Hardness

In heritage conservation, few words create more confusion than hardness. For many people, the word instantly recalls cracked bricks, spalled stone or flaking plaster — the legacy of cement repairs carried out with the best intentions and the worst results. Because of this, hardness has almost become a dirty word in conservation; something that should be avoided entirely as all hard mortars are dangerous by default and softness alone guarantees safety.

Yet hardness itself isn’t the problem. The real issue is misplaced hardness — the wrong strength, in the wrong place, for the wrong wall. Hardness, properly understood, represents a set of mechanical and physical behaviours that give masonry its ability to carry load, resist deformation, and stay intact. Without a degree of firmness, a wall would lose its internal bond and begin to behave like a loose stack of bricks.

The challenge, then, is not to eliminate hardness but to control it. A mortar too hard can crush or trap its masonry, while a mortar too soft can disintegrate and fail to support it. The goal in conservation is not softness for its own sake, but balance — finding the middle ground where strength, flexibility and breathability coexist. Understanding hardness is essential, because it lies at the heart of how walls move, behave and distribute their load, living through time.

Here are two very important concepts about hardness.

Hardness in mortars is not a single property but a mix of several measurable parameters that together describe how a material performs under stress and movement — both the body of the material and the wall-plaster interface. So these parameters are grouped into two categories: physical properties, which describe how the mortar behaves as a body, and bonding properties which describe how it interacts with the wall it binds to.

Physical Properties – The body of the material

Physical properties define how the mortar behaves as a bulk material under load, heat, and moisture. They govern strength, flexibility and breathability — the main traits that decide whether a mortar will move with the wall or fight against it. Here are the most important ones:

• Stiffness (Elastic modulus): this describes a material's elasticity, how easily the mortar deforms under load. A stiff mortar transmits stress directly into the masonry; a flexible one absorbs small movements safely. In old buildings, this flexibility metric probably the most important physical metric, more important than compressive strength.
• Compressive Strength (CS) - Push forces¹Jackson, Marie & Kosso, Cynthia & Marra, Fabrizio & Hay, Richard. (2006). Geological Basis of Vitruvius' Empirical Observations of Material Characteristics of Rock Utilized in Roman Masonry. Proc. Second Int. Congress of Construction History. 2. 1685-702. ²Fernandes, Francisco & Lourenco, Paulo. (2007). Evaluation of the Compressive Strength of Ancient Clay Bricks Using Microdrilling. Journal of Materials in Civil Engineering - J MATER CIVIL ENG. 19. 10.1061 ³Abed Soleymani, Mohammad Amir Najafgholipour, Ali Johari (2022). An experimental study on the mechanical properties of solid clay brick masonry with traditional mortars, Journal of Building Engineering, Volume 58 ⁴Goodman, R E, (1989). Introduction to Rock Mechanics, New York: John Wiley ⁵Suciu, Ovidiu & Cruciat, Radu & Ghindea, Cristian. (2014). Experimental Case Studies on Clay Fired Bricks Compressive Strength. Key Engineering Materials. 601. 10.4028: this measures how much weight a mortar can take before it crushes. It gives a sense of how much load a wall can carry, but it’s only one part of the picture. A mortar can have an ideal CS value on paper yet still be incompatible if it’s too stiff or dense for the wall it’s in.
• Flexural and tensile strength - Pull and Bending forces: these measure how the mortar behaves when it’s pulled or bent, not just when it’s squashed. These can be important in many applications.

Together, these physical characteristics determine whether the mortar will work with the wall or against it. A good mortar matches the wall’s properties without dominating or lagging behind.

The figures show how different materials behave in terms of strength, stiffness and breathability — some of the key traits that together define hardness.

At the soft end of the scale are air limes and weak natural hydraulic limes (NHL 2–3.5). These materials are gentle and flexible, with low strength and stiffness. They deform slightly under load and allow the wall to move without cracking. Their high vapour permeability makes them excellent for breathing, but too soft for structural work where greater cohesion is needed.

Moving up the scale, Roman-style pozzolanic limes and cocciopesto limes occupy the ideal middle ground. They are strong enough to stabilise masonry yet still softer and more flexible than most historic bricks or stones. Their stiffness is close to that of traditional masonry so stress is shared rather than concentrated. This is what makes them structurally compatible — firm enough to hold, soft enough to live.

