Calcium Hydroxide (Slaked Lime) Applications

What Is Calcium Hydroxide?

Calcium hydroxide — chemical formula Ca(OH)₂ — is an inorganic compound produced by reacting calcium oxide (quicklime) with water in a controlled exothermic process known as slaking. The resulting product is a fine, white, odorless powder with strong alkaline properties. Industrially, it goes by several interchangeable names: slaked lime, hydrated lime, builders' lime, and pickling lime (food-grade). Its CAS number is 1305-62-0, and as a food additive it carries the E number E526.

Industrial Applications of Calcium Hydroxide

Calcium hydroxide powers a remarkably diverse range of industrial processes. The following sections detail its primary application domains, ordered by global consumption volume.

1. Water and Wastewater Treatment

Water treatment represents the largest single end-use market for calcium hydroxide, accounting for approximately 24–30% of global demand.

In drinking water purification, hydrated lime serves three critical functions:

pH adjustment

● : Raises the pH of acidic source water to the 7.0–8.5 range required for distribution, preventing corrosion in pipelines.

Water softening (lime softening)

● : Precipitates dissolved calcium and magnesium hardness ions as insoluble carbonates, a process widely used in municipal plants.

Heavy metal removal

● : Elevates pH to precipitate dissolved metals — including lead, copper, zinc, and cadmium — as their hydroxide forms, which are then removed by sedimentation or filtration.

In municipal and industrial wastewater treatment, calcium hydroxide neutralizes acidic effluents from metal finishing, mining, and chemical manufacturing operations. It also aids in phosphate removal from sewage, helping prevent eutrophication in receiving water bodies. Compared to caustic soda (NaOH), hydrated lime offers a significantly lower cost per unit of alkalinity delivered — typically 50–70% cheaper — making it the reagent of choice for large-scale water treatment facilities.

2. Flue Gas Desulfurization (FGD)

Environmental air quality regulations worldwide mandate the removal of sulfur dioxide (SO₂) from power plant and industrial boiler exhaust. Calcium hydroxide is a primary sorbent in both wet and dry FGD systems:

Wet FGD (lime scrubbing): An aqueous slurry of calcium hydroxide is sprayed into the flue gas stream inside a scrubber tower. SO₂ reacts with the lime to form calcium sulfite (CaSO₃), which is subsequently oxidized to gypsum (CaSO₄·2H₂O):

Ca(OH)₂ + SO₂ → CaSO₃ + H₂O 2CaSO₃ + O₂ + 4H₂O → 2CaSO₄·2H₂O

The resulting synthetic gypsum is a valuable byproduct sold to the wallboard and cement industries. Wet FGD systems achieve SO₂ removal efficiencies exceeding 95%.

Dry and semi-dry FGD: A fine spray of hydrated lime slurry or dry powder is injected into the hot flue gas. The water evaporates, leaving reactive calcium hydroxide particles that capture SO₂. These systems are favored for smaller industrial boilers where a wet scrubber is uneconomical.

With mounting global pressure to cut industrial SO₂ emissions — particularly in China, India, and Southeast Asia — FGD continues to be a key growth driver for calcium hydroxide demand.

3. Construction and Civil Engineering

Calcium hydroxide has been a cornerstone of construction chemistry for millennia, and modern applications remain substantial:

● Mortar and plaster: When mixed with sand and water, hydrated lime produces a workable paste that slowly carbonates over time, absorbing CO₂ from the air to form calcium carbonate. This gives lime-based mortars a unique ability to self-heal fine cracks and accommodate building movement, unlike purely cement-based alternatives. Historic building restoration projects specifically require lime mortars for material compatibility with original masonry.

● Soil stabilization: In road construction and foundation engineering, adding 3–8% hydrated lime by weight to clay-rich soils triggers pozzolanic reactions that permanently improve soil strength, reduce plasticity, and decrease water susceptibility. This technique is standard practice for highway subgrades and embankment construction globally.

