In restoration, a compatible material is one that can function in a balanced manner with the original components of a historical building in physical, chemical, mechanical, thermal, and aesthetic terms, and can coexist with the building after the intervention is performed.
The most critical threshold here is the principle of "Reversibility", which is one of the cornerstones of conservation discipline. A compatible material must be removable from the structure in the future without causing any damage to the original stone, brick, or mortar of the building if a more advanced technique is required.
When this holistic compatibility is not achieved, interventions that initially seem successful eventually lead to irreversible structural damage, such as:
- Capillary and deep cracking,
- Salt efflorescence and blooming on the surface,
- Surface detachments and stone losses due to trapped moisture,
- Increase in microorganic and biological formations.
The Nature of Historical Buildings and Material Conflict
Historical buildings behave very differently from modern reinforced concrete structures. They are like living organisms: they breathe, transfer moisture, have a porous structure, and have established a microclimatic balance over hundreds of years.
When an incompatible, rigid material is added to this delicate balance, the system collapses. We see the most tangible example of this in cement conflicts: when high-strength Portland cement mortar or plaster is applied to a lime-based, flexible stone wall from the 19th century, the new mortar hardens excessively. During structural movement, stress concentrates on the stone. Consequently, while the mortar remains intact, the weakened historical stone fractures and turns to dust.
5 Basic Characteristics of Compatible Materials
Compatibility criteria, established through modern laboratory analyses (petrographic examinations, XRD mineralogical analyses, and pozzolanic activity tests), are grouped under 5 main headings:
1. Vapor Permeability and Capillary Compatibility (Breathability): Historical buildings must be able to expel the moisture within their structure. If the new material used does not allow water vapor to pass and blocks the pores, moisture becomes trapped inside the wall. When combined with freeze-thaw cycles, this leads to salt crystallization, plaster blistering, and the deterioration of the stone's surface structure. It is therefore critical that the restoration material has a high vapor permeability, similar to the original texture.
2. Mechanical Strength and Elasticity Compatibility: The hardness and compressive strength of the new material must not be higher than the original mortar and stone of the historical building. Excessively hard repair mortars applied over relatively soft and flexible materials, such as natural stone, Khorasan mortar, or historical brick, lead to micro-cracks and edge fractures. The goal is not to concretize the building, but to preserve its original mechanical flexibility.
3. Thermal Movement Compatibility: Every material expands and contracts with temperature changes. However, the thermal expansion coefficients of cement and historical lime mortar or kufeki stone are completely different. Between two contrasting materials moving at different rates, loss of adhesion (bonding) begins over time, gaps open between them, and surface detachments occur.
4. Chemical and Mineralogical Compatibility: Many modern materials (especially Portland cement) contain high levels of sulfates and alkali-soluble salts. When these chemicals react with historical lime systems and stones, they trigger new and destructive chemical processes that disrupt the stone's pore structure. The mineral structure of the new material must not be chemically foreign to the raw material of the original mortar.
5. Visual and Aesthetic Compatibility: Restoration is a cultural act as much as it is a technical one. The new intervention must not overpower the historical depth of the building, destroy its patina, or create an artificial "newly built" feel. However, while doing this, it must not create a "historical forgery" by imitating the original surface identically; it should respectfully present the trace of its own period.
Character Conflict Between Old and New Materials
We can clearly understand why we must avoid cement-based materials in interventions by looking at the contrasts between original mortars and modern materials under the microscope:
| Feature | Original Historical Material (Lime / Khorasan) | Incorrect Intervention Material (Portland Cement) |
|---|---|---|
| Mechanical Behavior | Flexible, dampens microscopic structural movements. | Excessively hard, rigid, and brittle; transfers stress to the stone. |
| Breathability Capacity | Has high vapor permeability (Porous). | Very low permeability, traps water and moisture inside. |
| Chemical Structure | Contains no free salts, mineralogically natural. | Sulfates and soluble salts in its content damage the stone. |
| Drying and Curing | Dries slowly, strengthens over time by absorbing carbon dioxide from the air. | Cures very fast, has high hydration heat, and causes shrinkage cracks. |
Why "Looking Similar" Is Not Enough
One of the most common mistakes in restoration is making decisions solely based on the color and texture of the material. Even if two materials look identical from the outside, their pore diameters, water absorption coefficients, elasticity moduli, and thermal behaviors can be completely different. Therefore, deciding on "compatibility" based on pure observation without performing quantitative (numerical) analyses in a laboratory environment carries significant risks.