If you own an old house, sooner or later you’ll face cracked pointing, crumbling plaster, or loose render. The instinct is often to “patch it up” with a bag of modern cement mortar. Unfortunately, this is one of the most common—and most damaging—mistakes in historic repair work.
This article explains, in plain terms, how different mortar types behave, why they matter, and which ones suit traditional buildings best. It’s based on comparative data from laboratory studies and field testing across seven key mortars, applied to a range of common masonry substrates.
Here are the plaster types selected for this study:
Because mortars or plasters are always applied to some surfaces, we have also included the key properties of the following wall substrates:
For a clear, holistic understanding of how plaster behave, we have identified the most important mortar properties, grouped into three key categories:
Together, these categories provide a comprehensive picture of how different plasters perform under real-world conditions. Understanding them helps explain why some mortars allow historic buildings to stand for centuries, while others quietly contribute to their decay.
To better understand how plasters behaves structurally, these properties have been divided into two groups. Together, these two groups provide a complete picture of the plaster’s structural performance, both as an independent material and as part of the wall system.
These properties describe the mechanical behaviour of the plaster mass—how the body of the material responds to stress, movement, and deformation. They determine the plaster’s overall mechanical compatibility with the substrate, indicating whether it will move with or resist the underlying masonry. If the plaster is too stiff (for example, Portland cement or NHL 5 applied over soft bricks), internal stresses can lead to cracking or brick spalling. Conversely, if it is too soft (such as air lime on a moisture saturated wall), it may lose strength and fail prematurely.
In this group, we have looked at the following physical properties:
Here are the above structural values for each plaster:
Explanation, what these figures mean in practical application:
Compressive Strength [MPa] - Push forces: air lime is soft and compressible, allowing old walls to flex and settle without damage. NHLs are firmer and progressively stronger, giving better structural support but risking stress on weak masonry if too hard. Cement is very strong and rigid, often stronger than the bricks or stones it joins, so cracks appear at the interface. Gypsum is weak and crumbly, fine for interior finishes only. Roman pozzolanic plasters are strong like brick but remain cohesive and forgiving, adding strength without creating tension in the fabric.
Tensile Strength [MPa] - Pull forces: air lime has low tensile strength but tolerates slight movement without cracking. NHL mortars are stronger and bind well, though higher grades become more brittle. Cement has excellent tensile numbers on paper, yet once a crack forms it fails sharply. Gypsum has little pull-resistance and easily fractures. Roman plasters balance strength and flexibility — thin yet resilient, able to stretch microscopically with the wall instead of tearing.
Stiffness (Elastic Modulus) [GPa]: air lime is flexible and self-healing; it yields before the wall does. NHLs stiffen as hydraulicity rises, moving less and transmitting stress to the masonry. Cement is extremely stiff — it locks movement and cracks under strain. Gypsum is moderately stiff but fragile, unsuitable for external or structural use. Roman plasters achieve a rare balance: firm enough to reinforce surfaces, yet elastic enough to move safely with historic masonry.
Shrinkage [%]: air lime shrinks the most as it loses water slowly during carbonation, often developing fine cracks if not cured carefully. NHLs shrink less, and the higher the hydraulic strength, the smaller the shrinkage — NHL 5 being the most stable. Cement has the lowest overall shrinkage but contracts rapidly as it sets, sometimes causing sharp cracking if restrained. Gypsum shows very little shrinkage in normal indoor drying but softens or swells when rewetted. Roman pozzolanic plasters, including Rinzaffo and cocciopesto, remain extremely dimensionally stable; their pozzolans control drying movement, virtually eliminating shrinkage cracks.
The perceived hardness of a plaster is a combination of several mechanical traits — compressive strength, stiffness, surface strength, and adhesion to the substrate.
These properties describe the plaster’s bonding with the substrate—how well it bonds and moves in response to environmental changes. Together, these properties indicate how effectively the plaster and the fabric will perform as a unified system over time. If the plaster and substrate expand or contract at different rates, or if adhesion is weak, stresses can build up at the plaster-fabric interface, leading to separation, detachment or surface damages to the historic building fabric.
In this group, we have looked at the following properties:
Here are the above structural values for each plaster:
Explanation, what these figures mean in practical application:
Thermal Expansion [×10⁻⁶/K]: air lime expands and contracts very little with temperature, matching most old materials. NHLs expand slightly more but stay compatible. Cement has a much higher thermal movement, causing stress at junctions and cracking thin renders. Gypsum moves modestly but only indoors where temperature swings are small. Roman plasters expand at nearly the same rate as brick and stone, remaining dimensionally stable under daily and seasonal temperature changes.