By contrast, Portland cement jumps sharply upward in both strength and stiffness. It is five to ten times harder and less deformable than lime-based materials. This rigidity blocks the wall’s natural movement, leading to cracking at joints and loss of breathability. Its low vapour permeability traps moisture and salts inside the wall, accelerating decay.

At the far extreme lies steel — incredibly strong, almost infinitely stiff, and completely vapour-closed. While essential in modern engineering, it has no place within the breathing, flexible world of lime and stone.

Bonding Properties – The plaster-masonry interface

Bonding properties describe how well the mortar connects to the wall. These surface-level interactions control how well the wall acts as a unified structure, rather than as two separate materials struggling against each other, and how it resists cracking, debonding and weathering.

• Thermal Expansion [×10⁻⁶/K]: describes how much a material expands or contracts when its temperature changes. If the mortar expands at a very different rate from the brick or stone, internal stresses form leading to cracks along the the mortar bed.
• Adhesion to Surface [MPa]: describes how strongly a material sticks or bonds to the surface beneath it. Too weak, and joints open or flake away; too strong, and the mortar can pull the face off soft masonry. The aim is a secure but forgiving bond that allows for slight movement.
• Surface Strength [MPa]: describes the hardness or durability of the material’s outer layer, the part exposed to weather. It needs enough strength to resist erosion from wind and rain, yet remain open enough for moisture to escape.

Every plaster grips the wall to some degree — that grip, known as adhesion to surface, must be less than what the wall’s surface can resist: its surface layer strength. If the plaster bonds too tightly, it can tear away the wall face when the structure moves or the salts expand. If it bonds too weakly, it will not stabilise friable or crumbling surfaces that need support.

Let's look at some real-world examples:

In summary: hardness is not a single metric, but a multitude of physical and bonding characteristics. Each metric plays a role, and the harmony between them determines whether the wall remains stable, dry and durable. No single number — not even compressive strength — can alone capture the true meaning of hardness in conservation.

Conservation practice offers a simple guideline for achieving this balance.

As a rule of thumb: the mortar’s mechanical strength — its compressive strength and stiffness — should be around 60–80% of the masonry’s strength.

Within this range, the mortar becomes the sacrificial “fuse” of the system, the one that breaks first if stress builds up, protecting the bricks or stones from damage. A mortar stronger than its masonry transfers stress into the stones or bricks and breaks them; a too weak one fails prematurely and leaves the wall unsupported. Staying within that 60–80 % range achieves the middle ground where the wall remains stable, flexible and durable.

Of course, this is not a fixed formula but just guidance. Every building, stone or brick type has its own character. The ratio is a guide to proportion, not a mathematical rule. It reminds us that mortar and masonry must match each other, sharing load and movement in harmony rather than in opposition.

The idea that mortars should match the walls is not new. The Romans understood this principle instinctively and applied it with remarkable precision⁶Vitruvius (1999), Ten Books on Architecture De Architectura. Rowland, I D. and Howe, T N, (editors), Cambridge: Cambridge University Press.. They recognised that no wall material exists in isolation — mortar and masonry form a single body, one providing confinement and the other cohesion. Their mastery of lime and pozzolans was not merely about achieving strength but about achieving balance. They adjusted the hardness of their mortars to suit the stones, bricks and environments they worked with, long before material science could measure such things.

Early Roman builders used lime mortars that were soft, porous, and rich in earthy materials. These suited the friable tuffs and weak volcanic stones of central Italy, which absorbed large amounts of water and offered little inherent strength. The mortars were gentle and accommodating, acting as the binding skin for an equally soft core. But as construction techniques advanced and the need for durability grew, Roman craftsmen began to refine their recipes, adding pozzolanic ash and crushed ceramics to create mortars that were still lime-based but significantly stronger and more cohesive.

This evolution wasn’t random — it followed a clear understanding that hardness must be relative to the wall fabric. Stronger stones demanded stronger mortars, while softer tuffs still required softer lime-rich mixes. Over the centuries, Roman builders achieved an extraordinary material harmony, pairing each stone type with the right mortar hardness.

