Hot Lime – The New Hot Topic - PAYE
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Hot Lime – The New Hot Topic

Hot Lime – The New Hot Topic

Spencer Hall ACR

What are Hot Lime mixes?

In essence ‘Hot-Lime’ refers to a mortar mix being produced using quick lime within the mix, rather than it being slaked first before being mixed with aggregates. Lime is produced by first burning chalk or limestone (CaC03) to form quick lime (Calcium Oxide – CaO). The quick lime is then slaked (by adding water (H20) which causes an exothermic reaction) and forms Calcium Hydroxide – Ca(OH)2. If no clay is present in the original limestone or chalk, the resulting lime is said to be ‘non-hydraulic’.

Traditional lime putty (or non-hydraulic) mortars tend to be produced using ‘mature’ lime; this is a lime that has been slaked and stored under a layer of water for (normally) a minimum period of 3 months and is considered to be the purest form of non-hydraulic lime.

A common misnomer is that a hot-lime mix is applied actually physically hot… it can be, but this is not the factor by which this is defined. The key to a hot-lime’s definition is that it has been mixed/produced from a non-slaked lime (as in it completes its exothermic reaction as it is being mixed with its aggregate). As a result, most hot mixes are produced in close proximity to the location where they are to be used.

Why are they advocated? Analysis etc

Hot-mixed lime mortars have a number of different properties to mortars prepared from mature lime putty due to the effect of the heat in the reaction and the high alkalinity of the lime. The quicklime reacts with water generating heat, and simultaneously binds together with the sand or aggregate to produce a mortar.

Following analysis, no historic lime mortars have been found to have an Mpa higher than 3 (Mpa = Megapascals – the denomination by which to measure tensile strength).

  • Modern NHL (Naturally Hydraulic Lime) mortars however tend to have a significantly higher Mpa;
  • NHL 2 after 28 days will have a typical Mpa of 4.0
  • NHL 3.5 after 28 days will have a typical Mpa of 5.5
  • NHL 5 after 28 days will have a typical Mpa of 7.0

There are concerns that the standardised 3:1 ratio often used for making lime mortars is a fallacy that has been widely adopted incorrectly. A mortar of 3 : 1 aggregate : quick lime actually produces a ratio of nearer 2 : 3 which is obviously much weaker than the standardised 3 : 1 aggregate : lime mixes that have become industry standards over the years.

A colleague who is a conservation scientist once explained to me that the analysis of many old historic lime mortars gives a mix ratio for sand : lime putty, by mass of 1.5 – 2 : 1, (i.e. about 1 – 1.3 : 1 by volume).

He concluded that this was because they were generally ‘hot mixed’ as 3 parts of sand with 1 part of quicklime by volume. Over the years this 3 : 1 sand : quicklime mix ratio, by volume, has incorrectly been recorded as 3 : 1 sand : lime putty, by volume.

We can convert a sand : quicklime mix ratio to a sand : lime putty mix ratio as follows: Aggregate : Quicklime = 0.44 x Aggregate : Lime Putty (by volume) Using this conversion factor we see that the 3 : 1 sand : quicklime by volume mix ratio of old, is actually equivalent to only 1.32 : 1 sand : lime putty, i.e. 3 : 1 = 0.44 x 3 : 1 = 1.32 : 1 …also known as the 3:1 myth.

What are the advantages/disadvantages?


  • Hot lime ratios inevitably create a weaker mortar, which act in a greater sacrificial manner than harder, richer mixes
  • Hot-mixed limes produce a fat, workable mortar that adheres well to masonry.
  • The addition of quicklime to mortars binds in fine particles as well as minimising the shrinkage of excessively clay-rich earth (which may be used as an aggregate).
  • The heat generated during slaking quicklime aids the incorporation of additives to moderate the properties of the mortar e.g. for improved workability or water resistance. Tallow, casein or similar proprietary materials will melt, dissolve or ‘saponify’ (convert to ‘soap’ by reaction with alkali) during this reaction, allowing a more thorough dispersal within the mortar.
  • Quicklime expands as it slakes, so the volume of a hot-mixed lime mortar increases during and for a period, after mixing.
  • Hot-mixed lime mortars take up water rapidly as they slake and so the mortar stiffens up quickly, allowing for more extensive build rates.
  • Hot lime mortars firm up at a much faster rate than normal fat lime mixes – this can make working time limited.
  • Hot mixes are logical and practical for small scale vernacular projects, but I would suggest unworkable on larger scale commercial projects
  • Quicklime is designated as a ‘hazardous’ material, but is not ‘dangerous’. It can be used on site providing appropriate health and safety procedures are followed. It is however highly caustic and a reactive material (during slaking) or mixing which can cause irritation or burns if it comes into contact with skin or the eyes. As with all lime mortars full PPE must be used at all times when handling quicklime. Sugar-water eyewash MUST be made available wherever mortars are being prepared (saline eyewash is ineffective for use with lime). These considerations could make using ‘hot lime’ mixes difficult in certain situations.


The various historical methods of preparing hot-mixed lime mortars are still the subject of on-going research and some debate, and there may well have been considerable local and regional variation. Until this can be proven and the approaches standardised, we choose to remain interested but somewhat distanced from the subject. As with all project work however, each solution should be judged on its own merits – if therefore a hot lime mix is specified for a project we would of course embrace the opportunity to work with it and garner greater experience.

Further reading & useful links;

‘Mortars Renders & Plasters’ – Practical Building Conservation, English


Building Limes in Conservation – Building Limes Forum, Donhead.