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Alteration Processes

Alcali-Silica rection

Concrete can suffer active alkali-silica reaction (ASR) when, concrete aggregates contain microcrystalline quartz particles. Certain rock varyeties like slates, metaquartzites, gneisses, schists, and other rocks containing low temperature silica/silicate minerals and/or strained quartz that are potentially reactive with cement alkalis.

If we have reactive (silica based) aggregate(s) and water plus alkalis the ASR reactions occurs and, if one of these components is not present, the reaction does not occur.
The ASR can be identified with petrographic exams: its effect is easy to be identified at a micro scale: little cracks, alkalic gel exudations, distress/physical disruption of the concrete among the cracks. Once the silica-gel is formed, from the reaction of silica with high alcaline materials, if wetted, the gel expands and swells, causing tension and compression forces inside it.


Blistering is one of the most common types of failure related to the adhesion of the paint. Sometimes the blisters are dry and sometimes filled with liquid. The blisters can be both large and small, often shaped as hemispheres. The size usually depends on the degree of adhesion to the substrate, or between the coats, and the internal pressure of the gas or the liquid inside the blister. Blistering can be caused by a number of different conditions:
Soluble salts contaminating the substrate or contaminating the surface between coats. No coatings are 100% water proof. The moisture vapour passing through the coating can dissolve salt into a concentrated solution. Pressure in the high concentration liquid will cause blisters. This phenomen is called osmosis.
Contamination of the surface (e.g. oils, waxes, dust, etc.) will not allow proper adhesion of the coating. The moisture vapour tends to be concentrated in these areas of low adhesion. In this case, the blisters are so-called “dry” blisters.
Poor or inadequate solvent release from the coating. Entrapped solvents can increase the water absorption and moisture vapour transmission of the coating and lead to blistering. Solvent odour is usually connected with retained solvents. If the blistering is widespread on a construction: reblast and wash before a new system is applied. For local areas: blast or carry out other mechanical cleaning before recoating



Corrosion means the breaking down of essential properties in a material due a to chemical reactions with some agent.

In the most common use, the word means metals reacting with water and oxygen and the production of typical oxide salts of the original metal. In particular, corrosion is oxidation of the iron, rust.
Corrosion also includes the dissolution of ceramic materials and can refer to discoloration and weakening of polymers by the sun ultraviolet light. Conservation experts, moreover, use the expression “stone corrosion” to identify the natural stone hydrolysis-oxidation mechanism.


Removal of materials by dissolving them away from solids is called leaching. The chemical process industries use leaching in may processes but the term leaching is more common to identify the asportation of some component by percolation of a solvent in a solid material. Leaching is typical of soils and rocks (see, for exemple, this link, about the leaching of some iron salts from a Chalcopyrite and the change of the leached salts into other minerals) and, typically, leaching produce a stain, a chromatic alteration, that can be often identified as efflorescence or concretion).
In building, we often have the percolation of water through mortar screeds (i.e. floor substrate) and the leaching of lime from it (cementitious screeds are full of free lime). When the dissolved lime (Ca(OH)2) reaches an evaporating surface (wherever it is) it turns to calcium carbonate and the efflorescence turns to a concretion as it hardly bonds to the surface. Note that acid waters can leach carbonated limes too. This deterioration process is typical of limestones in polluted environments. When we have sulphate rich athmospheres, limestones are slowly turned to gypsum by aggressive water and gypsum is washed away or forms crusts in places not reached by rain.

Leaching means loosing mass.
This mainly means changes in mechanical properties (compression or tensile resistance of a mortar, for exemple).
The water run off on a surface can smooth it and can round its corners

White Rot attack

White rot fungi produce unique extracellular oxidative enzymes that degrade lignin, as well as related compounds found in explosive contaminated materials, pesticides, and toxic wastes. To elucidate the genetic basis of this technologically important behavior, we have sequenced the thirty million base-pair genome of the white rot fungus Phanerochaete chrysosporium to high draft using a whole genome shotgun method. This is the first basidiomycete genome to be sequenced.

Lignin plays a key role in the carbon cycle as the most abundant aromatic compound in nature, providing the protective matrix surrounding the cellulose microfibrils of plant cell walls. This amorphous and insoluble polymer lacks stereoregularity and, in contrast to cellulose and hemicellulose, it is not susceptible to hydrolytic attack. Although lignin is a formidable substrate, its degradation by certain fungi was recognized and described nearly 125 years ago. Collectively referred to as white rot fungi (since they degrade brown lignin, and leave behind white cellulose), these are the only microbes capable of efficient depolymerization and mineralization of lignin. All are basidiomycetes, a fungal group that includes both edible mushrooms as well as plant pathogens such as smuts and rust.

Phanerochaete chrysosporium has been the most intensively studied white rot fungus. White rot fungi secrete an array of peroxidases and oxidases that act non-specifically via the generation of lignin free radicals, which then undergo spontaneous cleavage reactions. The non-specific nature and exceptional oxidation potential of the enzymes has attracted considerable interest for application in bioprocesses such as organopollutant degradation and fiber bleaching.