The historic evolution of Roman mortars⁷Jackson, Marie & Deocampo, Daniel & Marra, Fabrizio & Scheetz, Barry. (2010). Mid-Pleistocene volcanic ash in ancient Roman concretes. Geoarchaeology. 25. 36 - 74. 10.1002/gea.20295. clearly reflects this progression:

• Between 210–80 BC, mortars were very friable and earthy, typically grey and weak in cohesion. Buildings such as the Temple of Castor and Pollux were constructed with soft, porous tuffs and low-strength mortars. These tuffs could absorb 15–23 % of their weight in water and lose up to half of their dry compressive strength when wet. Unsurprisingly, many structures from this period have survived only as ruins — not because of design faults, but because both stones and mortars were equally fragile.
• By 80–40 BC, Roman mortars had become denser and less friable. Their darker grey colour reflected a reduction in earthy components and an increase in reactive volcanic material.
• From 40–20 BC, the technology improved dramatically. Mortars became cohesive and durable, incorporating finely crushed tuffs and the first red pozzolans — innovations introduced during the time of Julius Caesar. This marked the turning point from weak, air-lime mortars to early pozzolanic limes that could withstand higher stress and limited moisture exposure.
• Between 20 BC and 40 AD, Roman builders produced mortars of excellent quality, non-friable and firm, often red or reddish-brown in colour. The stones, too, had improved: builders increasingly used denser materials with moderate compressive strength (50–100 MPa). This was the start of the Pax Romana, a period of stability and prosperity that encouraged large-scale stone construction — a golden age for both architecture and material science.
• From 40–90 AD, Roman mortars became firmer and more consistent, grey to reddish-grey in tone. During this era, monumental works like the Colosseum were built. After the Great Fire of Rome in 64 AD destroyed much of the city, Emperor Nero’s building reforms led to new fireproof materials. Wood was largely replaced by stone, and the fire-prone travertine stone gave way to more stable and heat-resistant materials. Mortars in this period were engineered to match these denser, more resilient stones, ensuring the masonry behaved as a single body under both heat and load.
• Finally, between 90–180 AD, Roman mortar technology reached its peak. The mixes were rock-hard, durable, and finely balanced, completely free from earthy impurities. Builders skillfully adjusted hardness across the structure: foundations and heavy vaults contained denser, stronger mortars enriched with white or red pozzolans, while lighter vaults incorporated 20–30 % pumice for reduced weight and flexibility. Roman masons had achieved a practical mastery of material compatibility — mortars that were never harder than the stones, but perfectly adapted to each element’s role.

The results speak for themselves. Many Roman walls, arches and domes have survived nearly two millennia in harsh climates and active seismic zones. Their endurance comes not from extreme strength but from perfect proportion — mortar and masonry working together as one system.

The Romans achieved what conservation professionals strive for today: a structure where hardness is relative, not absolute, and where every material is tuned to the one beside it. They didn’t measure compressive strength; they observed behaviour. They understood that longevity lies in balance — in mortars strong enough to hold yet soft enough to yield — and they built accordingly. This understanding of balance and relative hardness underpinned the success of their buildings for centuries.

Modern builders, however, work in a different world. The rise of reinforced concrete and steel changed how we think about structure itself. We no longer build by feel and proportion, but by calculation and precision. Materials are engineered for strength, not flexibility; for certainty, not adaptability. And while this approach revolutionised modern architecture, it also created a deep misunderstanding when applied to historic masonry.

Modern and historic walls may both carry load, but they do it in entirely different ways. The difference lies not only in materials but in philosophy — one built on rigidity and control, the other on flexibility and balance.

Modern Walls — Strength Through Rigidity

Modern buildings made of reinforced concrete or steel frames behave like rigid skeletons. Their strength comes from the precision of their design: each beam, column, and slab is calculated to carry a defined load along a fixed path. The loads move vertically from the roof through the columns down to the foundation — a system engineers call point loading.

In a point-loaded structure, the material is so stiff that it resists bending or distortion. The weight therefore concentrates at specific points or lines, creating narrow zones of stress. This system is efficient and predictable — ideal for engineered materials that can tolerate enormous compression and tension without deformation. But it is also unforgiving. Because there is no flexibility built into the system, any movement beyond its design limits — settlement, thermal expansion, vibration — results in cracks or structural failure.

