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What is Hydraulic Lime?

By B. Turner
Updated: May 17, 2024

Hydraulic lime is a type of lime, or calcium carbonate, which is used to make mortar and plaster products. The lime is heated, then mixed with an aggregate material like sand or stone. Once this mixture is blended with water to form mortar, it can be used in many types of masonry construction projects. The mortar can be placed between bricks or blocks to bind them together, or may even be applied to the surface of the masonry to create a plaster or stucco-like application.

A specialized form of limestone is used to make hydraulic lime. The limestone is mixed with clay then fired in a kiln to high temperatures. This process removes much of the moisture from the lime and also produces mineral by-products known as silicates. The remaining limestone and silicates are combined to form hydraulic lime.

To understand how hydraulic lime works, it is important to first understand how traditional hydrated lime works. When hydrated lime is mixed with water to form mortar, it has a consistency similar to peanut butter. As the mortar reacts with the air, it absorbs carbon dioxide, causing it to harden or cure. Hydraulic lime mortar, or the other hand, begins to harden when it is exposed to water. Over time, it will also absorb carbon dioxide from the air to experience a second phase of hardening or curing. Due to these differing reactions, hydraulic and hydrated lime are very different products and are not interchangeable.

Hydraulic lime offers a number of benefits over traditional lime mortar blends. The most important is its ability to cure and harden when wet, which means it can be used in many applications where other mortar products would fail. It also has low elasticity, resulting in fewer cracks due to expansion and contraction. Hydraulic lime is also very porous, allowing excess moisture to escape rather than collecting inside the wall structure. This makes it very popular for historic preservation applications, where traditional hydrated lime would crack or crumble over time due to trapped moisture.

This product is available in a number of different varieties, which are chosen based on the application. Natural hydraulic lime Type 2 (NHL-2) is designed for interior applications and some exterior uses where the mortar will be protected from moisture. NHL-3.5 is suitable for most exterior applications, while NHL-5 is designed for the toughest outdoor applications, such as floor slabs, underground work, and chimneys.

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Discussion Comments
By SkyWhisperer — On Mar 09, 2012

@MrMoody - Limestone is supposed to be good for your soil too. I can see we have some limestone in our front lawn, especially where there is a lot of landscaping. We have hard, dry clay and the limestone helps keep the clay together so that it will fold into clumps. It also helps to correct the pH balance of the soil. If the soil is too acidic it will make it more alkaline.

By MrMoody — On Mar 08, 2012

I think that for mortar lime in fact may be one of the best solutions because of its resiliency. I can’t believe how easily traditional concrete mortar cracks.

One of my home improvement projects involves repairing the front gate outside. What happened is that the gate post (which is made of brick) began to crack in inclement weather. As a result the metal latch which fastens to the post can no longer be held securely in place.

I actually need to apply fresh mortar to that part of the gate so I can snap the clamp back in, and the gate can open and close normally. I am thinking to use hydraulic lime however since it appears that it will withstand the changes in temperature better than the usual mortar.

I also like the fact that it’s more malleable, like lime putty plaster which is used in a lot of different projects.

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