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Natural Hydraulic Lime

Hydraulic lime (HL) is a general term for varieties of lime (calcium oxide), or slaked lime (calcium hydroxide), used to make lime mortar which set through hydration: thus they are called hydraulic. The other common types of lime mortar set through carbonation (re-absorbing carbon dioxide (CO2) from the air) and are sometimes called air lime. Hydraulic lime provides a faster initial set and higher compressive strength than air lime and eminently hydraulic lime will set in more extreme conditions including underwater.


​In Conservation Technology Ltd. we sell two varieties of NHL (Natural Hydraulic Lime) Secil Natural Hydraulic Lime and Roundtower Natural Hydraulic Lime. 

Secil has been producing Natural Hydraulic Lime for over 100 years in Portugal. Secil NHL 2, NHL 3.5 and NHL 5 all conform to the most recent European standards EN459-1:2010 and are supplied in 25kg bags, or used in a variety of mortars and plasters, all tried and tested in the Irish market.   These limes are suitable for Historic Buildings and New build.  Breathable, flexible and ecological.  Stoneware Studios have collaborated with Secil to bring this high-quality range to the Irish market.  Secil has similar strengths to the French NHL’s and is suitable for plastering, rendering, bedding, pointing, limecrete, etc.

Why Choose Secil  Natural Hydraulic Lime?

Secil Natural Hydraulic Lime (NHL) features the same properties it used to have, decades ago, before the introduction of Portland cement. It is an exclusive result of the chemical composition of its natural raw materials. Our Natural Hydraulic Lime features a unique brownish colour due to the characteristics of the stone that we extract from our quarry, which is the same used since the beginning of the unit’s existence. Regular tests are carried out in our laboratory to ensure our NHL maintains the same specifications and quality.


Common Pointing Lime Technical Data Sheets

Secil NHL 2 Technical Sheet 

 Secil NHL 3.5 Technical Sheet 

Secil NHL 5 Technical Sheet 

Secil NHL Safety Data Sheet

Secil NHL limes are also used in Secil pre-mixed mortars, customised for bespoke applications (based exclusively on Natural Hydraulic Lime):


Secil Special Solution mortars (not based exclusively on Natural Hydraulic Lime):

  • Reabilita RA05 – this macroporous plaster is for the treatment of salt infestation, efflorescence and cryptoflorescence,  including salts arising from rising-damp and cement related sulphates.

  • Reabilita RR20 – This is a plaster system designed for the repair of early cement renders.


Roundtower Limes has been used in thousands of jobs from Derry City Walls to Mizen Lighthouse. Manufactured by Socli, the largest producer of Naturally Hydraulic Lime in France, this range of limes gives first-class results, full breathability and vapour permeability, great flexibility, minimal shrinkage and, most importantly, stability over time. This lime is unblended and has been a critical element to the success of thousands of projects throughout Ireland and the world.


With its two lime quarries and manufacturing plants, Socli is one of the leading companies in the manufacture of Naturally Hydraulic Lime in the World.
Created in 1863, the factory of Sauveterre-la-Lémance is one of the oldest in Europe in the production of Natural Hydraulic lime while the factory of Izaourt (Founded 1903) is also one of the most important producers of Natural Hydraulic lime in the World by virtue of its four vertical kilns, extensive test laboratories and superb training centre.

Roundtower Natural Hydraulic Limes are natural products. They contain no additives and are not blended. Roundtower Natural Hydraulic Limes are suitable for building restoration and conservation. Equally, they are very suitable for new construction and engineering works. Roundtower Natural Hydraulic Limes are recommended for mortars, plasters, concrete and grouts. There are three grades of Roundtower Natural Hydraulic Lime: NHL2, NHL3.5 and NHL5.


What is Naturally Hydraulic Lime?

Natural Hydraulic Lime (NHL) is obtained by the calcination of argillaceous or siliceous limestone. The burning process produces calcium oxide and calcium silicates and calcium aluminates. The resulting material is ground, hydrated and carefully packaged. The resulting Natural Hydraulic Lime, when put in contact with water, will set and harden. Carbon dioxide present in the air also contributes to this hardening process.


Natural Hydraulic Lime is mixed on-site with suitable aggregates to give compatible, flexible, breathable and beautiful plasters and mortars. In conjunction with this, it's very special hydraulic set gives the added benefit of robustness and long-term durability.


Why choose Roundtower Natural Hydraulic Limes?

There are many reasons for using Roundtower Natural Hydraulic Limes:

  • Natural colours are suitable for both conservation and new build. Excellent sand colour reproduction.

  • Compatibility & Suitability – a variety of strengths and colours available to suit existing mortar, whatever age.

  • Suitable compressive strength – range allows for a choice of compressive strengths. Compressive strength is achieved gradually, allowing for movement.

  • Stability. Some limes are prone to cracking in the medium term (3months-2 years) as carbonation continues to take place. Roundtower has an even distribution of free lime that allows for a stable continuous build-up of strength with no stresses and associated cracking.

  • Low alumina content – less susceptible to attack by salts.

  • Resistance to salts – no gypsum or cement make sulphate attack and
    alkali-silica reactions impossible. Excellent performance even in the marine environment.

  • Purity – No addition of any kind.

  • No need for blending – no need for pozzolans, cement, plasticizers, water retainers, waterproofers etc.

  • Great workability – Free-lime content gives excellent workability

  • Self-healing properties – Free-lime content also gives the mortar self-healing properties.

  • Versatility of use – rendering, plastering, pointing, grouts and other building mortars

  • Reworking – all Roundtower Natural Hydraulic limes can be re-worked within 24 hours.

  • Economy – low bulk density means a good economy and superior volumes of mortar.

  • Elasticity – no expansion joints, minimal shrinkage and cracking.

  • Permeability – condensation dispersion, reduces rot, healthy living environment.

  • Resistance to weather – quick setting means less need for protection from adverse weather conditions compared to Fat lime mortars.

  • Resistance to bacteria & vegetable growth – alkalinity of lime inhibits growth.

  • Insulation – permeability reduces moisture in a structure.

  • Recycling – building materials used with Roundtower Natural Hydraulic Limes can be recovered and re-used.

  • CO2 Absorption – Natural Hydraulic Lime re-absorbs CO2 during carbonation of the free lime.

  • Variety of international reference projects.

  • First-class quality technical back-up service.

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