What is Hydrated Lime?

Hydrated lime, sometimes referred to as “slaked lime” or more likely just “hydrate”, is a versatile chemical with many uses across numerous industries and applications. Technically, hydrated lime is calcium hydroxide, Ca(OH)₂, a derivative of quicklime – another versatile chemical with even more uses.  As the name implies, water is used to hydrate quicklime in a process known as slaking. When quicklime is slaked, water is added to facilitate an exothermic reaction, changing quicklime to hydrated lime. This process is carried out using large, specialized equipment known as hydrators. These industrial hydrators continuously add the exact amount of water needed to quench the chemical reaction between quicklime and water. Hydrators not only control the water and quicklime additions, but they also provide the necessary agitation to properly hydrate the quicklime into hydrated lime.

Although water has been added in the process, physically, the end result of hydrated lime is a dry, fine powder with a relatively low density and high purity and is very chalky white in color. The bulk density of hydrated lime can range from about 25 to 40 lb/ft3 with 35 lb/ft3 being an average value. In layman’s terms, it looks and feels like baking flour but is obviously intended for very different usage.

Hydrated lime can be high calcium, magnesian, dolomitic, or hydraulic. High calcium hydrated lime contains almost entirely calcium hydroxide and is typically what is referred to as hydrated lime. If there is an elevated amount of magnesium, present as magnesium hydroxide (Mg(OH)₂), then the hydrated lime is referred to as magnesian. If the amount of magnesium hydroxide approaches fifty percent, then the hydrated lime is referred to as dolomitic. The magnesium oxide (MgO) present in magnesian and dolomitic quicklime requires increased pressures to properly hydrate, resulting in even more specialized operations/equipment. Hydrated lime that is referred to as hydraulic contains considerable amounts of silica, alumina, and iron, usually from clay deposits in the source material, and results in a material that is used for building applications. Air-entraining additives can add to the durability of the final product when working with hydraulic lime. These additives result in the formation of microscopic air bubbles that can help resist additional forces, such as in freezing conditions. ASTM Standard C207 defines hydrated lime for masonry purposes as:

• Type N — normal (high calcium) hydrated lime
• Type S — special (dolomitic) hydrated lime
• Type NA — normal (high calcium) air-entraining hydrated lime
• Type SA — special (dolomitic) air-entraining hydrated lime

What is the Difference Between Lime and Hydrated Lime?

Lime is a general term for many calcium-based materials, including hydrated lime, lime slurry, lime putty or lime kiln dust (LKD). However, when the term “lime” is used, it almost always refers to quicklime. In short, quicklime is produced when limestone (CaCO₃) is calcined, or cooked, at high temperatures for long periods of time, resulting in a reactive oxide – calcium oxide (CaO).  As previously mentioned, quicklime can be further processed into hydrated lime by adding the right amount of water and agitation. “Lime” does not apply to limestone or any carbonate form of lime, nor agricultural lime (aglime), although it is often wrongly used in this way.

Uses of Hydrated Lime

What is Hydrated Lime Made of?

Chemically, hydrated lime is calcium hydroxide, Ca(OH)₂.  It consists of calcium oxide and water. When these two ingredients are combined in the proper proportions, the result is hydrated lime, or “hydrate” for short.

Is Hydrated Lime Safe?

As with any chemical, hydrated lime must be treated with respect and used as it is intended for desired outcomes. It is an alkaline material with an elevated pH of 12.4, so proper PPE should be used to keep it out of the eyes, respiratory tract, digestive system and off the skin. It is stable and only perishable when CO2 is present and absorbed.

Still have questions? Contact us today for additional information on hydrated lime, or to see if it’s the right product for your next job site.