● Autoclaved Aerated Concrete (AAC): Hydrated lime reacts with silica-rich materials (fly ash, sand) under high-pressure steam curing to produce lightweight, thermally insulating concrete blocks that are increasingly popular in green building projects.

● Whitewash and lime paint: A traditional, low-cost coating still used in agricultural buildings, food processing facilities, and developing-market housing for its antimicrobial properties and breathability.

4. Agriculture and Soil Management

In agriculture, calcium hydroxide serves as a cost-effective soil amendment and farm management tool:

● Soil pH correction (liming): Acidic soils — prevalent in tropical and subtropical regions with high rainfall — lock up essential nutrients and reduce crop yields. Agricultural-grade hydrated lime raises soil pH into the 6.0–7.0 optimal range, improving the availability of nitrogen, phosphorus, and potassium. Its finer particle size gives it a faster reaction time than ground limestone (CaCO₃), making it suitable for situations requiring rapid pH correction within a single growing season.

● Livestock and poultry sanitation: A 10–20% lime slurry (limewash) applied to barn floors, walls, and equipment provides effective, low-cost disinfection against common pathogens. The high pH environment is inhospitable to bacteria and fungi, and the dried residue poses minimal toxicity risk to animals compared to stronger chemical disinfectants.

● Crop protection: Lime-based sprays can deter certain insect pests and fungal diseases on fruit trees and vegetable crops, offering an alternative to synthetic pesticides in organic farming systems.

● Pond and aquaculture management: Hydrated lime is used to sterilize pond bottoms between production cycles and to buffer water pH in fish and shrimp farming operations.

5. Food Processing (Food Grade E526)

Food-grade calcium hydroxide meets stringent purity standards — typically ≥96% Ca(OH)₂ content with heavy metal limits measured in parts per million — and is approved for use as additive E526 in the EU, UK, and many other jurisdictions:

● Nixtamalization: The most iconic food application. Dried maize kernels are cooked and steeped in a calcium hydroxide solution, a process developed by Mesoamerican civilizations thousands of years ago. This alkaline treatment softens the pericarp (outer hull), improves dough texture, and crucially releases bound niacin (vitamin B₃), making it bioavailable. Masa — the resulting dough — forms the basis for tortillas, tamales, corn chips, and related products consumed worldwide.

● Pickling: Food-grade calcium hydroxide (pickling lime) is used to maintain crispness in pickled cucumbers and other vegetables by reinforcing pectin structure in the cell walls. The lime must be thoroughly rinsed away before the final pickling brine is added.

● Sugar refining: In the carbonation stage of beet and cane sugar processing, calcium hydroxide is added to raw sugar juice to precipitate non-sugar impurities (proteins, organic acids, color bodies), which are then removed by filtration through the carbonation with CO₂.

● Other food uses: pH adjustment in fruit preserves, calcium fortification in beverages, firming agent in processed fruits and vegetables, and as a processing aid in traditional foods including Chinese century eggs and Japanese konnyaku.

6. Chemical Manufacturing and Metallurgy

● Chemical intermediate: Calcium hydroxide is a feedstock for producing calcium stearate (a lubricant and stabilizer in PVC processing), calcium hypochlorite (bleaching powder for water disinfection), and various calcium-based greases.

● Steelmaking: In electric arc furnaces, hydrated lime is injected to remove phosphorus, sulfur, and silica from molten steel as slag-forming fluxes.

● Pulp and paper (Kraft process): In the chemical recovery cycle of Kraft pulp mills, calcium hydroxide (lime mud) is regenerated from calcium carbonate in a lime kiln and reused to causticize green liquor (sodium carbonate → sodium hydroxide), an essential step that makes the process economically viable.

● Leather tanning: Used in the unhairing and liming stages to swell hides and remove hair and epidermis before tanning.

Conclusion

Calcium hydroxide remains one of the most versatile and economically significant industrial chemicals in global use. Its unique combination of strong alkalinity, moderate solubility, and low cost relative to alternative alkalis (NaOH, KOH) ensures its continued relevance across water treatment, environmental protection, construction, agriculture, food processing, and chemical manufacturing.