Adhesion to Surface [MPa]: air lime bonds naturally by carbonation, sticking well to rough or porous surfaces but gently enough to detach safely later. NHLs adhere more firmly as their hydraulic components grow, sometimes too tightly for friable stone. Cement bonds extremely hard (up to 10 times stronger than line) — and once it grips, it can tear off the wall face if it fails. Gypsum sticks strongly to dry, smooth backgrounds but loses adhesion when damp. Roman plasters form a mineral bond that ties and strengthens the wall surface without over-binding it.
Material Surface Strength [MPa]: air lime surfaces stay soft and sacrificial, slowly weathering away to protect the masonry beneath — ideal for traditional, breathable wall systems. NHLs form harder, denser skins that resist erosion better, but may become brittle and develop surface cracking over time if overexposed. Portland cement creates very hard, dense faces that strongly resist abrasion and rain but are impermeable and unyielding, often leading to trapped moisture and detachment from softer masonry. Roman plasters, by contrast, develop a firm yet breathable mineral surface — tough enough to withstand weathering and salts, but still vapour-open and compatible with historic substrates. Gypsum finishes are smooth and fine but remain soft and fragile, easily damaged by impact or moisture.
To fully understand how plasters interact with moisture, we need to assess their behaviour with both water vapour and liquid water — to what extent different plasters allow walls to breathe and release moisture, and how they respond to rainwater or prolonged wet conditions.
There are two key parameters that describe how plasters handle liquid water: the A-value and the W-value. Both refer to liquid moisture uptake, but they capture this at different stages of the process.
Because the early suction is always much faster than the slow, steady flow that follows, A-values are typically about ten times higher than W-values.
Here are these values for each plaster type:
Explanation, what these figures mean in practical application:
Vapour Permeability (Breathability) [μ]: air lime is extremely breathable and lets trapped moisture escape easily. NHLs remain fairly vapour-open, though higher grades (NHL 5) slow the drying. Cement is dense and blocks vapour movement, trapping damp behind. Gypsum is vapour-open but unstable if moisture condenses. Roman plasters, including Rinzaffo, combine high vapour permeability with liquid resistance — they breathe like lime but stay drier.
Moisture Buffer Value (MBV) [g·m⁻²·%RH⁻¹]: air lime naturally regulates indoor humidity, absorbing and releasing moisture slowly like a sponge. NHLs buffer moderately, their smaller pores limiting capacity. Cement barely responds to humidity changes — once wet, it stays wet. Gypsum absorbs vapour quickly but saturates and softens. Roman plasters buffer about the same as lime (depending on the mix) but keep indoor humidity steady by allowing a constant vapour flow without retaining water inside.
Water Absorption (A-value) [kg·m⁻²·h⁻⁰·⁵]: air lime absorbs water rapidly through open pores, then dries fast when conditions change. NHLs take up less water as their structure becomes denser. Cement hardly soaks water at all but traps any moisture that enters through cracks. Gypsum absorbs eagerly then softens or dissolves. Roman plasters have the lowest liquid uptake — their micro-porous network blocks water but still allows vapour escape, keeping the wall dry.
Sustained Rain Resistance (W-value) [kg·m⁻²·h⁻⁰·⁵]: air lime cannot resist rain for long — it wets deeply under steady exposure. NHLs improve resistance but still darken and hold some moisture. Cement appears waterproof but cracks allow water behind the layer. Gypsum disintegrates when exposed to prolonged wetting. Roman plasters excel: they shed liquid water completely while remaining vapour-open, preventing rain from driving into the wall.
Chemical properties explore the two-way chemical interaction between plasters and the wall fabric.
On one hand, they describe how well the plaster resists chemical influences from the substrate — particularly salts commonly found in historic masonries, which are a leading cause of plaster disintegration. Damage can also result from chemicals leached into the wall fabric from previously applied (modern) incompatible materials.
On the other hand, the constituents of the plaster itself should not harm the underlying masonry. Many modern plasters contain additives that can react adversely with historic substrates. This overall relationship is referred to as the chemical compatibility of the plaster.
In this group, we have examined the following chemical properties:
Here are the above values for each plaster:
Explanation, what these figures mean in practical application:
Salt Resistance: air lime suffers badly from salt crystallisation, flaking and powdering over time. NHLs handle salts better but still degrade under strong cycling. Cement traps salts behind its dense face, causing masonry to spall. Gypsum dissolves entirely when salts form. Roman plasters, especially Rinzaffo, are virtually immune to salt damage. Salts remain in solution and migrate toward the wall–Rinzaffo interface, where they stay without crystallising. This keeps both the plaster and the masonry safe, preventing internal pressure build-up and surface flaking.
