Measurement of salinity in building materials: a synopsis of current practices and potential innovations

This article provides an overview of the various techniques used to measure the salt content of building materials. The authors discuss the importance of salinity measurement, as high salinity can lead to degradation and damage in building materials. They then consider the traditional methods of salinity measurement, including conductivity measurement and chemical titration. While reliable, these methods are often time-consuming, destructive and may not provide a complete picture of the salt distribution within the material. The authors then focus on emerging non-destructive techniques that offer the potential for in-situ salinity measurement. These include X-ray Dffraction (XRD), Thermal Analysis and Fourier Transform Infrared Spectroscopy (FTIR). Each method is discussed, with the authors highlighting its advantages, limitations and potential applications. The article concludes with a discussion of the future of salinity measurement in building materials. The authors propose a multimodal approach combining traditional and new methods to achieve more accurate and comprehensive salinity measurements. They also emphasise the need for further research and development in this area, particularly to improve the accuracy and reliability of non-destructive techniques.


Introduction
To maintain the structure of a building over time, it is necessary to understand the chemical and physical processes that affect its durability and stability.However, important factors such as the salts present in the walls and plasters of buildings are often overlooked.The most common salts are sulphates, chlorides, carbonates, nitrates and, less commonly, nitrites.The dominant cations are calcium, sodium, magnesium and potassium, which are readily soluble in water as crystalline substances [1].
Salts in buildings come from many sources.They can be present in building materials such as bricks, mortar, concrete or plaster.Salt can also come from the ground when moisture from the ground penetrates the walls, or from the air when the building is exposed to salty air, especially near the sea.Other sources of salt include rainwater, sewage, fertilisers and air pollution [2].For salt to appear on the surface of the wall, it has to get there.This usually happens through capillaries in the building material, where water containing dissolved salts penetrates the structure of the wall through small pores and capillaries.The damaging effect of salts on buildings is mainly due to crystallisation or hydration processes, which create large mechanical pressures in the pores of the material when there is no freedom of movement to increase the amount of the chemical compounds.When salts crystallise from a saturated solution in the pores of the building material, a so-called crystallisation pressure is created, and when salt crystals hydrate (combine with water molecules) in the pore space, a so-called hydration pressure is created [3].The presence of soluble salts not only changes the appearance of the structure when they become visible on the surface as efflorescence, but can also cause serious damage when they crystallise in pores and capillaries.All soluble salts can seriously deteriorate the condition of porous materials, leading to the breakdown of their structure and irreversible loss of cultural heritage.For this reason, it is common to observe spalling, flaking, alveolation, disaggregation or decohesion on historic stones, often caused by the presence of large quantities of sulphates, chlorides, calcium, sodium, potassium and magnesium nitrates.The effect of crystallisation of soluble salts in a given material depends on the type of salt, the atmospheric conditions and the characteristics of the stone.Various factors, such as the size and distribution of voids in the stone, determine its resistance to the pressure generated by the salts during the crystallisation and hydration processes.Identification and characterisation of the salts present in damaged masonry is essential both for diagnosing the causes of deterioration and for developing conservation measures [4][5].
The deterioration of buildings due to salt crystallisation is a complex phenomenon.Although there is a large body of literature discussing and describing this deterioration, most of the work is primarily qualitative.Some phenomena, such as crystallisation pressure, are mentioned, but in-depth quantitative research is lacking.The identification and characterisation of salts present in damaged masonry is very important both for diagnosing the causes of deterioration and for developing conservation measures.

The preparation of wall/plaster samples determines the method of salt determination
Methods for determining salts in building walls vary and depend on the specific needs of the study.Different research methods are used for this purpose.The first challenge in studying salinisation is to prepare samples for analysis.This involves taking samples from walls and plasters and preparing them properly for testing.There are several ways to take samples, for example taking surface samples by rubbing with absorbent materials.Another method is to take a drilling sample, where a hole is drilled into the wall and the material removed is collected and analysed.Samples must be properly prepared for analysis, which may include drying, grinding to a powder, dissolving in a suitable solvent and then filtering or evaporating [6].
The method of salt identification is dictated by the way wall/plaster samples are prepared.These methods can be divided into 2 groups.The first group includes water extraction from plasters, where water soluble salts are determined.The second group is the examination of plaster samples in the solid state.The methods used for chemical determination of salts in plasters are presented in figure 1.

