As we know, water is a polar solvent for excellence, is a typical dipole molecule (due to the greater electronegativity of oxygen compared to hydrogen, It produces two charging poles). For this reason, various molecules of H2Or do you have randomly, but orient themselves, held together by Intermolecular Association, the hydrogen bonds, that you establish between the opposite poles of loads. This dipolar structure is the characteristic that makes it also ionizing, the fact that the value of dielectric constant is high, It also gives the ability to dissociate. Upon contact with H2Or substances already if the Ionians, as the sali, You can detach and go into solution as free hydrated ions. THE H2Or may be withheld by solids in various ways, the first distinction that we can do is water essential, e.g.. the water of crystallization in Chalk Case4 2H2Or in which the molecules of H2Or form an integral part of the Crystal structure of the compound and are present in stoichiometric quantities; and water not essential, which is not related chemically to the mixture, e.g.. the water absorption (that is the amount of water lost from the surface of a solid), the water absorption (represents a condensed phase withheld from the interstices or colloidal solids or capillary gels), occlusion water (is that trapped microscopic cavities of crystalline solids), Finally the H2Or it can be dispersed in a solid form a solid solution.
All processes of deterioration depends on instability of the air-water-system artifact, i.e. by the amount and the rate at which water exchange materials with the environment, According to their specific characteristics of porosity and Hygroscopicity.
The air temperature of the water varies constantly in course of the day and even more so in’ARC a year. The amount of steam H2Or in the air depends on its time not only as a result of temperature changes and the related phenomena of condensation, but especially in relation to local weather events, with continuous air exchange more or less hot and humid from outside to inside and vice versa.
The water vapour that forms on the surface of the Earth is continuously in the atmosphere and, Depending on the temperature the air, can absorb a given quantity of steam. Maximum moisture content at a given temperature is called the saturation concentration to which corresponds a maximum vapour pressure or saturation pressure (ps).
A typical meteoric humidity parameter is humidity, the relationship between the actual concentration of H2Or in the air and the concentration necessary for saturation to a certain temperature. More cold air will lower its water content, If it cools hot and humid air will reach a temperature at which the air is saturated with H2OR; further lowering the temperature, part of the water vapour will turn into liquid. This temperature is defined as the temperature of dew or condensation temperature. The mass of water vapour in unit volume of moist air is the absolute humidity UA. Molar UM moisture, Instead, is the number of moles of water vapor occupies a volume of dry air. In turn the relationship, multiplied 100, between molar moisture and humidity air saturated steam molar in the same conditions of temperature and total pressure, humidity percentage is defined (UP).
The relative humidity percentage, the relative humidity (UR) represents the relationship, multiplied 100, between UA and UA air of air contains steam under the same conditions of temperature and total pressure.
The air may contain, at a certain temperature, a maximum amount of water vapour that represents the 100% of UR, If the concentration of moisture goes over this value the water will condense on cold surfaces or to remain in the air in over-saturation conditions in the form of fog. Condensation can occur either because of a general lowering of temperature, both as a result of migration of water vapor in areas with lower temperature. According to the structure of materials, condensation can occur in the form of droplets, If it's impermeable surfaces such as ceramics, or in the form of dark spots generated by the development of mold, fungi and bacteria if moisture is absorbed by the porosity of the material such as plaster, bricks, porous stones, Chalk, etc.
Moisture can be gained from materials, In addition to absorption and absorption, for capillary condensation in extremely fine pores with the formation of a meniscus. Capillary condensation comes from the fact that the vapor pressure (p ") above a curved meniscus is different from the vapor pressure of a flat water surface (p '). In particular if the curvature of the meniscus is negative then p ">p’ It has capillary condensation, the case of hydrophilic materials. If the curvature is positive p "<p’ There is no condensation, the case of waterproof materials. More capillaries are thin, plus the meniscus will curve and lower will be the vapor pressure inside the capillary; If the vapor pressure of air is higher than that of the capillary, a part of the water vapour will condense in the air the meniscus.
Surface condensation of moisture occurs on masonry walls in cold with temperatures below the dew point temperature, When coming into contact with warm moist air.
The capillary rise
The contact of the foundations with the water in a highly humid soil or even with a flap table, causes the vertical-lift H2Or inside of the building as a result of capillary absorption. The height of capillary rise is inversely proportional to the diameter of the pores of materials, eg brick walls whose porosity varies between 1 and 10 micron, the theoretical maximum lift height will vary between 15 cm and 1,5 m.