Phanerochaete chrysosporium has several features that might make it very useful. First, unlike some white rot fungi, it leave the cellulose of the wood virtually untouched. Second, it has a very high optimum temperature (about 40 C), which means it can grow on wood chips in compost piles, which attain a very high temperature. These characteristics point to some possible roles in biotechnology.


Surface characteristics alteration

Chromatic Alteration:
Usually defined with other expressions (stain, deposit etc.) that also explain how the alteration is produced. Use this expression when you are not able to see and identify the object and understand the basic failure process.

Local chromatic alteration (it is mostly used when the surface becomes darker or gets a particular colour). We can use also the terms “mark” or “spot” when the change is localized.
Note that on the façades you often have a complex combination of chromatic alteration processes due to:

  • particulate deposit
  • differential removal of the deposits due to rain run off
  • algae uneven growth due to differential wetting of the façade
  • oxide pigmentation (iron or copper - the most used)
  • bird excrements

The English term “bleach” has the same root of the terms blanche, blank, bleak and probably black (!). The connection seems to be “burning, blazing, shining, whiteness.” That the same root yielded words for “black” and “white” is probably because both are colorless, and perhaps because both are associated with burning.
A bleaching is usually the contrary of a stain, that is the evidence of a region of a surface that has become whiter or just more clear or with less colour than other parts. Also Depigmentation, that means loose of colour. and also fading, when the effect is more smooth.
Sometimes a low uniformity character of a paint is desired, as with lime-based paints (it. “velature”)

A place or area that has been burned, damaged by fire.

The term is normally used among geologists to identify special rock formation. In our case, we use this term to identify calcareous efflorescences produced by the carbonation of soluble lime over the surface of a building or by the leaching of non-soluble lime by acid rains.
We use it when we have thick lime deposition over (or under) a surface, like the following ones.

A crust is the result of the acid alteration of a limestone or a lime based render (gypsum is formed from the calcium carbonate parts). We usually have a “black crust” because the porous gypsum surface adsorbs water as well atmospheric pollutants (among which, most, black carbon residual from internal combustion engines and traffic).

It is a material accumulated on a surface not strongly bonded to it, that, usually, can be easily taken away.
Usually, deposits are subjected to local and differential runoff of water on the surface of a building, that changes its appearance.
Deposits occur easily over horizontal surfaces and near them.

Added materials (deposits) become encrustations when their weathering consolidate them and they are very adherent to the surface. NB Encrustation is not a crust!
Note also that, very often, deposits or encrustations on a façade are washed away.
Note that, in some cases, we have also the corrosion and the erosion of the surface due to the chemical and mechanical action of running water (rain). In this case we have more or less uniform deposits and encrustations cut by vertical whiter stripes from where deposits are constantly removed during rain events, expecially where rain colected on horizontal surfaces is drained or channeled.

Thin semi trasparent deposit very adherent, artificially or naturally added, to the surface.
Do not confond with “patina” that is a natural alteration of the surface that is not due to a decay or weathering, just a non-negative chromatic alteration (the aging of the material)

Surface cristal-like deposits, usually white and powder like or filament like. When the salts take a glassy or marble like form, very well sticked to the surface, we have a concretion (similar to an encrustation).
Often combined with disgregation and powdering of the surface. Due to efflorescence, of course, but if you do not see any salt over the surface or under it (detachment and bulging or blistering), then you do not have any efflorescence, just salt attack induced disgregation (and perhaps staining and may be biological growth).

Biological Growth:
Growth of vegetation on a surface (from algae, lichens, mosses, to trees and higher plants) may form microbe films or lawns, microbe camps, penetrate into slits, cracks, crevices and pores, or work its way through the material creating their own new cavities.
Not only vegetals: insects make holes or create deposits (spidernets), birds

A natural alteration of the surface that is not due to a decay or weathering, just a non-negative chromatic alteration (the aging of the material). You can have a patina over a metal or over a stone.
(do not confond with film)

Form (geometry) or position alteration

The tipical deformation of a beam or of a plate subjected to a load perpendicular to its length or surface.

The term seems to come an ancient norman word used to say “blowing” “swelling”. A blister is a usually small pocket between two sealed layers in which some liquid or gas is confined. Very often, the blister is just a first degradation stage and it evolves in a film crack and its exfoliation.
Blistering is a typical roof membrane problem, but also a paint (film, tipically sintetic films) problem.
The pushing force can be caused by water or other solvent evaporation, or by salt criptoefflorescence or, at least, because of temperature.
When the blister itself is very big, then it is more clear to use the term bulging

Rigid movement of one part or component from its original position in the building.

Reduction of size. It is usually not visible without instruments. But as it turns out in cracks, it becomes easily seen.

