactivity test as a measure of lime’s quality