Lime has been used for over 10 thousand years in one form or another, for everything from foundations to plaster to paint.

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There are two basic types - so-called "air" limes that cure from contact with the air, and "hydraulic" limes that cure with water.

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Air limes are the original ones - used to build everything you see that's older than 150 years, from castles and cathedrals to garden walls. They are very flexible, permeable, and great to work with, but you have to be careful to cure them properly and they are not as durable as hydraulic limes, so in exposed area like chimneys they can erode quickly.  You can make them more durable with the addition of "pozzolans" - fine, lightly burnt clay that makes the lime set faster and become more durable.

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Hydraulic limes were developed for engineering purposes in the 17th and 18th centuries.  They come in three main strengths, the numbers corresponding to their crushing strengths:

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NHL 2 - good for sheltered areas and soft stones/bricks.

NHL 3.5 - good as a general building lime for most applications.

NHL 5 - good for very exposed areas (it can set underwater).

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To compare, the cement you buy in every builder's merchant is 12 to 20 times stronger (and more depending on how you mix it)  This also means it's almost impermeable and very brittle.  Great for long-span concrete structures where it's high strength and rigidity can be used to advantage, not so good on older, softer, more flexible buildings.

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By combining these types of limes with different sands, fibres, clays, and other ingredients (casein powder, for example, to make it self-levelling), you can get a huge variety of properties, colours, and textures.

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