What is Natural Hydraulic Lime (NHL) and why is it different to locally produced Non-Hydraulic Lime products?

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The term “hydraulic” as it applies to lime materials, relates to their capacity to set by “hydrolysis” in the presence of water vs. “carbonation” reaction with CO2 from the air that occurs more slowly over a period of weeks or even months as the material dries. Hydraulic limes therefore are those that will initially set (and re-set!) in the presence of water whilst Non-Hydraulic Limes remain reliant upon the slower carbonation or “drying” process to achieve their set.

Naturally Occurring

Essentially, as its name suggests, Natural Hydraulic Lime (NHL) is a naturally occurring lime material generally associated with building conservation or as a green, eco-building material. Its use dates back to the times of the ancient Egyptians, Greeks and Romans but declined during the 19th century with the emergence of Ordinary Portland Cement (OPC).

Practical Construction

Modern research has however delivered a greater understanding as to the unique properties of these traditional materials and their practical application in both restorative and modern construction.


NHL contains traces of natural clay-like materials known as “pozzolans” that promote early setting whilst the product remains wet and after which, as the material dries, its hardness continues to develop by carbonation. The set of Non-Hydraulic Lime such as that produced locally in Western Australia, is dependent upon “carbonation” which takes place much more slowly only as the material dries over a period of months. The problem that arises, particularly in areas of high erosion potential, renders and mortar prepared with non-hydraulic lime will tend to crumble and fail ahead of reaching their fully set hardness through the drying process.

The introduction of substitute "pozzolanic" materials such as fly ash to non-hydraulic lime in order to promote hydraulicity has produced varied results. It has therefore become common practice to add cement to Non-Hydraulic Lime mixes to ensure an early set. Whilst this may be suitable for use with current-day hard-fired bricks, the addition of cement to lime mortars makes them much harder and impervious to moisture transfer. The resultant entrapment of water within the softer material of older bricks and stone masonry units such as limestone or sandstone then promotes their eventual decay by de-crystalisation.

Practical Advantages

  1. Fast Setting – can be used on-site just as efficiently as modern cement.
  2. Hydraulic – Will set under water making it excellent for use in masonry exposed to severe weather and marine environment construction.
  3. No Shrinkage – Shrinkage cracking virtually eliminated due to hydraulic setting.
  4. Breathable – Vapour permeability allows for the absorption and evaporation from masonry protecting against salt damage and de-crystalisation.
  5. Flexible – Allows for movement and thermal expansion.
  6. Adaptable – Available as NHL 2, NHL 3.5 and NHL 5 grades to allow for design of mortars suitable for masonry of varied density and degrees of weather exposure.
  7. Standardised Product – NHL 2, NHL 3.5 and NHL 5 are standardised materials that will produce predictably consistent end-product.
  8. Workable – Mixed correctly, NHL mortar, render or plaster are essentially or the same consistency and workability as modern cement.
  9. Sympathetic – Being softer than modern cement, NHL mortar will not wear away surrounding masonry over time with bricks and stones more easily reclaimed if dismantled.
  10. Aesthetic – Indistinguishable from non-hydraulic lime mortars producing the same traditional appearance.


CLPM imports and distributes the Singleton Birch range of NHL 2, NHL 3.5 and NHL 5 into Australia. Please call us on +61 8 9371 3877 for quantity pricing.  

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