The Mix That Makes or Breaks It — Mortar in Historic Facade Repointing part 2/3

Part of the series: Joints, Mortar and the Art of Repointing Historic Facades

Before we talk about joint styles — and there are more than most people realise — we need to talk about what goes into them.

Mortar.

It sounds simple. Sand and something to bind it. But the choice of binder, the ratio of the
mix, the grain size of the sand, and the hardness of the final product relative to the brick
or stone around it — these decisions determine whether a repointing job protects a
historic facade for the next fifty years or begins destroying it within ten.

This is not an exaggeration. Wrong mortar on a historic building is one of the most
common and most costly mistakes in the sector. And it happens every day, on buildings
that deserve better.

This is the single most important principle in historic masonry repointing, and it is
violated more often than it should be.

A mortar that is harder than the brick or stone it sits between will not flex with the
natural movement of the wall. As the building expands and contracts with temperature
and moisture changes — as every building does, every day — the stress goes
somewhere. If the mortar is harder than the masonry unit, the stress goes into the brick
face. It spalls. It cracks. It deteriorates.

If the mortar is appropriately softer, the stress goes into the mortar joint — which is
exactly where it should go. The mortar weathers before the brick. That is its job. It is the
sacrificial element of the wall, designed to be renewed periodically while the
irreplaceable historic masonry behind it endures.

Cement mortar on historic brick is the most common violation of this principle. Portland
cement produces a mortar that is, in most cases, significantly harder than the soft, limeburned
brick used in 18th and 19th century construction. The consequences are visible
on buildings across Europe — spalling brick faces, cracking at the joint edges, moisture
trapped behind an impermeable mortar film that has nowhere to go.

Before Portland cement became widely available in the late 19th century, virtually all
mortar was lime-based. Lime had been used in construction for thousands of years, and
the buildings that survive from those centuries are, in many cases, a testament to how
well it works when applied correctly.

Air lime (calcium lime, CL) is the simplest form — produced by burning limestone and
slaking the result with water. It hardens slowly through carbonation — absorbing CO₂
from the air over months and years. It remains relatively soft and flexible, and is vapourpermeable
— it allows moisture to move through the wall rather than trapping it.

For many historic buildings, particularly those built before the mid-19th century, air lime
is the most historically appropriate binder available. Its softness makes it compatible
with older, softer brick. Its permeability makes it compatible with the moisture
management logic of traditional wall construction.

The limitation of air lime is its slow strength development and its vulnerability to wet
conditions during curing. It is not a product that can be applied in cold or wet weather
and forgotten about. It requires attention, appropriate curing conditions, and patience.

Natural Hydraulic Lime (NHL) is produced from limestone that contains naturally
occurring clay minerals. These minerals give the lime hydraulic properties — the ability
to set in contact with water, rather than purely through carbonation.

NHL is classified by strength: NHL 2, NHL 3.5, and NHL 5, with higher numbers
indicating greater hydraulic content and faster, harder set. NHL 2 is relatively soft and
slow-setting — appropriate for sheltered locations and softer masonry. NHL 5
approaches the behaviour of a weak cement in some respects and should be used with
caution on historic buildings.

For most historic facade repointing in northern Europe, NHL 3.5 is a reasonable default
— strong enough to be practical, soft enough to be compatible with most historic brick,
and permeable enough to allow the wall to breathe.

The key advantage of NHL over air lime is workability and speed. It sets faster, is less
sensitive to weather conditions during curing, and produces a mortar that is more
consistent and easier to control in practice.

Trass is a volcanic rock — a natural pozzolan — quarried primarily in the Eifel region of
Germany. When ground to a powder and combined with lime, it reacts with the calcium
hydroxide to produce a hydraulic binder of considerable durability.

Trass lime has been used in historic construction for centuries — the Romans used
pozzolanic additions to lime in their most demanding applications. Its hydraulic
properties make it particularly valuable in exposed or damp locations where pure air lime
would struggle.

For historic facade repointing, trass lime offers a combination of properties that makes
it attractive: good compatibility with historic masonry, reasonable strength
development, good vapour permeability, and a long track record of performance on
historic buildings.

It is not always easy to source, and its properties vary depending on the source and
processing of the trass. But for craftspeople working regularly on historic buildings, it is
worth understanding and worth having access to.

Portland cement is not always wrong in a historic context. For certain applications —
below ground level, in very exposed locations, in structural repairs where speed of
strength development is critical — a cement addition or a weak cement mix may be
appropriate.

What is not appropriate is standard cement mortar applied to historic brick or stone
facades as a direct substitute for lime-based mortar. The hardness, the impermeability,
and the chemical incompatibility with historic masonry make it a poor choice that
causes damage over time.

