Lime mortars have had a resurgence in popularity in Britain since the 1990s with the use of Natural

Hydraulic Limes (NHL) becoming prevalent in conservation work. However, it has become apparent

that NHL mortars lack many of the qualities they were originally thought to possess, they are often

too hard for the situation they’ve been used in, not sufficiently vapour permeable and visually

different from historical mortars.

Prior to the 1990’s there was no significant construction use of hydraulic limes in Britain outside of

high strength situations such as military or civil engineering projects (Henry, 2018.) NHL mortars

require water to activate the curing process and unlike the air limes generally found in Britain they

tend to set very hard and are generally unsuitable where a lime mortar is called for in a domestic

historical setting. The qualities that made hydraulic limes suitable for major engineering works -

hardness and the ability to set underwater, also make them unsuitable in a context where a vapour

permeable, softer sacrificial mortar is needed.

Historically, the majority of lime mortars used in Britain were hot mixed quicklime mortars, that

would be mixed on site using locally sourced aggregates with some suggestion that 400 yards would

be about the maximum distance bulky items like aggregates would travel to site (Artis, 2018.) Hot

mixed mortars are the simplest form of lime mortar, mixing a fired limestone (quicklime) with damp

sand to slake the lime and then create a mortar. This document is not intended as a step-by-step

instruction on how to make a hot mix mortar, but as useful background information for specifiers

and contractors to understand the benefits of hot-mix mortar and to help develop a mix that’s

suitable for a particular setting.

The below information is intended as an overview on the use of hot mix lime mortars and should not be considered an exhaustive guide on the subject. Before any practical work is undertaken with hotmix mortars it is the responsibility of any contractors to ensure they are adequately trained in the safe use of the materials and equipment needed.

The need for historically sensitive hot mix mortars is predominantly concerned with managing the

build-up and movement of moisture in a building and avoiding the impacts that mismanaging

moisture can have. It’s estimated 60% - 80% of solid masonry buildings in Britain have been built

using hot mix lime mortars (Artis, 2018) These mortars are highly vapour permeable allowing

moisture to move freely through the fabric of the building, this level of ‘breathability' allows the

building to buffer moisture, balancing indoor humidity by absorbing moisture and drawing it through

the lime in the wall and pushing it to the exterior face of the building’s walls where it evaporates.

Hot mix mortars are twice as breathable as NHL 3.5 based mortar, (Brown, 2017) a common

specification for mortars used for repointing of historic buildings. Repointing a building, that was

built using hot-mix mortar, with an NHL mortar, or worse still cement, will disrupt or completely stop

the movement of moisture from inside the building to outside. Moisture becomes trapped in the

fabric of the building as it can’t escape past a hard NHL or cement mortar. Moisture trapped in the

fabric of the building has several negative impacts, firstly as the walls become wet the building will

lose heat quicker than a dry building, this translates to lower occupant comfort and higher than

necessary fuel bills. Secondly, as the mortar in the walls become soaked the moisture will try to

escape from the building through the masonry (brick or stone) rather than through the mortar. With

soft clamp bricks, limestone or sandstone the masonry will become soaked and susceptible to rapid

erosion known as spalling, where the front of the bricks blows out whilst the mortar remains intact.

This is a reversal of how the brick/mortar relationship should work where the easily replaceable

mortar should be the sacrificial element that saves the masonry from damage. Thirdly the use of

over-hard mortars (NHL or cement) can trap moisture to the extent that it can’t escape from the

building at all (neither through the mortar or the brick/stone) in this instance the mortar in the walls

becomes saturated and over a period of several years the mortar becomes soft and weakened.

A mortar should be softer than the brick or stone that the wall is built from. Hot mix mortars tend to

be relatively soft (2 - 3 N/m2) compared to NHL mortars and research conducted by Historic England

has shown that NHL mortars become several times stronger than their 28 day lab test strength –

NHL 2 becoming up to 6 N/m2, NHL 3 becoming up to 10 N/m2 and NHL 5 reaching up to 15 N/m2

(Henry, 2018.) Most clamp fired red bricks have a compressive strength of around 3 - 4 N/m2 making a hot mix mortar the right choice for a large number of red brick buildings built before the early 20th

century. A soft mortar should be considered a sacrificial part of the building that can be replaced

every 50 - 100 years. Using over-hard mortars (NHL or cement) will lead to deterioration in the

masonry rather than in the mortar which will lead to masonry being replaced rather than simply

repointing.

A balance needs to be struck between a mortar being soft enough to not damage the building and

being robust enough to have a working life of several decades. Hot mixed quicklime mortars achieve

this by combining quicklime, a relatively weak binder, with aggregate in a manner that makes a lime

rich mortar. Prior to slaking quicklime and aggregate are combined in a ratio of 1:3, (1 lime : 3 sand)

however during the slaking process the lime almost doubles in volume, giving a lime to aggregate

ratio closer to 1:1.5 (Artis, 2018)

Mortars made with lime putty appear to offer a solution to contractors that avoid the health and

safety issues associated with site-prepared hot mixing. Lime putty is made from slaked quicklime, as

is the case with hot mixing, but the putty contains a large amount of water, to mix a lime putty

based mortar in a sufficiently rich mix to achieve a lime to aggregate ratio of 1:1.5 results in a mix

that is too sloppy to work with. The result is that lime putty mortars end up being made with

insufficient lime in the mix as this is the only way to get a workable mortar, and whilst these mortars

can work they can also take a very long time to cure and can be susceptible to failure in exposed

locations. (Henry, 2018) The majority of bulk-bag pre-mixed mortars available in the UK are made

using lime putty.