To manage this rigidity, modern buildings rely on precise joints and expansion breaks that allow them to move in controlled ways. The structure itself, however, remains fundamentally brittle: it is strong by design, but no adaptability.

Old Walls — Strength Through Flexibility

Historic walls built of lime and masonry follow a very different logic. They behave less like a skeleton and more like a woven fabric made of many small, cooperating parts. Instead of transmitting forces in straight lines, old walls share loads through countless micro-adjustments within their thickness.

When weight presses down from above, the lime mortar compresses slightly, allowing the masonry to form tiny arching actions that spread the load sideways as well as downward. The forces don’t travel through a single path but disperse through the wall in all directions. This is known as distributed loading — a natural self-balancing mechanism that allows old masonry to absorb movement, settlement, and even vibration without cracking apart.

In this system, lime mortar isn’t just glue — it’s the cushion that allows the wall to adapt. It compresses under pressure, recovers as loads shift, and redistributes stress through the masonry around it. The result is a structure that may not be mathematically perfect but is remarkably resilient. It tolerates small imperfections, absorbs energy, and heals itself over time through re-carbonation.

This is why so many old walls have survived centuries of movement, frost, and repair. Their flexibility is their strength. They are stable not because they resist movement, but because they accommodate it.

Two Systems, Two Different Approaches

The modern and the historic systems speak different structural languages. Modern walls depend on precision and stiffness, while old walls depend on adjustment and continuity. One concentrates load — the other shares it. One needs control — the other thrives on balance.

This distinction between point loading and distributed loading is fundamental to understanding why so many modern repairs fail when applied to historic fabric. Materials like cement, concrete, or steel belong to the point-loaded world; they are designed for precision and rigidity. Old masonry belongs to the distributed world; it survives through flexibility and shared compression.

When a modern material is introduced into an old wall, these two worlds collide. The inserted rigid modern materials block the natural flow of stress through the masonry. Loads that once spread gradually across many stones now become trapped, concentrating along the line where the old and new materials meet. Cracks start at these junctions, radiating outward like fault lines between two incompatible systems. Moisture follows the same path, trapped where the dense, impermeable patch interrupts the wall’s ability to breathe.

Many of the cracks and distortions we see today in historic buildings — often blamed on settlement, vibration, or “age” — actually trace back to these interventions. The real breakdown of structure often begins after the introduction of modern materials. Cement pointing, concrete banding, rigid steel ties or impermeable plinths change how the wall behaves, locking movement that was once harmless. Instead of slowing decay, they accelerate it.

Over time, this mismatch sets up a destructive cycle. As the wall moves naturally with temperature, moisture or foundation shifts, the hard elements resist. Stress accumulates, causing the surrounding lime and stone to fracture. Water then penetrates the new cracks, becomes trapped by the impermeable repairs, and begins to erode the softer adjacent material. What was once a slow, natural process of weathering turns into rapid mechanical decay — triggered by materials meant to prevent it.

The irony is striking: the attempt to “strengthen” the wall creates the very weaknesses it was supposed to cure. The result is a structure that appears stronger but behaves more weakly — a contradiction born not of neglect, but of misunderstanding the nature of historic masonry.

To repair an old wall successfully, one must understand and respect how it carries its load. The goal is not to impose rigidity but to restore cohesion and flexibility — to allow the wall once again to act as a single, flexible body. Yet many consolidation mistakes arise from failing to understand how masonries actually work. They rely on gentle movement and shared loads between many small parts, not on the kind of stiffness modern materials impose.

Modern repair methods tend to swing between two extremes — too strong and too soft — each producing its own kind of failure.

1. The “Too Strong” Approach — The Structural Engineer’s Reflex

The first and most common mistake is the over-strengthening of historic walls. Modern structural engineers, trained on reinforced concrete and steel structures, instinctively equate safety with strength. When faced with cracked or leaning masonry, their reflex is to “reinforce” it — to make it harder, stiffer and stronger.

