What is the difference between hydraulic and non-hydraulic lime?

Limes when used in building can be categorised into two types, either hydraulic (Natural Hydraulic Limes) or non-hydraulic (lime putty). The main difference between the two is the way is which they set.

Hydraulic limes set by hydrolysis, a reaction caused by water. It causes a faster and harder set, therefore these limes are more often used for exterior work, especially in exposed or damp conditions. Hydraulic limes are available as a bagged powder and in differing degrees of strength.

Non Hydraulic Limes have various names such as Fat Lime, Lime Putty or Air Lime. They set by carbonation. This causes a much slower set and the lime remains softer and more breathable. Our non-hydraulic lime putties are matured on site and used to produce our Medium, Coarse and Fine Stuff plasters and mortars.

Hydrated lime (not to be confused with hydraulic lime) which is sold in most builders mechants is a dry form of lime putty and is a poor sustitute for lime putty and is not recomended for use in lime mortars. It is mainly used as a plasticiser in cement mortars. A cement mortar with a small amount of hydrated lime is not a lime mortar.

Selecting the correct lime for the job 

The correct type of lime should be selected according to the job. Lime putty is always recommended for use with cob, strawbale and timberframe.  Also in areas with soft brick or stone unless the location is in a particularly exposed location in which case a NHL 2 or 3.5 is recommended.  For hard brick or stone a NHL should be used, the strength of which is determined by the exposure of the location. A NHL3.5 or 5 is advised for below ground works, footings and rendering or pointing to cellars.

Recently there has been a resurgence in the use of hot-lime mixes. These are usually mixed by professional conservation specialists on site and are a mix of quicklime, sand and water. This produces an exothermic reaction and consequently a hot mortar mix which is usually used immediately. Research suggests that this can produce a stickier mortar better bonded to the aggregate and substrate.

Hydraulic Lime (NHL)

NATURAL HYDRAULIC LIME (NHL) is produced by burning and slaking limestone which is quarried with a proportion of silica and trace elements.

It is the silica and trace elements that when burnt, become reactive with water and will cause an initial hydraulic set of the mortar when mixed with water.

It is the initial hydraulic set that makes the HYDRAULIC LIME MORTARS easier to use for the contact work and for conservation work where time scales and ease of use are important. The work still needs protection from wind, rain, frost and drying, but for much shorter periods of time than with lime putty mortars.

Natural Hydraulic Lime must have no additions such as Pozzolans or cements to conform with the EU Norms (EN/BS 459, 1-2-3).

Here at Lime Stuff, we stock SECIL Natural Hydraulic Limes.

Non Hydraulic Lime (Lime Putty)

Lime putty or non hydraulic lime is produced by slaking fresh Quicklime (lump lime) in an excess of water which is then left to mature for at least three months. The resulting fat lime putty hardens as a result of exposure to the air/re-carbonation and does not set under water. This lime is often regarded as the most appropriate lime to use in the conservation of old buildings where maximum permeability is required. 

We produce a FINEST MATURED BUXTON LIME PUTTY and a FINEST MATURED CHALK LIME PUTTY. 

Our lime putty is blended with sand to produce our own LIME RENDERS and LIME MORTARS. It is also sieved and diluted to make our LIMEWASHES.

Pre-mixed Mortars

Our PRE-MIXED LIME MORTARS and PRE-MIXED LIME PLASTERS are made by mixing lime putty with sand. We supply a range of standard plasters or can make up batches of special mixes to match existing plaster.

Lime plaster mixes are supplied ready to use but will require re-working or knocking up before use.

Pre mixed plaster/render is supplied with or without HORSE HAIR. HAIRED RENDERS should not be stored. The hair is added to mature render just before dispatch and the render should be used as soon as possible after delivery. Hair is required in scratch coats to give tensile strength.

Limewash

Traditionally LIMEWASH was the principal finish applied externally and internally to historic buildings, quite often applied directly to the masonry or brickwork and more commonly to pre-applied lime coatings (i.e., harling, plaster, render etc.,). Although often thought of as a decorative coating, the limewash was first of all a protective layer to the lime coatings and masonry substrate. On new lime renders and plasters it unifies and protects the surface particularly while strength is developing within the new plaster.

As with all lime coatings, limewash is a breathable coating allowing evaporation of moisture and water vapour. Limewash is also a repairing material, being used to fill small shrinkage cracks on the lime coverings. Limewash can also be used in conjunction with various aggregates to make shelter coats for friable masonry and will act as a sacrificial protective coat.