Low Hygroscopicity of the bricks suggests that the theoretical maximum lift height should be higher in masonry made of hygroscopic materials (as is the case with all mortars and most building stones). In reality it is difficult that we can never find in front of walls where the maximum height of the humidity has reached near-theoretical values, because in a masonry real height actually reachable from the humidity is always less than the maximum potential; i.e. the height that the water would reach if it prevents evaporation of the same masonry surfaces below. This evaporation causes, gradually growing the level of ski lifts, decrease moisture content of masonry, up to a threshold where if growth no longer occurs, don't you have a scarcity of water at the base of the wall, but the prevalence of evaporation on capillary absorption.
If we are confronted with cases of high levels of humidity lifts (exceeding 2 m), This may depend on factors that prevent or slow evaporation on the lower surface of the wall, e.g.. cases of little porous materials, skirtings, furniture or furnishings various massed against the sides of the masonry; preventing normal ventilation of the lower parts of the surfaces.
The various forms in which it manifests the H2Capillary or are: moisture stains, posting of plasters, destruction of mortars free, crystallization of salts, degradation of stones, bricks, etc.
As we said at each temperature matches a certain value of maximum concentration of water vapor in the air. When the steam, in terms of maximum concentration, comes into contact with a colder surface (that have a temperature equal to or lower than the dew point temperature of water vapor) you have the phenomenon of condensation, the formation of a thin layer of H2Or liquid on the surface cooler. This veil of water will be more or less rapidly sucked inside the capillary wall. Condensation moisture may form inside of the wall, as long as the temperature of the material is less than that of dew for the water vapor content from the wall itself, This phenomenon is termed interstitial condensation is frequent in ancient thick walls large enough. The most frequent cases of interstitial condensation occur in areas of higher inhomogeneities of masonry. Condensation can also occur inside of the pores, in the presence of high relative humidity and with extremely small pores (with a radius of less than 0,5 micron).
The formation of H2Or condensation within walls or stone materials usually gives rise to a rapid migration of water towards drier areas, However, if in the zone of the condensation temperature drops and ice forms: moisture will migrate to the ice, because contact with areas that are in any case the drier. In an environment with high relative humidity, the application outside of the wall materials that pose anyway vapor barrier of water, encourage the formation of interstitial condensation, to increase the water content of the wall.
Unlike the humidity lifts and the localized rain infiltration or leakage from pipelines, It is rare for condensing humidity being able to accurately locate the area of the phenomenon. The visible effects of removal (and not the entry of moisture) they are usually efflorescence or whitish halos indefinite contours.
Most often occurs when the condensation determined by seniority, on the surface or inside of stone materials, a certain amount of H2Or that you can depend on rising humidity or infiltration. And’ difficult to reconstruct the actual mechanism of damage from moisture, When you want to distinguish with accuracy the origin, There is one case where moisture condensation is pretty easy to diagnose: When the phenomenon affects well demarcated zones, in which the condensed water stagnates in surface or penetrate very little. In this case there are dark spots that faithfully repeat the solid body shape, immediately below, ' colder’ because it has high compactness and thermal conductivity (of beams iron or stone blocks covered by layers d’plaster of small thickness).
Even the corners are cooler areas of this type, that is well demarcated and poorly absorbent. The damage occurs with dark spots, instead of white, because the dust in the atmosphere builds up more in cold surfaces than on those hot.
From the conservative point of view, all damage from moisture occur not at the time when the material absorbs water (except for mechanical damage from dilation or swelling and minor mechanical resistance moisture produces many hygroscopic materials), but when the water evaporates and dries. A porous body that is exposed to air after immersion in water for quite a long time from being permeated, dries by evaporation until you reach a specific hygroscopic moisture content. In some cases the process of evaporative drying may go beyond the condition of equilibrium between hygroscopic moisture content and UR, (e.g.. If a material is heated dry more than would the relative humidity present in the air). Apart from these exceptions, the evaporation of moisture from a material is always due to a decrease in air UR; considering that in our climates, the relative humidity rarely falls below the 30%, No hygroscopic material is never completely dry.
Obviously the speed of evaporation increases in the presence of a current of air, But whatever the speed the evaporation remains cost until the entire area concerned by the phenomenon is uniformly soaked in every point by the same quantity of moisture. This condition of uniformity cannot be long-lasting because the pores small exert greater strength and so the water evaporates before the pores larger and earlier by buchi, fractures, etc.
