What is Hydrated Lime?

Meaning of Hydrated Lime

Hydrated lime is a type of dry powder made from limestone. It is created by adding water to quicklime to convert the oxides to hydroxides. Combined with water and sand or cement, hydrated lime is most often used to make mortars and plasters. Its chemical name is calcium hydroxide or Ca(OH)2.

Contact with hydrated lime can cause eye irritation.

When making hydrated lime, the manufacturer must first make quicklime. Quicklime is obtained directly from calcination in raw limestone by the calcination process and is composed of calcium oxide and magnesium oxide.

In this process, the limestone is first broken down to reduce its size. It is then washed and taken to ovens to be heated through a three-step process: preheating, calcination and cooling.

Once cooled, the quicklime is crushed and then water is added.

Combined with water and sand or cement, hydrated lime is most often used to make mortars and plasters. Depending on the production method, high calcium or dolomite hydrated lime can be created.

Dolomite hydrated lime can be created under normal atmospheric pressure or under high pressure. Different processes produce different percentages of water, calcium and magnesium in the finished lime.

In high pressure processes all the magnesium and calcium are hydrated, while in normal pressure processes only part of the magnesium and calcium is hydrated.


There are several kinds of hydrated lime. Types S and SA denote special high-purity limes and are most often used in mortars. Types N and NA indicate normal files and are not approved for use in construction materials, but may be used in the production of asphalt and paper.

Types SA and NA are aerated limes, so their structure is more porous than their non-aerated counterparts.

Hydrated limes are classified by the amount of water they hold and their maximum air content. S types retain 85 percent water, N types 75 percent. Types NA and SA are classified with a maximum air content of 14 percent, and types N and S have an air content of seven percent.

Also called slaked lime, hydrated lime is a good bonding agent and is waterproof. It is also strongly alkaline, with a pH of 12.4.

For this reason, it can be used to neutralize acid, in water and wastewater treatment, and to stabilize soil. This also means that it will cause skin and eye irritation if it comes into contact with the body, and inhalation of dust can cause health problems.

Gloves, eye protection, and masks should be worn at all times when working with these substances.

Type S and SA limes are commonly used in construction mortar mixes.

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