When investigating the causes of damage to monuments located in the immediate vicinity or in the area of centers conducive to air and water pollution with aggressive substances, great attention should be paid to the impact of atmospheric conditions and the impact of the parameters of the internal climate of the rooms, and in special cases also the surrounding aquatic environment.
The effects of chemical aggression on a given building material depend on the intensity of the aggressiveness factor in a given environment. In many countries, zone classifications or severity levels with numerical indicators have been developed in this field. Most often, this intensity is determined by concentration, which is expressed in a gaseous environment in mg/m3, and in the aquatic environment in mg/l.
The aggression of the air environment is usually defined depending on the degree of atmospheric pollution with dust and sulfur dioxide (SO2). For aquatic environments, these substances are included, that are most common in the environment.
During preliminary studies of the impact of the aggressiveness of a given environment, the most useful in conservation works will be a five-point scale characterizing changes in, which occur in masonry materials or structures in the process of chemical corrosion.
Depending on the degree of aggression of the air environment, that surrounds the monuments, the following five environments can be distinguished: non-aggressive, slightly aggressive, medium aggressive, highly aggressive and overly aggressive. A non-aggressive environment can be considered the microclimate of closed spaces inside the building and the atmosphere away from industrial centers and big cities. In this environment, there are no signs of damage on the face of masonry materials.
Highly aggressive and overly aggressive environments are found primarily in large cities and in the vicinity of industrial centers, especially chemical plants, hut, coking plant, heating plant, gasworks, cement plant, etc., that pollute the atmosphere and water with aggressive substances. In this case, on the surface of stone and brick, you can see clear changes progressing over time: corrosion develops not only from the outside, but also causes the rapid decomposition of the structure of masonry materials.
Weakly and moderately aggressive environments include urban areas and those located intermediately between a non-aggressive environment and an environment with a higher degree of aggression. Here in turn, depending on the concentration of aggressive substances, slight surface changes of the material can be noticed, progressing very slowly, or marked epidermal changes, and even partial structural changes progressing over time.
The classification of environmental aggression specified above is not exhaustive in terms of comparison, because it does not use specific indicators showing the degree of saturation of a given environment with aggressive substances and does not specify the nature of the action of these substances on materials and structures. These are very complex issues and not yet sufficiently explored, therefore requiring the assistance of appropriate scientific institutions in the research.
As for the chemical origin of aggression, it is depending on the predominance of factors causing corrosion that aggressions are distinguished:
- sulphate, characterized by it, that in mineral materials, due to the formation of sulfuric acid salts, the structure bursts without loss of mass,
- carbonate, causing changes in structure due to attack by aggressive carbon dioxide or carbonic acid,
- acid, caused by acids and salts of strong acids, which is usually accompanied by a loss of mass due to the dissolution of mortars,
- leaching, causing the leaching of certain mineral substances by soft waters.
In some cases, ammonium aggression is also distinguished, magnesium, basic or solo, or of a complex nature, where two or more factors act simultaneously. The assessment of the degree of harmfulness of a given environment and the mechanism of chemical transformations in the elements and materials of a historic building can only be carried out by specialized facilities based on a full chemical analysis. These establishments may also specify, what protective measures are the most rational and economical.
For the protection of cultural property, the most effective method of staving off aggression will be the removal of all sources causing environmental degradation, and in cases impossible to cause, that the concentration of harmful substances does not exceed the permissible level, i.e. such, which will not harm people, plants or materials.