Soluble Salts Introduced: air lime is chemically clean and adds no salts to the wall. NHLs introduce tiny traces from their natural clay content, rising slightly with hydraulic strength. Cement carries high sulphates and alkalis that contaminate masonry; gypsum also introduces soluble sulphates. Roman plasters are made from washed sands and pure pozzolans, contributing no harmful salts and even drawing old ones outward.
Chemical Compatibility: air lime is chemically neutral and fully compatible with historic materials. NHLs are safe when weak and less so when strong — their high pH and reactive hydrates can stress soft bricks or limestone. Cement is chemically aggressive, promoting salt and alkali reactions. Gypsum reacts chemically with lime in damp environments, forming salts. Roman plasters remain inert and pH-stable, coexisting safely with any old masonry.
After looking at all these physical, moisture-related and chemical properties, one natural question arises: Which plaster is the best?
The truth is — there is no such thing as “the best plaster". That’s the wrong question to ask.
Every plaster behaves differently depending on where and how it is used. A mix that performs beautifully on a dry internal wall might fail within months on a damp exterior. The way a plaster handles moisture, salts, or movement depends entirely on the plaster's composition, the wall’s condition, the local climate and the substrate behind it.
The right question, therefore, is not “Which plaster is best?” but rather: Which plaster is best for a particular type of job?
This is the only question that makes sense in real conservation work — because the answer always depends on context.
To make sense of this, we looked at a range of typical renovation scenarios. Each of these scenarios reflects a real-life situation encountered in historic buildings — from a dry sitting room wall to a saturated basement or a storm-battered seaside façade. By analysing how each plaster type behaves under different conditions, we can move beyond simple claims of “better” or “worse,” and instead identify the right materials for the right job.
The renovation scenarios are:
Next, for each renovation scenario we translated all physical-chemical plaster properties - such as strength, stiffness, vapour permeability, capillary behaviour, salt resistance and chemical compatibility - into a simple suitability score from 1 to 10, as shown below:
These scores are not absolute, but relative, context dependent. A plaster that scores “10” in one scenario may score “3” in another — not because it is a bad product, but because its natural characteristics don’t suit specific applications.
When suitability scores are viewed as a whole, clear patterns emerge. No single plaster performs well everywhere, but each one excels in the right setting. The results show how the fundamental chemistry of a material governs its real-world behaviour — and why matching the plaster to the job is so essential.
Overall, the pattern is clear: the softer and more lime-rich a material is, the more breathable and forgiving it remains. The more hydraulic and dense it is, the stronger and more water-resistant it becomes.
Conservation success depends on choosing the right balance between strength, breathability, water and salt resistance for each wall and application.
Soft, lime-rich mortars allow a building to breathe and age gracefully. Hydraulic and pozzolanic plasters provide resilience where damp and salts persist. Dense, cementitious mixes may offer mechanical strength but often at the cost of compatibility and reversibility.
Here is a concise summary of each material, its key features, pros and cons, and its best suitability in conservation and repair.
Air Lime (CL90)
The gentlest and most traditional plaster. Aair lime is soft, highly breathable, and chemically pure — ideal for weak or historic masonry. It sets slowly by absorbing carbon dioxide from the air, creating a flexible and self-healing matrix. This allows walls to move, breathe, and dry freely, preserving fragile materials without stress.
Pros: extremely vapour-open; flexible; chemically clean; fully compatible with all traditional substrates.
Cons: mechanically weak; prone to shrinkage; absorbs liquid water easily; deteriorates quickly in damp or salty environments.
Best suited for: sheltered or interior plastering, limewashing, and low-stress repairs where breathability and flexibility matter more than strength.
NHL 2 (Natural Hydraulic Lime – Feebly Hydraulic)
A gentle lime containing small amounts of natural clay and silica, NHL 2 sets partly by hydration and partly by carbonation with air. It retains much of the softness and breathability of air lime but gains faster set and slightly higher strength.
Pros: excellent vapour permeability; low shrinkage; good adhesion; compatible with weak masonry; modest salt resistance.
Cons: slower setting in cold or humid conditions; weaker mechanical performance in exposed façades.
Best suited for: internal and external plasters on soft brick or stone, slightly damp or sheltered walls, and mixed lime repair work.
NHL 3.5 (Moderately Hydraulic Lime)
A balanced lime with moderate clay and silica content, NHL 3.5 offers good mechanical strength while remaining fairly vapour-open. It is often considered the “all-rounder” among limes — strong enough for external renders, yet flexible enough for most historic masonry.