As they form these points more or less humid dry, the humidity of the surrounding areas tends to flow towards them, moving laterally and in depth, rather than to the surface. This results in a slower evaporation rate and a shift towards the inside of the front of moisture.
This will slowly evaporate, at a rate inversely proportional to the distance that separates it from the surface; the point at which it produces this difference in the speed of evaporation is called critical moisture content.
To understand the damage caused by the phenomenon of evaporation must examine the inside of porous body, more precisely the area in which the moisture front. We saw that at a certain temperature and RH, the evaporation rate is constant, When the moisture front coincides with the surface of the material, as soon as the front retreats, the evaporation rate decreases.
For each material the hygroscopic moisture content is related to the relative air humidity, i.e. increases and decreases with it.
The water inside the walls is always present in the form of saline which can separate anhydrous salts or hydrates that cause damage to materials. Migration and recrystallization of soluble salts carried by water is the primary mechanism of alteration of stone materials. The shape and extent of the damage depends on the porosity and Hygroscopicity of the material. THE H2OR, especially if very pure, causes the dissolution of salts which meet on its way to settle elsewhere; also the water coming from the floor loading of salts which it contains and, continuing its journey through the porous materials to the surface of evaporation, carries them in his path. The same happens for the H2O of infiltration, on his journey back, from top to bottom.
The solubility of the salts is highly variable, There are practically insoluble substances such as silver chloride and very soluble salts such as calcium chloride or sodium sulfate. Transferring of salts depends, then, by their solubility and water flows inside of porous materials, streams that in turn are influenced by the presence of salts which may give rise to viscosity variations, changes of surface tensions and differences in osmotic pressures.
Soluble salts, that can be found in stone artifacts, can have different origins:
– are an integral part of the walls first;
– are inherent in the processes of manufacture of the materials;
– derived from reactions and exchanges with the atmosphere that envelops the artifact;
– originate from the soil on which the monument was built;
– derive from the action of biodeterioration;
– are generated by electro-chemical corrosion of metals;
– formed as a result of the use of products for cleaning and consolidation of the stone, or to techniques of restoration inappropriate or unsuitable.
The salts which have more often found in stone materials are:
Sulphates they are very mobile salts, dissolved in natural waters that represent one of the major constituents, are also contained in alloying elements aircraft and hydraulic, in rocks, in polluted atmospheres, in marine atmospheres:
– calcium sulfate, it comes in various forms as hydrated:
Chalk (Case4 2H2OR)
– magnesium sulfate, This hydrated in various forms, It occurs with more frequency in rural environment as:
epsomite or epson salt (MgSO4 7H2OR)
kiersite (MgSO4 H2OR)
– sodium sulfate , This hydrates or anhydrous form, It occurs mainly in the urban environment as:
mirabilite (Na2I KNOW4 10H2OR)
thenardite (Na2I KNOW4)
Let's clarify the possible origins of sulfates:
to) are present in agricultural soils, circulating water can dissolve them and if these dates by capillarity, e.g.. in a masonry make you sulfates containing;
(b)) sea water contains small amounts of sulfates, especially magnesium sulfate. Sea winds carry, for miles, suspensions of seawater particles with these salts;
(c)) Sometimes the materials used for preparation of plaster may contain small amounts of sulfates as impurities. Even the materials used erroneously for restoration can result in the presence of sulfates (e.g.. stucco or plaster injections);
(d)) may have microbial origin, There are some types of microorganisms able to metabolise sulphur reduced forms quantitatively in sulfates, alongside other micro-organisms which produce sulfides (that is a reduced form of sulfur);
and) air pollution. As A result of combustion of hydrocarbons, sulfur content in it is converted into sulfur dioxide (SO2), the latter reacting with oxygen form the sulfur trioxide (I KNOW3), in turn this with water to form sulfuric acid (I KNOW3 + H2O = H2I KNOW4) which attacks the calcium carbonate into sulfate. These transformations can take place directly on the material stone industry, or sulfuric acid can form in the air and can get in contact with the wall by reacting with the calcium carbonate. Another possibility is that sulfuric acid formed in the air is neutralized by basic substances present in it such as ammonia, forming ammonium sulphate, or calcium carbonate (in atmospheric dust) resulting in calcium sulfate. There is also the possibility that sulfur dioxide is absorbed directly as such and as sulfurous acid on limestone materials; might as well be the formation of calcium sulphite, only then can oxidize to sulphate.
Sulphates are dangerous because, not only are soluble in H2OR, but can exist in different hydration layers and can crystallize with different amounts of H2O depending on the temperature and relative humidity.
