Different Types of Lime Mortar Explained
Lime mortar is a traditional type of mortar made out of lime and water. It is also called natural lime mortar and should not be confused with the popular type of mortar, Portland cement mortar. There are two types of lime mortars: one type hardens when exposed to air, and the other one hardens when mixed with water. In other words, these two types are called non-hydraulic and hydraulic lime mortars. There are also sub types under the hydraulic lime mortar category.
Non-hydraulic Lime Mortar
Non-hydraulic lime is so named because it does not require water in order to harden. This type of lime is basically taken from calcium carbonate in its purest forms, i.e. limestone and chalk. The material is burned in a kiln and made to produce quicklime or calcium oxide. Quicklime becomes calcium hydroxide once it is mixed with water. In order to reform calcium carbonate, calcium hydroxide is made to react with carbon dioxide in the air.
Non-hydraulic lime can be produced in two different forms, i.e. lime putty and hydrated lime. Take note that hydrated lime is different from hydraulic lime, although the words are somewhat similar. Hydrated lime can sometimes be hydraulic when it hardens after being mixed with water, and non-hydraulic when it does not harden after being mixed with water. Lime putty is also considered to be non-hydraulic and is made simply from lime and water.
When non-hydraulic lime mortar is set in a mason unit, it takes quite a long time to harden. The setting process may take weeks or even months or years to achieve its optimum strength. The slow process is due to the fact that it requires carbon dioxide in the air to reform back to its original state. Lime putty will retain its form as long as it is kept underwater and away from the air. When it is ready for use, it can be taken out of the water and exposed to the air to harden.
Hydraulic Lime Mortars
Hydraulic lime mortars are designed to harden when they come in contact with water. In the production of a calcium oxide, impurities in the material form aluminates and calcium silicates that cause the material to harden when mixed with water. The calcium oxide is then mixed with a sufficient amount of water in the kiln to produce calcium hydroxide, while allowing the aluminates and the calcium silicates to set. Once the calcium hydroxide is formed along with the aluminates and the calcium silicates, they are bagged to prevent moisture from seeping in and hardening the mortar. Unlike non-hydraulic lime, hydraulic lime can easily harden after it is mixed with water.
Hydraulic lime is graded depending on the rating and strength of the material. Hydraulic limes are describes as eminently hydraulic, moderately hydraulic, and feebly hydraulic. In modern times, these grades are referred to as NHL 5 for eminently hydraulic, NHL 3.5 for moderately hydraulic, and NHL 2 for feebly hydraulic. NHL stands for Natural Hydraulic Lime.