The materials chosen for such interventions — cement, concrete bands, epoxy resins, steel beams and tie rods — all share one trait: they are many times stronger and stiffer than the lime-based mortars and soft bricks of the original fabric. They stop the wall from moving and breathing. Instead of stabilising it, they block its natural ability to distribute stress.

For case in point, here are some figures:

The contrast is striking.

• Traditional materials such as lime mortars, bricks and stones sit within a narrow and compatible range of strength and stiffness. Their compressive strength typically lies between 5 and 20 MPa — enough to carry load, but still soft enough to compress and adjust. Their stiffness is relatively low and forgiving (good flexibility), allowing small amounts of movement without damage. In practical terms, a lime mortar deforms several times more under load than concrete or steel would, which is exactly why old walls can move and settle without cracking.
• Modern structural materials, however, are in a completely different league. Portland cement mortars and reinforced concrete can be two to six times stronger and about ten times stiffer (less flexible) than the historic masonry around them. Their elastic moduli rise into the tens of gigapascals, meaning they barely deform under load. Steel goes even further, being hundreds of times stiffer than any lime-based material. When modern materials are inserted into old walls, they simply cannot share movement or stress — they dominate and overstress the historic fabric.
  

  The physical mismatch is further worsened by other properties. Cement and concrete are dense and almost vapour-tight (non-breathable), preventing moisture from escaping and trapping dampness in the wall. Their chemical composition introduces salts that can react with the original lime or stone, while steel elements expand as they rust, physically bursting the masonry that surrounds them.

  So while these materials appear strong on paper, their strength comes at the cost of compatibility. They might look “strong” on paper, in old masonries that strength becomes a liability. In a living wall that depends on flexibility, breathability and shared compression, such materials behave like rigid islands. They block the natural movement of the structure and concentrate stress at their edges — causing the very cracks and failures they were meant to prevent. The harder, denser and more impermeable a material is, the more violently it conflicts with the wall’s natural movement and moisture balance.

At first, the intervention may look solid, even reassuring. But over time, the rigidity of the inserted material begins to work against the wall. Seasonal expansion, small movements, or thermal shifts concentrate stress along the joint between the new and the old. Cracks form precisely where the systems meet, marking the boundary between incompatible materials. The harder component always wins — until it causes the masonry around it to lose.

This is why over-strength repairs often accelerate decay rather than prevent it. They ignore the wall’s original logic: flexibility, friction, and shared load paths.

Old masonry doesn’t fail because it’s weak; it fails when it loses its ability for its blocks to work together.

2. The “Too Soft” Approach — The Cautious Conservator

At the other extreme is the overly cautious repair — the belief that the safest mortar is always the weakest. Some conservators, reacting against the harms of cement, specify mortars so soft that they have little structural function. These lime-rich mixes are vapour-open and kind to historic materials, but when the masonry itself has lost cohesion or must bear load, such mortars are too soft to reconnect or stabilise it.

A very weak mortar cannot transmit load effectively between bricks or stones. It may shrink, crumble or wash out under prolonged wetting and frost. The wall remains essentially unbound — cosmetically repaired but mechanically fragile. What appears as a safe, gentle intervention actually leaves the building vulnerable to continued movement and water penetration.

Soft mortars are essential in repointing or plastering where the goal is sacrificial protection, but they cannot replace structural cohesion when the wall’s core has failed. When the problem is structural, the remedy must be structural too — not aggressive, but adequately strong to re-establish continuity within the wall.

Both extremes — the engineer’s reflex and the cautious conservator’s — fail for the same reason: they think in absolutes. One side believes strength solves everything; the other believes weakness avoids all harm. But old masonry thrives in neither rigidity nor weakness.

Successful consolidation means finding the middle ground, where the repair materials restore friction, cohesion and continuity without overpowering the masonry. The mortar must be strong enough to reconnect, yet soft and porous enough to remain a true partner to the wall.

This is where structural lime mortars, such as the Roman-style pozzolanic mixes, come into their own. They bridge the gap between the extremes — offering moderate strength with flexibility, durability with breathability. These materials do not fight the wall’s natural behaviour; they rejoin it, bringing back the balance that made the structure endure for centuries.