At each State of hydration is a different specific volume, whenever the transformation from one layer to another, a change in volume. When, for the increase of UR, you have the phenomenon of hydration, Salt increases in volume by exerting a pressure (hydration pressure) on the walls of the capillaries in the porous structure of a material. When this pressure exceeds the strength of these walls are broken, the whole structure becomes more porous and therefore more susceptible to the further action of moisture and salts.
– sodium carbonate, is hydrated in various forms:
natron or natrite (Na2CO3 10H2OR)
The calcium carbonate, Besides being present as a constituent element in the marble and limestone, We find it also in the frescoes and murals in General, because it is formed by carbonation (Ca(OH)2 + CO2 > CaCO3 + H2OR) of lime in the plaster. And’ very little soluble in H2OR, However, it may be solubilized as baking when in the water there is a sufficiently high amount of carbon dioxide. The CO2 melting in H2OR, in the damp wall, gives rise to the formation of carbonic acid, which reacts with calcium carbonate, which is bicarbonate to form more soluble. It establishes a balance between these different chemical species: carbonates – carbon dioxide + water – baking; but gradually as the wall begins to dry out and evaporation occurs, the balance shifts in favour of the formation of calcium carbonate, which being slightly soluble, settles quickly on the surface. We have two negative effects:
to) a white patina that tends to cover the surface,
(b)) the impoverishment of the inner layers, that lose part of their calcium carbonate.
Nitrites and nitrates. Nitrites are not found very often in stone materials, They oxidize rapidly into nitrates. The decomposition of organic material produces nitrite nitrogen: You can find these types of salts on a stone if there are infiltrations of sewage or from areas where there is organic material decomposition Street.
– The calcium and magnesium nitrate crystallize only when UR reaches lower values to 50% :
nitrocalcite (Ca(NO3)2 4H2OR)
nitromagnesite (Mg(NO3)2 6H2OR)
Nitrates can have the same source of nitrites, or may come from agricultural land or in the vicinity of the stone artifact there is some old tombs. Also in hydrocarbon combustion produces different organic molecules and nitrogen oxides, the latter through a complex series of reactions forming nitric acid that, reacting with the calcium carbonate, from the formation of calcium nitrate. Nitrates are highly soluble in H2O and their action towards the porous structure is similar to that of chlorides.
– The calcium and magnesium chlorides fail to crystallise under normal conditions even if present in large amounts, due to their high Hygroscopicity:
antarticite (Baby2 6H2OR)
bischofite (MgCl2 6H2OR)
Especially the chlorides of sodium and calcium are mainly made to artifacts from the sea. In some cases it can be caused by impurities of the materials used for preparation of plaster, in particular the sand. In addition, some industrial activities (for es. the combustion of certain types of coal) can cause the presence of gaseous hydrochloric acid into the atmosphere. Salts are very dangerous, soluble and often very deliquescent; they can absorb moisture from the atmosphere and hold onto the stone material. When crystallize up deposits that develop very porous capillary forces able to suck water. Their presence lowers the transition temperature of hydrates and makes then easier changes from one State to another for hydration salts of sulfates. Finally, When they are in solution, have a high mobility and can penetrate, alterandole, in many crystal structures; These salts help to destroy the porous materials, that quickly tend to pulverize.
The cement may contain some alkali soluble salts, In addition to sulphates, nitrites and nitrates, added to the wording to get the particular characteristics of the final product. If the wall, where does a responders with cement, is subject to moisture movement within the hair structure, It may happen that a proportion of soluble salts migrate to cement the original plaster. The different between cement and porosity Malta original can promote the crystallization phenomena occur against the latter, If it is more porous. If concrete action is indispensable and irreplaceable, You should use the one where the alkali content is less than 0.2–0,3 %.
The degradation due to salts is caused by dissolution phenomena, crystallization, hydration and Hygroscopicity; so if you are in a situation of thermodynamic stability damage encountered will be insignificant: There are cases where no special alterations were found in the walls despite the presence of salts, as the temperature and humidity conditions have remained unchanged for long periods of time. On the contrary the solubilization/crystallization phenomena, hydration/dehydration and the hygroscopic variations may cause the destruction of the material.
To resolve quickly and swiftly any problems of moisture and mould in walls, the most suitable product, eco-friendly, quickly and safely is without any doubt IgroDry.
The unique and innovative product ’ can heal in a few days all types of masonry.
Go to learn about exclusive offers you great opportunities.
From the wonderful thesis of Dr. Irene Franzin for UIN (International art college)