Frequently Asked Questions...

0Q: What is the difference between Hydraulic Lime and Hydrated Lime?

A: Natural Hydraulic lime (NHL) is a lime binder made up of impure lime in a powder form that sets under water in a process called hydration. NHL's also have a free lime content which sets in the air. 

Q: What is Lime Putty?

A: Lime Putty (Calcium diHydroxide) is a pure lime binder made from pure quicklime and slaked under an excess of water.

Q: Does Lime Putty need to be matured for 3 months?

A: No - modern quicklime is fired very efficiently and slakes instantly therefore when using a powdered quicklime there is no requirement to mature lime putty.  

Q: What does 'NHL' mean?

A: 'Natural Hydraulic Lime' however sometimes NHL is also referred to as 'Non-Hydraulic Lime' just to confuse matters.

Q: Do they produce Natural Hydraulic Lime in the UK?

A: No - There is no homegrown supply of Natural Hydraulic Lime anymore. It is all imported from Europe.

Q: Is lime dangerous?

A: Lime is classified as hazardous as it is an irritant but with the correct PPE (Personal Protective Equipment) you can manage the risk. 

Q: Can lime putty mortar or plaster be used outside?

A: Non-hydraulic lime putty mortars and plasters are sacrificial in their design and should be softer than the historic fabric they are going to be used on. They are best suited to conservation projects of special architectural significance where maximum permeability and flexibility is desired to protect and conserve the historic fabric. It is advised to use a skilled and experienced craftsperson with the pre requisite knowledge and experience in using lime putty based mortars, they are not recommended for DIY use. Skilled practitioners will often gauge in NHL 5 up to 20% or 5:1 (Lime Mortar:NHL 5): into the mix externally to provide additional hydraulic properties in normal conditions in low levels of exposure.    

Q: Can Lime be used in the Winter months?

A: It is recommended to only use lime products from April 1st and September 1st. Work should be protected from: wind, rain and frost for the entirety of the winter months. 

Q: Can you add cement to lime?

A: Although it was common in the late 19C and early 20C to gauge in Portland Cement into lime mixes it is generally regarded in conservation circles as inappropriate. Roman Cement or Natural Cement can be gauged into non-hydraulic lime mixes @ 10% or 9:1 (Lime Mortar:Natural Cement).

Q: How can you tell if a mortar is cement or lime?

A: Lime Mortars and Plasters are very soft in relation to cement mortars and can be scratched with a fingernail. Lime based materials will fizz veraciously in acids such a vinegar. 

Q: What is Hot Lime Mortar?

A: Hot Lime mortar has come to mean a few things in recent years but essentially is a lime mortar which has been produced by mixing the sand/aggregates and quicklime together to make a mortar. This mortar can then be used immediately when hot or left to cool and be used later on. Sometimes 'Hot Lime' just refers to another name for Quicklime.   

Q: Do you need to protect lime work?

A: Yes it is essential to protect all lime work. Hydraulic limes need continually wetting for 72 hours to enact the proper hydraulic set. Non-Hydraulic/ Lime Putty mixes need a long slow cure to achieve carbonation and should not be allowed to dry out too fast.  Optimum conditions for lime work are 15C with a relative humidity of 75%.

Q: Is using lime specialist work?

A: Yes lime is very specialist work which requires year to grasp and master. It is always advised to use a competent craftsperson who is experienced in using lime with the requisite skills, technique and specialist qualifications to carry out the job in full.

Q: Is using lime expensive?

A: It can be as there is more involved and can take longer to do and there are fewer craftspeople around who know how to use it properly. However material wise there isn't much in it as you don't get any waste with lime as it can be reused as doesn't set as fast.

Q: Is animal hair e.g horse hair ok to add to Lime Plaster?

A: Due to chemical treatment for anthrax, the acid washing breaks down the natural keratin in the hair. Imported horse hair is only to be added to lime plaster immediately before use and NOT stored in lime as it will further degrade and become not fit for purpose.

Q: Is Polypropylene Fibre a good alternative to using natural animal hair?

A: Yes, Polypropylene Fibre (PPF) is the industry standard for reinforcing lime plasters and renders. It is much stronger, dispersers better in the mix and is chemically inert and won't rot or degrade once added to high-alkali lime.

Q: Is Hydraulic Lime appropriate to be used on historic masonry?

A: Yes, Natural Hydraulic Lime (NHL) has been used for centuries for the repair of historic masonry. Different brands of NHL have varying properties and levels of strength. Use an NHL with a high free-lime content and low compressive strength i.e. NHL 2 on softer more porous fabric. The Lafarge NHL 2 has a free lime content of 44% and typically has a <4Mpa compressive strength whereas a typical woodfired brick would have a compressive strength of 50Mpa so the mortar will always be softer than the fabric. NB. For very sensitive very friable historic masonry of architectural significance use a lime putty based mortar which will act more sacrificially. 

Q: Can you apply lime plaster over gypsum or plasterboard?

A: Yes, however you would need to prime the surface first with an aggregated primer such as Baumit Premium Primer then after 24hours can apply a lime plaster such as the Baumit Klima Glatt or Tector Natural Finish.

Q: Can you repoint with lime over sand and cement mortar?

A: No It is not advised.