If cement must be used — and there are situations where it is the pragmatic choice —
the proportion should be minimal, the overall mix should remain softer than the
masonry, and the implications for moisture management should be understood before
application begins.

The sand in a mortar mix is not just filler. Its grain size, shape, and mineralogy affect the
final strength, colour, and texture of the joint. For historic repointing, matching the
aggregate of the original mortar is often as important as matching the binder.

A mortar with fine, rounded sand will look and behave differently from one with coarser,
angular aggregate — even if the binder proportions are identical. On a historic facade
where the original pointing has a specific visual texture, getting the sand right is part of
getting the result right.

Historic mortars were not always simply lime and sand. Craftspeople across Europe
added other materials to improve specific properties, and understanding these additions
helps explain why some very old mortars have performed remarkably well over
centuries.

Ash — wood ash or coal ash was sometimes added to lime mortars as a pozzolanic
material. Like trass, ash contains silica compounds that react with calcium hydroxide to
produce additional binding compounds. The result is a mortar with improved hydraulic
properties and better resistance to moisture — useful in exposed locations or at the
base of walls where rising damp was a concern.

Shell lime — in coastal regions, lime was often burned from seashells rather than
quarried limestone. The resulting lime has slightly different properties from rock lime,
and mortars made with it have a specific character that is worth understanding when
working on buildings in those areas.

Hair and fibre — animal hair, straw, and other fibrous materials were added to lime
mortars and plasters to control cracking during drying. The fibres act as a form of
reinforcement, distributing shrinkage stress across the mix. Hair mortar is particularly
associated with lime plasterwork but appears in some pointing mortars as well.

Brick dust — crushed or ground brick, like ash and trass, is pozzolanic. It was widely
used in Roman mortars and continued to appear in historic construction through the
medieval period and beyond. It also contributes colour — a warm pink or red tone that is
characteristic of certain historic mortars.

These additions were not random. They represent accumulated practical knowledge
about how to make a mortar perform better in specific conditions. When analysing an
original historic mortar before repointing, finding evidence of these additions tells you
something important about what the original builders were trying to achieve — and
should inform what you use to replace it.

This point deserves to be stated clearly, because it is frequently ignored in practice.

Modern mortar admixtures — plasticizers, accelerators, retarders, waterproofing
agents, air-entraining compounds — are routinely added to site-mixed mortars for
convenience. They make mortar easier to work with, extend its open time, speed up its
set, or improve its flow. For new construction with modern materials, some of these
have their place.

On historic facades, they do not.

Plasticizers — the most commonly added admixture — work by introducing air into the
mortar mix, making it more workable without adding water. The resulting mortar is less
dense, less strong, and often less vapour-permeable than one mixed without them. On a
historic wall that depends on the breathability of its mortar joints to manage moisture, a
plasticized mortar can disrupt the moisture balance of the entire wall assembly.

Waterproofing admixtures are perhaps the most counterproductive of all. A mortar that
repels water from the outside also traps moisture inside the wall. On a historic building
where moisture management depends on the wall being able to dry out through the
joints, a waterproofed mortar creates exactly the conditions it claims to prevent.

The rule is simple: lime mortars for historic masonry do not need admixtures. A
well-proportioned lime mortar, mixed properly with appropriate water content, is
workable, durable, and compatible with historic masonry without any additions. If the
mix seems difficult to work with, the answer is to adjust the proportions or the water
content — not to add a plasticizer

On protected historic buildings, mortar colour is rarely a free choice. Heritage
authorities typically require that replacement mortar matches the existing historic
mortar as closely as possible — in colour, texture, and finish.

For unprotected historic buildings, the choice is wider, but the principle is the same: the
mortar should complement the masonry, not compete with it. A mortar that is too light
draws the eye to the joints rather than the brick. A mortar that is too dark does the
same. The goal, on most historic facades, is a joint that reads as part of the wall rather
than as a feature in its own right.

Pigments can be added to lime mortars to adjust colour — iron oxides are the most
common and the most stable. But the starting point should always be a careful analysis
of what is already there.

The last thing you add is the water. Always use clean tap water! NEVER use rainwater, well water, or water from a river or pond! It sounds logical, but it does happen. Its to soft, and contains too many impurities and affects the quality of the mortar very negatively!

Next in this series: the joint styles themselves — from the most common to the
most rarely seen, and what each one demands from the craftsperson who
executes it.

RestoreFacade Guild — for the craftspeople who understand that what goes between
the bricks matters as much as the bricks themselves.