The final reason to use hot mix mortars is one of character and appearance, NHLs and cement

mortars tend to look hard, can either be too light or too dark and have a character that is at odds

with the softer appearance of a hot mix mortar which will likely contain varied aggregate and small

visible quantities of unslaked lime.

In addition to the above reasons for using hot mixed lime mortar there are several practical benefits

to using them. Hot mixes are very sticky or ‘fatty’ compared to NHL mortars which tend to be very

lean and ‘dead’ feeling. Quicklime hot mixes tend to be lime rich and the hot mix process traps a

certain amount of water vapour in the mix (Copsey, 2019) which produces a lighter, fattier mix than

NHL mortars. The sticky nature of hot mixed limes means less of the material falls from a trowel

whilst repointing or rendering, making the work quicker and leading to less waste than with NHL

mortars.

Hot mix mortar also has a quicker initial set than NHL or lime putty mortars , especially when used

hot, (Artis, 2018) this can be especially useful when used with hard masonry eg engineering bricks,

smooth reds or hard smooth stone like granite, or where the volume of material needed to repoint a

joint is particularly big, eg a stone wall with large irregular joints.

Hot mix can also have a slow final set time, depending on various factors (time of year, masonry

substrate and mix content) it’s possible to work with hot mix lime, leaving it for several hours or

even over night before pointing is rubbed up. This slowness of set allows a great deal of flexibility

and so long as the appropriate aftercare is administered to the work a successful result can be

achieved.

Hot mixing on site also has advantages over NHL or bulk bag pre-mixed lime putty mortars.

Site mixed hot mortars are the most affordable lime mortars available. Bag for bag quicklime is

comparable to NHL, however once quicklime has been slaked it expands to approximately double its

initial volume making it a more cost-effective option. Similarly, site mixing is also considerably

cheaper than premixed mortars that have a manufacturer’s profits and transport costs to add to the

initial material costs, as well as the tendency for lime putty mixes to be too lean and therefore

lacking durability.

One of the greatest reasons to use site prepared hot mixed mortars is the ability to control every

aspect off the mix, ensuring the quantity, location and timing of a mix can be organised to suit the

individual needs of a particular job, avoiding the unnecessary waste and expense associated with

pre-mixed bulk bag mortars. 

Site mixing also allows a specifier and contractor to develop a mix that’s exactly right for a particular

location. Adjusting the type, size and colour of aggregates until the correct mortar is arrived at, there

is more on this subject below.

When quicklime comes into contact with water there is an instant exothermic reaction and the lime

can reach temperatures of up to 400c in seconds, not only is heat produced but the reaction is often

as violent as it is rapid with steam and lime spitting from the mix.

Quicklime will react with any water, whether it’s been intentionally introduced as part of a slake or

it’s in the eyes, nose or on the skin of people nearby when a slake is taking place. PPE is of the

utmost importance to avoid burns, respiratory damage and damage to eyes. PPE is often treated as

optional on sites with contractors making it available to workers but turning a blind eye if they

choose not to use it. With hot mixing this can’t be the case, it’s potentially very dangerous with the

ability to cause life changing injuries very quickly. PPE must be worn. Furthermore, consideration

should be given to other people on site when hot mixing is taking place, ideally nobody other than

those involved with the mix will be nearby, but an assessment of conditions should be made to

ensure a cloud of quick lime doesn’t envelop people nearby who aren’t directly involved in the

slaking and mixing process.

Quick lime also presents dangers for storage and handling, incorrectly stored bags that have been

opened, either intentionally or by mistake, pose a fire hazard on site and consideration should be

made to ensure quicklime is stored away from water sources and flammable materials.

Despite the dangers, once they are known and understood correct procedures to mitigate the

dangers of working with lime can be put in place allowing people to work safely. Broadly speaking

the precautions that CDM 2015 would expect when working with cement or NHL lime are applicable

to working with quicklime and making hot mix mortars, however it is crucial that everyone working

with hot mixes has been adequately trained and that workers adhere to that training and the correct

use of PPE.

Specifying a hot mix mortar is an opportunity to develop a mix that closely matches the physical

characteristics of an existing mortar. This is especially important when repointing as an incorrect mix

can adversely affect the performance and appearance of a building. Due to the difficulties associated

with hot mix, as outlined above, many contractors have turned to using pre-mixed bulk bag mortars.

These mortars are generally made using lime putty with a variety of different aggregates to produce

different coloured mortars, however simply using ‘Yorkshire’ mix or ‘Heritage’ mix from a specialist

merchant rarely produces a mortar that’s historically accurate or sufficiently robust. In the instances

where visually matching a mortar is important specifiers can analyse the existing mortar and develop

a mix that closely matches the existing mortar.