3. The Right Kind of Strength — The Roman Approach

Structural Roman lime mortars such as Betoncino MGN or Modena M5 MGN were developed precisely to occupy the middle ground between weakness and over-strength. They behave like lime but carry enough internal strength to reconnect a disjointed wall. Their pozzolanic chemistry forms minute silicate bonds that give firmness and early set while keeping the open pore structure and flexibility of traditional lime.

A structural lime mortar doesn’t try to make the wall hard; it makes it coherent again. Within the masonry it fills voids, binds units, and restores the friction that allows loads to pass safely through the fabric. Because it remains slightly elastic, it compresses and recovers with the wall instead of forcing it apart. It also stays vapour-open, allowing moisture to move through rather than trapping it behind impermeable skins.

Mechanically, these mortars work through a balance of three key traits:

• Cohesion and internal bonding: the ability to stay intact under compression and mild tension, maintaining continuity inside the joint.
• Elasticity and strain tolerance: a capacity to deform slightly under stress, absorbing movement without cracking.
• Compatibility of stiffness: close to that of common brick and stone, so stress is shared and distributed rather than concentrated.

In practice, a pozzolanic lime of around 15 N/mm² compressive strength provides just that equilibrium: strong enough to stabilise a weakened wall, yet soft and open enough to remain in tune with historic materials. Unlike cement, it doesn’t create rigid islands; unlike pure air lime, it doesn’t lose cohesion under load.

Proof Written in Stone

The best evidence for this balanced strength lies not in modern testing, but in the walls that still stand. Roman mortars built on exactly the same pozzolanic principles have survived nearly two millennia of weather, earthquakes and neglect. Their resilience has been studied extensively in the last few decades and provides tangible proof of the system’s long-term success.

• The Pantheon in Rome (built c. 120 AD) still holds the world’s largest unreinforced Roman concrete dome — a perfect demonstration of pozzolanic lime’s strength and adaptability. Its 43-metre span has stood for nearly 1,900 years without reinforcement, its mortar remaining intact and flexible enough to accommodate minute movements through centuries of earthquakes.
• The Colosseum (c. 80 AD) stands on the same principles. Despite centuries of ground motion and the loss of many stone blocks, its core mortars — made of lime and volcanic ash — continue to bind the remaining fabric together. Analytical studies show that the pozzolanic matrix in these mortars has developed stable calcium-aluminosilicate crystals, still active and resilient after almost two millennia.
• The harbour concrete at Pozzuoli and Portus Cosanus: is perhaps the most remarkable example of all. Submerged for two thousand years, it remains cohesive and strong. Recent research revealed that these pozzolanic mortars continue to strengthen slowly over time through self-healing reactions between lime and seawater — a natural, long-term durability unmatched by modern Portland cement.

These examples all demonstrate the same principle: moderate strength with flexibility outlasts rigidity every time. The Roman system worked not because it was hard, but because it was right.

Modern pozzolanic lime mortars such as Betoncino MGN and Modena M5 MGN follow that same principle — a carefully tuned mineral system that reintroduces strength in harmony with breathability and movement. Their heritage is not theoretical; it’s standing proof, written in stone and lime across the walls of the ancient world.

Understanding the principles of compatible strength is one thing; applying them correctly on site is another. In practice, structural lime mortars are used not as decorative finishes, but as functional consolidants — materials that restore the internal unity of a wall while keeping its natural movement and breathability. The same balance that made Roman structures endure is now used to stabilise historic masonry without resorting to rigid modern substitutes.