The term enters the English languge from the old French “bouge”, that meant “leather bag”, “wallet”: from Latin “bulga”. It has the same origin of “budget”, that means “the amount in my (leather) wallet” and means a local swelling of the surface. In building façades, this swelling is usually due to a detachment of the outer layer, due to internal forces. It is different from the spalling (rigid parts expelled outward).
When it is very little, it is better to use the term blistering. When you don’t have a film or layer swelling outside, but rigid pieces forced to detach from the surface, it is better to talk about spalling or expulsion.
(Italian: “rigonfiamento”, sollevamento superficiale localizzato del materiale).

Loss of mass/parts

It means big (deep) pits and is tipical of very soft sedimentary stones (limestones) and bricks. In italian weathering vocabulary pitting is used only for metal corrosion. In English you can easily say “pitting”.

Little (few millimeters) holes usually with a emispherical form, little and near one to the other.
Pitting is not only used to describe corrosion form of steel and metals but also for stones.
Pitting Corrosion is is one of the most damaging forms of corrosion of metals.

It means the degradation of an original material, from which thin layers are separated from the substrate.
“Lamina” is a latin word that means “thin piece of metal or wood. The field in which the term is most used (scientifically) is that of Failure Analysis of Composite Materials. Note that the most simple among composite materials in building is plywood.
Everything that is made in layer can delaminate. Natural stones, metals, plastics and, above all, reinforced material, when reinforcement is layed on a surface. Paints can delaminate, but when the detached “laminae” curls, then we can more clearly use the term exfoliation”.

The term comes from latin “erodere” that means to “gnaw away,”: ex=”away” + rodere= to “gnaw” (see rodent). Similar to abrasion, still from latin “abradere”, that means to scrape off: ab=”off” + radere=”to scrape” (see razor), but the eroded surface is usually more smooth.
Usually, the term is used to identify the loss of mass from a surface that has been caused by a polishing or abrading action by some agent, at least by water run over a surface.
Note that water often works both as chemical and mechanical erosion agent. Chemical erosion is basically influenced by material solvability. Lime (carbonate) stones are the most affected by chemical erosion and rain acidity.
The eroded surface remains compact but you have a change in its geometry or at least a reduction of the thickness of the object. When the surface is not compact is preferable to use powdering, sanding, etc, following the form of the parts that the surface looses.
Sometimes you can use expressions that indicates the effect of the erosion: i.e. “rounded corners” or “thickness reduction”.


It has a german-norman origin: flak meant “loose or torn piece” (the very fla meant “to skin”). A flake is a small fragment - usually with a plane form - of something broken off from a whole.
When we say flaking or flakes me mean irregular little parts, detached or still partly attached to the original surface, with a certain thickness (not a thin film). The formation and detachment of flakes is alwais related to a surface crack more or less similar to a crazing.
When the flakes are very thin, is better to use the term “scaling” .

Loss of coherence, disintegration, starting from the surface, resulting in the separation of very little parts or particles.
Regarding to the dimension of the particles detachable from the surface, we can say also:
-Sanding, when the particles are sufficiently rough;
-Chalking or powdering when the powder that will soil our hand touching the surface is very fine (and whity).

The scales are the skin plates of fish or snakes (from the german “skælo”, that meant to split, divide, but with a common root with many other languages, i.e. latin “culter” or knife and others).
We can use the term “scaling” to identify very thin parts or flakes falling or detaching from a surface, after a - usually - diffused cracking of a film layer. It is used also for Concrete.
When you have a scaling paint originated froma a regular network of cracks, it is often used the term “alligatoring” (it. “coccodrillatura”).
Scaling is a typical effect of an internal mechanical action that disrupt a layer:
-Freewze-thaw attac i.e. on a concrete pavement;
-Salt attack

Mechanical damage

Cracking or crack? A crack is a break line; to crack means to create a break into an object. The material can be completely broken into two or more separated pieces. In other cases a local network of cracks can just weaken the material the object is made of, keeping its integrity.
Usually, the term “crack” should be used to identify deep breaks. When the break is just shown on the surface, is more often used the expression “haircrack”.
Many things can crack:
-A wall;
-The rendering of a wall, due to an improper work or setting;

  • A RC parapet;
    -A wood beam;

Very diffused - usually randomly diffused - haircracks. Typical of ceramic glaze.
It is just an appearance fault but, from the cracks a degradation process can start.
The term is used also for Concrete.

Hair crack:
A surfacial crack with a width of less than 0.15 mm.

The scales are the skin plates of fish or snakes (from the german “skælo”, that meant to split, divide, but with a common root with many other languages, i.e. latin “culter” or knife and others).
We can use the term “scaling” to identify very thin parts or flakes falling or detaching from a surface, after a - usually - diffused cracking of a film layer. It is used also for Concrete.
When you have a scaling paint originated froma a regular network of cracks, it is often used the term “alligatoring” (it. “coccodrillatura”).

Shimmy hole:
Holes in a wood surface produced by xilofagi insects.