Introduction to Lime - Types of Lime

Slaked/High Calcium/Putty/Air/‘Fat’ or Non-Hydraulic Lime

This lime is produced by slaking fresh Quicklime in an excess of water. It is also known as non-hydraulic lime because it requires exposure to air in order to carbonate and does not set under water.

This lime is regarded as the most appropriate lime for old buildings or ‘softer’ substrates where maximum permeability, capillarity and flexibility is required.

Premixed ‘wet’ products are made from lime putty with the addition of an aggregate (or
alternative) and/or fibre

Natural Hydraulic Lime (NHL)

This ‘hydrated’ lime is different to the ‘bag’ lime as it is produced from limestone naturally containing clay and other impurities which enable it to set without exposure to air; it is therefore used for providing a faster initial set in more extreme conditions including under water. There are many types of hydraulic lime and they are categorised according to the following (NHL - Naturally Hydraulic Lime):

• Feebly hydraulic lime (NHL 2) – contains less than 12% clay which means it is slower to set (about 20 days in water);
•  Moderately hydraulic lime (NHL3.5) – contains 12%-18% clay which gives a slightly faster set (15-20 days in water);
• Eminently hydraulic lime (NHL5) – has up to 25% clay content which means that a much faster set is achieved (hours/days).

These limes have become increasingly popular over the last decade because of their comparative ‘ease of use’ and ‘strength’, compared to fat lime products, but see Choosing Lime for ‘trade-off’ chart. It is important to understand the building, Its location and construction materials to provide an informed specification for which lime to choose.

Hydraulic Lime (HL)
These contain lime and other materials such as cement, blast-furnace slag, fly ash and limestone filler. Manufacturers are not required to state the composition of hydraulic lime.

Formulated Lime (FL) 

These contain lime with hydraulic properties comprising air lime and/or NHL with added hydraulic and/or pozzolanic material – classified according to its compressive strength.

Natural/Roman cement
Natural or ‘Roman’ cement was extensively used in the 19th century (nothing to do with the Romans). It is based on a naturally occurring limestone containing clay and unlike the fat lime products above, was not slaked, just ground. This product comes from a single, naturally occurring mineral, unlike Portland cement which consists of ingredients blended together. It was appreciated for four main reasons: fast set, strength, aesthetics and durability. It was patented in 1796 and used for run moulding, precast moulds, artificial stone, stonework joints (especially canals), floors (terrazzo) and fresh plaster sculpting. It is still available today, ‘Prompt’ is an example.

Pozzolans
Pozzolans are ‘heated’ materials such as brickdust, GGBFS (Granulated Ground Blast Furnace Slag), Pulverised Fuel Ash (PFA) and Calcined Clay that contain silica, alumina and iron which become reactive towards alkalis including lime. They help to give a fat lime mortar a ‘faster’ set (essentially creating an ‘artificial’ hydraulic lime) and are therefore helpful in many situations to broaden the applications for fat limes. It is believed that many historic mortars were ‘fat’ or ‘non-hydraulic’ because those that have been analysed contained these ‘heated materials’ which changed the performance of the mortar.