Hydraulic and Non-Hydraulic Lime

What is the Difference Between Hydraulic and Non-Hydraulic Lime?

Lime Mortar for building purposes can be split into two distinct categories – Hydraulic Lime Mortar and Non-Hydraulic Lime Mortar. This blog post looks at the difference between Hydraulic and Non-Hydraulic Lime and will help you choose the right lime mortar for the job you need it for.

Non-Hydraulic Lime Mortars

Non-Hydraulic Lime Mortars are also referred to as Hot Lime Mortars and are perfect for pointing and building with softer stones and bricks. The softer mortar has a higher porosity, and better flexibility than hydraulic mortar, as it relies solely on natural carbonation.

These come wet as ready mixed lime mortars, all you need to do is rework them and they are ready to use.

You can also turn these into a hydraulic mortar by the addition of a pozzolan. This then turns the mortar to an FHL (Formulated Hydraulic Lime) as oppose to an NHL (Natural Hydraulic Lime).

The use of FHL has been around since the Romans who used volcanic ash to formulate the lime when building the Colosseum, this adds extra strength to the mortar, but does slightly lower the breathability and flexibility. Increasing the amount of pozzolan added to the mortar will increase the mortar’s strength and lower the breathability slightly. This enables you to use the same mortar for several different applications where the strength and a better weather resistance is needed.

Hydraulic Lime Mortars

Natural Hydraulic Lime (NHL) comes from limestone containing natural impurities of clay and other minerals. The amount of impurities within it determines how hard it will set.

There are three European classifications of Natural Hydraulic Lime: NHL 2, NHL 3.5 and NHL 5. These grades are based on the compressive strength of laboratory mortars after 28 days. These are often misleadingly termed feebly hydraulic, moderately hydraulic and eminently hydraulic.

The strength of a Hydraulic Lime Mortar varies depending on the manufacturer of the hydraulic lime. Some Natural Hydraulic Lime will test higher in the qualifying band, and some test lower. Strength will also vary depending on the mix ratio and the type of aggregate used.

Below is a table of what varying strengths NHL can achieve:

NHL works by setting in the presence of water, which is why it’s known as Hydraulic Lime.

Eco Lime generally recommend the use of Hydraulic Lime where the need for a higher breathability and lower strength is outweighed by the desire for an earlier and harder set. With any mortar used, the best mortar is the mortar that is the softest and most flexible for the job required.

Using our Premixed Hydraulic Lime Mortars ensures a greater consistency of strength, as these are formulated to strength with the aggregates already added.

These come in a great selection of colours, which will ensure you get the right mortar in the right colour without having to worry about batching consistency.