Overview of analyses

Analysing mortar involves taking a sample of an existing mortar and breaking it down into it’s

constituent parts to assess the quantity of lime and the type of aggregate used. Below is an overview

of what that process involves.

1.  Gather a sample or the existing mortar that is to be replicated. The greater the sample

weight the more accurate the analyses will be. 1kg of dried mortar is a suggested minimum

sample size.

2.  Break the sample up using a hammer or mortar and pestle taking care not to lose any

materials. Place the sample in a non-plastic bowl, then cover the sample with brick acid. The

brick acid will dissolve the lime content in the sample but leave the aggregate. Leave the

sample to stand until the sample has been fully broken down.

3.  Remove the sample from the acid using a spoon and place it into another bowl or dish and

rinse with water.

4. Spoon the sample from the water bowl and place into a coffee filter, then rinse the sample

again with clean running water. Be careful through this procedure not to drop any grains of

the sample.

5.  Once fully rinsed, place the sample on a clean baking tray, spread the sample out and place

it in a warm place to dry naturally. Using a hairdryer may disperse the sample so placing it

near a radiator is a safer option.

6.  Once the sample is dried it can be weighed again, the weight difference between the original

sample and the dissolved sample is the amount of lime that was in the mortar.

7.  The sample can now be passed through a series of grading sieves which will accurately

determine the different size, type and quantity of aggregate in the mortar.

The analysis should reveal the quantity of lime and the type and colour of aggregate in the mortar.

With this information to hand a specifier and/or contractor can set about finding suitable

aggregates, this may involve excavation on site or multiple trips to specialist building merchants,

however it’s worth bearing in mind that historically most aggregates were sourced from close to the

construction site and if possible, this would be a good place to start looking.

Lime burning to produce quicklime was a common practice in Britain prior to the 20th century, it

was typically a localised cottage industry with a degree of variety in the production process with the

source rock, kiln type, scale of production, fuel type and burn length all influencing the final product.

Many of the limes produced would have included partially burnt quantities of limestone along with

un-burnt fuel, ash and general dirt from the kiln. Slaking of such varied quicklime (which would

usually be in a lump the same size and shape as the rock was went it entered the kiln or ground up

into kibbled lime) would give equally varied results when made into a mortar and it would be

common for pozzolanic impurities like ash and limestone dust, along with unslaked lumps of lime to

be present in a final mortar. These impurities helped form the particular character of many historic

mortars and making a new mortar that replicates an old mortar isn’t necessarily viable.

Modern production methods mean that commercially available quicklime is a pure, consistent

product lacking any of the natural pozzolanic impurities found in mortars made from locally burnt

limestone. A positive of modern, consistent quicklime is that it allows for the controlled

development and consistent manufacture of lime mortars, allowing quantities to be amended and

batches tested until a suitable mortar is arrived at.

Assuming there is little variation in the performance of modern quicklime, then most of the variation

of a mortar mix will come from changing the aggregate. Historically most aggregates came from

close to a construction site where they were used, either on site itself or from a local small-scale

quarry. Many of these quarries are no longer in use so the specifier has a potentially time-consuming

job sourcing suitable aggregates from builder’s merchants and success will likely be determined by a

mix of local knowledge and trial and error.

The whole process of analysing a mortar, sourcing appropriate aggregates, mixing sample batches

and testing for suitability is a labour-intensive process and the benefits of accurately replicating an

historic mortar will need to be balanced against the financial costs involved.

If a building needs hot mix mortar for the reasons outlined in the section - Benefits - vapour

permeability, sacrificial nature of the mortar, etc but a full analysis isn’t necessary or viable then a

basic mortar for brick laying and pointing can be made using easily available aggregates and

quicklime. Whilst this approach may not deliver a perfect visible match it will produce a mortar that

performs correctly for the building and will almost certainly be better and more affordable than the

bulk bag alternatives. 

Purpose: For general building or repointing with traditional clamp bricks or soft stone eg limestone,

sandstone.

Masonry preparation: Dampen thoroughly before applying mortar

Mortar: Calbux 90 Quicklime / Builder’s sand / Sharp sand / GGBS (Pozzolan)

Ratio: Mixed by volume - 1 quicklime : 2 builder’s sand : 1 sharp sand : GGBS 10% of quicklime

volume

Mixing: Best results are achieved with a top loading forced action mixer although the mortar can be

mixed successfully by hand. A belle mixer is not suitable for mixing.

Purpose: For building or repointing with traditional clamp bricks or soft stone eg limestone or

sandstone below DPC (Floor) level or in exposed positions, flaunching on chimney stacks, bedding

ridge tiles etc

Masonry preparation: Dampen thoroughly before applying mortar

Mortar: Calbux 90 Quicklime /NHL 5/ Builder’s sand / Sharp sand /

Ratio: Mixed by volume - 1 quicklime : 1 NHL 5: 3 builder’s sand : 1 sharp sand

Mixing: Best results are achieved with a top loading forced action mixer although the mortar can be

mixed successfully by hand. A belle mixer is not suitable for mixing.