• Rebedding and Repointing: for re-bedding loose stones or bricks, or for deep joint repairs, structural limes such as Betoncino MGN or Modena M5 MGN offer the ideal consistency. They create a strong yet flexible joint that transfers load evenly across units. In bedding applications, this means a firm seat for each stone that still allows micro-adjustment under movement. For repointing, the mortar’s moderate hardness ensures it remains slightly sacrificial, failing before the masonry — strong enough to protect the masonry, yet soft enough to be renewed in future cycles of maintenance.
• Structural Rendering and Jacketing: in more complex consolidations, structural lime mortars can be used as reinforced coatings or jackets to strengthen walls externally or internally. A 30–40 mm layer of Betoncino MGN applied with basalt or AR-glass reinforcement meshes significantly increases shear resistance while remaining vapour-open and chemical-free. This approach stabilises walls suffering from bulging, lateral spreading or vibration fatigue without trapping moisture or locking the structure into rigidity. The result is a breathable composite system — a wall that is consolidated, not encased.
• Anchoring and Pinning: in cases of local instability, structural lime mortars can work in combination with fibreglass anchors inserted into the masonry. Each anchor is embedded using an injectable lime slurry such as Calce F3 MGN, connecting the wall’s outer and inner leaves through breathable, mineral bonds. Unlike steel, these anchors do not corrode or expand, and they are flexible and move in harmony with the surrounding materials. When combined with a structural lime coating, they create a discreet, reversible reinforcement that respects the wall’s integrity.
• Grouting and Injection: where walls have voids, delamination or separation between leaves, pozzolanic lime grouts such as Calce F3 MGN can be injected to restore continuity. These grouts flow into cracks and cavities, re-establishing friction between disconnected units without creating hydraulic pressure or chemical stress. Because they set gradually and remain slightly compressible, they fill gaps without bursting the masonry apart — a common risk with cementitious grouts. Once set, the wall regains its monolithic behaviour while retaining the breathability of lime.
• Protective and Surface Layers: finally, pozzolanic lime systems can act as protective skins against moisture ingress. A finishing layer such as Rinzaffo MGN applied over the wall head or exposed faces creates a waterproof yet breathable barrier. It prevents saturation — the root cause of frost damage and salt crystallisation — while allowing vapour to escape freely. This kind of protection stabilises the wall’s moisture regime, ensuring that the newly consolidated structure can dry naturally and age slowly.

The Result — Strength in Harmony

When applied with care, these techniques create what the Romans achieved two thousand years ago: a structure where materials cooperate rather than compete. The wall becomes a living system again — firm, dry and stable, yet still able to breathe and move.

Unlike cement-based systems, structural lime consolidations do not impose a new character on the building; they restore the old one. The materials remain mineral, reversible and fully compatible, ensuring that a repair today will not become tomorrow’s problem.

In structural conservation, safety lies not at the ends of the scale but in its centre. Walls don't fail because they are soft, they fail because they lose cohesion — and they fail again when rigid materials prevent that cohesion from returning.

Structural lime mortars offer the right kind of hardness: strength that works with the masonry, not against it. They restore friction, balance and breathability while allowing the wall to flex and age naturally. Two thousand years of proof stand across Europe’s ancient buildings and harbours — monuments not to brute force, but to intelligent material harmony.

Strong enough to hold, soft enough to live — that is the enduring lesson of Roman lime and the guiding principle of every good heritage repair today.

References

• 1
  Jackson, Marie & Kosso, Cynthia & Marra, Fabrizio & Hay, Richard. (2006). Geological Basis of Vitruvius' Empirical Observations of Material Characteristics of Rock Utilized in Roman Masonry. Proc. Second Int. Congress of Construction History. 2. 1685-702.
• 2
  Fernandes, Francisco & Lourenco, Paulo. (2007). Evaluation of the Compressive Strength of Ancient Clay Bricks Using Microdrilling. Journal of Materials in Civil Engineering - J MATER CIVIL ENG. 19. 10.1061
• 3
  Abed Soleymani, Mohammad Amir Najafgholipour, Ali Johari (2022). An experimental study on the mechanical properties of solid clay brick masonry with traditional mortars, Journal of Building Engineering, Volume 58
• 4
  Goodman, R E, (1989). Introduction to Rock Mechanics, New York: John Wiley
• 5
  Suciu, Ovidiu & Cruciat, Radu & Ghindea, Cristian. (2014). Experimental Case Studies on Clay Fired Bricks Compressive Strength. Key Engineering Materials. 601. 10.4028
• 6
  Vitruvius (1999), Ten Books on Architecture De Architectura. Rowland, I D. and Howe, T N, (editors), Cambridge: Cambridge University Press.
• 7
  Jackson, Marie & Deocampo, Daniel & Marra, Fabrizio & Scheetz, Barry. (2010). Mid-Pleistocene volcanic ash in ancient Roman concretes. Geoarchaeology. 25. 36 - 74. 10.1002/gea.20295.