Pros: balanced strength and permeability; good weather resistance; moderate salt tolerance; predictable setting.
Cons: less breathable and more rigid than softer limes; requires compatible substrates to prevent cracking.
Best suited for: external renders and structural mortars on medium-hard brick or stone, and for general conservation work in temperate, damp conditions.
NHL 5 (Eminently Hydraulic Lime)
The strongest of the natural hydraulic limes, NHL 5 contains a high proportion of reactive clay and silicates, producing a dense, fast-setting binder. It provides excellent mechanical performance and weather resistance but reduced breathability.
Pros: durable; low shrinkage; good frost resistance; suitable for high-load or exposed situations.
Cons: less vapour-open; more brittle; may over-strengthen soft masonry; modest salt resistance.
Best suited for: strong, stable substrates such as hard brick, sandstone, or natural cement façades; marine or exposed structures where moderate breathability is acceptable.
Portland Cement
An industrial, high-temperature burnt cement with very high early strength and low porosity. While useful in modern applications, it is chemically and physically incompatible with historic walls. Its rigidity and lack of breathability trap moisture, leading to spalling and stone decay.
Pros: strong; hard; quick-setting; widely available.
Cons: too dense and inflexible; blocks vapour; traps salts and moisture; chemically aggressive and incompatible with lime-based masonry.
Best suited for: modern concrete buildings or non-historic substrates. Not suitable for traditional or heritage applications.
Gypsum Finish Plaster
A fine, fast-setting interior plaster made from gypsum. It gives a smooth, decorative finish and is widely used in modern construction, but it is not appropriate for traditional masonry. Gypsum is highly sensitive to moisture, dissolves when damp, and reacts chemically with lime and salts. In heritage buildings it can trigger efflorescence, softening or surface detachment.
Pros: smooth surface; fast setting; easy to apply on modern, dry substrates.
Cons: low durability; dissolves in damp conditions; reacts with lime; introduces sulphate salts; chemically incompatible with historic masonry.
Best suited for: modern, dry internal walls on cement backing or gypsum board. Not for heritage or lime-based masonry of any kind.
Pozzolanic Lime Mixes (Roman-type)
A lime binder blended with natural volcanic sands and ashes (siliceous pozzolans) that react with lime to form stable hydraulic compounds. This reaction begins soon after application, giving the plaster an early set and strong bond even in damp conditions. The result is a cohesive, breathable, and salt-resistant material that combines the flexibility of lime with the durability of a hydraulic mortar.
Pros: vapour-open but water-resistant; excellent salt tolerance; early hardening with good mechanical strength; long life span; compatible with all heritage substrates.
Cons: slightly denser than air lime; requires careful mixing to maintain consistent reactivity; typically more expensive than simple lime mortars.
Best suited for: damp or saline masonry, conservation renders and base coats where controlled drying, salt protection, and long-term chemical stability are essential.
Cocciopesto-Lime Mixes (Roman-type)
A traditional lime and crushed brick (terracotta) mix, cocciopesto creates a micro-porous, salt-resistant render that buffers moisture effectively. It behaves like a breathable, thermally stable, and flexible lime plaster, often paired with Rinzaffo MGN for maximum protection.
Pros: extremely breathable; excellent moisture-buffering and indoor humidity regulation; compatibility; good salt and frost resistance.
Cons: strength depends on mix.
Best suited for: as a second coat or finish in damp and humid environments, cellars or coastal façades — especially when applied over a Rinzaffo MGN salt-resistant base coat.
Rinzaffo MGN (Roman-type)
A pozzolanic–hydraulic Roman plaster specifically designed for damp, salt-laden, or permanently humid masonry. Its fine microporous matrix completely blocks liquid water while remaining vapour-open, allowing moisture to escape safely as vapour. This keeps the wall dry and prevents destructive salt crystallisation within the masonry. Once cured, Rinzaffo forms a cohesive, mineral skin that reinforces the surface rather than sealing it.
Pros: completely salt-resistant; blocks liquid water; highly vapour-open; stabilises friable surfaces; fully reversible and chemically neutral.
Cons: denser and stronger than soft limes; unnecessary for dry interiors
Best suited for: any historic wall exposed to moisture — including saturated, salt-contaminated masonry, basements, cellars, and marine or coastal conditions.
Choosing the right plaster is therefore not about finding the strongest, newest or hardest material — it’s about finding the most compatible materials for the wall.
A good plaster should work with the building, not against it: letting moisture move, accommodate movement, manage the salts and keep history intact.
Here are some other related pages that you might want to read to broaden your knowledge in this field.
Here are the some recommended materials / products that can help solving or dealing with some of the problems discussed on this page.
