Lime Classification: The 3 Conventional Types Explained

Lime, primarily calcium oxide ($CaO$), obtained from heating limestone ($CaCO_{3}$), is a crucial material in various industries, especially construction.

Conventionally, lime is classified based on its chemical purity and the presence of impurities, which significantly affect its properties and uses. The most common classification divides lime into 3 main types.

Understanding the Classification Basis

The classification primarily depends on the percentage of calcium oxide ($CaO$) and the types and amounts of impurities present, such as magnesium oxide ($MgO$), silica ($SiO_{2}$), and alumina ($Al_{2}O_{3}$).

The Three Conventional Types of Lime

Here are the 3 conventionally recognized types of lime:

• High Purity Lime (or Fat Lime / Pure Lime):

  This type of lime typically contains a very high percentage of calcium oxide, usually more than $90\%$. It is derived from pure limestone. When mixed with water, it undergoes a vigorous slaking process, producing a smooth, putty-like paste. This is often referred to as 'fat lime' because of this property. It has high plasticity and is suitable for finishing plasters and general masonry.

• Magnesian Lime:

  This lime contains a notable amount of magnesium oxide ($MgO$), typically ranging from $5\% \text{ to } 30\%$, alongside calcium oxide ($CaO$). It is produced from dolomitic limestone, which contains both calcium carbonate ($CaCO_{3}$) and magnesium carbonate. Magnesian lime slakes less vigorously than pure lime and can exhibit slight hydraulic properties depending on impurities.

• Impure Lime (or Hydraulic Lime / Poor Lime):

  This category includes limes that contain significant amounts of impurities, mainly silica ($SiO_{2}$) and alumina ($Al_{2}O_{3}$), typically ranging from $5\% \text{ to } 30\%$. These impurities are often derived from clay present in the limestone source. The presence of these compounds allows the lime to set and harden even when exposed to water, a property known as 'hardeness' or 'setting'. It is classified based on its hydraulic properties (e.g., natural hydraulic lime - NHL).

Therefore, based on these conventional distinctions related to purity and composition, lime is typically classified into 3 primary types.