Methods for the measurement of salt content in water extracts from plaster and walls of buildings
There is no clear indication of which method best reflects the true salinity of building materials.The simplest method of measuring salinity in water extracts from plaster and building walls is to measure conductivity.Conductivity meters, sometimes called conductometers, are devices that measure electrical conductivity, or the ability of a material to conduct an electric current.Their principle of operation is simple: conductivity is measured by passing an electric current through the material and measuring the resistance it encounters [7].In the context of diagnosing problems with salt in buildings, conductivity meters can be useful because salt solutions conduct electricity better than pure water.Therefore, areas of the wall that are wet and contain a lot of salt may have a higher conductivity than areas that are wet but do not contain a lot of salt [8].Conductivity meters are a diagnostic tool that do not provide a direct measure of salt content, but can help identify areas that may require further investigation.Conductivity measurements can give an indication of the presence of salts, but usually require confirmation by more direct methods of chemical [9].Another simple method for determining salt ions in water extracts from walls is titration.Titration is a technique used in analytical chemistry to determine the concentration of a specific substance in a solution.Different titrants are used to determine the amount of salts present.Silver nitrate is used to measure the concentration of chloride ions and barium chloride is used to measure the concentration of sulphate ions.Calcium and magnesium are determined by titration with ethylenediaminetetraacetic acid (EDTA) in the presence of calcein and murexide as indicators.The amounts of Fe2O3 and Al2O3 are determined by titration with barium diphenylaminosulphonate and dithizone as indicators [10].Titration methods are simple but time consuming.In addition, no standard chemical analysis has been developed, as several studies have found different concentrations of analyte depending on the type of mortar [11][12][13].Some salts may react with each other or with other constituents, which may affect the results.A disadvantage of titration methods is also the sensitivity of the indicators, which are not always perfectly sensitive to specific salts, which can lead to inaccurate results.Some substances present in extracts can interfere with titration, for example by reacting with the titrating solution or by affecting the operation of the indicator.In addition, some salts may require a very specific pH range for titration, which may be difficult to maintain [14].
Spectrophotometric methods are another methods for determining ions.The analytical technique used to determine the concentration of a substance in a solution based on the interaction of light with that substance is called spectrophotometry.There are different types of spectrophotometry.UV-VIS spectrophotometry measures the absorption of light of a specific wavelength by a solution.If the salt in the water extract absorbs light in the UV or visible range, this technique can be used to determine it.These methods can be used to determine both cations and anions.However, not all ions can be detected directly by UV-VIS spectrophotometry, as not all ions absorb light in the UV or visible range [15].For example, nitrate ions (NO3 -) can be detected by the Griess method, in which nitrates are reduced to nitrites and then the nitrites are reacted with Griess reagent to form a compound of intense red colour, which can be detected spectrophotometrically. Similarly, phosphate ions (PO4 3-) can be detected by the phosphomolybdenum method, in which phosphates are reacted with molybdenum acid to form a compound of intense blue colour, which can be detected spectrophotometrically [16].
All of these methods require proper sample preparation and spectrophotometer calibration, as well as the preparation of a calibration curve.At present there are fully automated UV-VIS spectrophotometers that do not require the preparation of a calibration curve and have a fully automated sample preparation process, precise application, manufactured by companies such as Hach Lange or Hannach.Their disadvantage is the cost of analysis.
The problem with spectrophotometric determination of salts in water extracts from walls/plasters is interference.Many substances can absorb light in the same wavelength range as the ions being analysed, which can cause interference and falsely high results.For example, organic chemical compounds present in the extracts can absorb UV and visible light, which can affect the obtained results [17].
Atomic Absorption Spectrophotometry (AAS) measures the absorption of light by metal atoms in a gaseous state.It is particularly useful for determining concentrations of metals such as sodium, potassium, calcium, magnesium that may be present in salts.Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) measures the emission of light from metal atoms or ions in plasma.This technique is also used to determine metal concentrations.ICP-OES, AAS and ICP-OES methods allow for the determination of only metal ions, they are costly [18].
Currently, the standard analytical equipment is ion chromatography (IC), which is a reliable method for analysing many different anions and cations in solutions (or water extracts of building materials) to meet international standards and regulations.The IC technique identifies ions and provides accurate quantitative analysis.IC gives very accurate and reliable results, but requires constant care and calibration before each series of measurements.Phase association data processing shows how the chemical and mineralogical characteristics of the leached solution can be interpreted in terms of the relative amounts of a few key elements.IC has somewhat limited accessibility and higher cost [9].
The research methods discussed above have their limitations, mainly due to the diversity of water extracts from buildings materials, which can vary greatly in terms of chemical composition.This can make standardisation of analytical procedures difficult.

Methods for measuring the salt content of building materials in the solid state
Another approach is to study the salt content of plaster samples in the solid state.
Electrical methods are often used to study salts in plasters and walls.They are based on measuring the electrical properties of materials, which can change depending on the presence of salts.However, it is important to remember that the accuracy of these methods is lower than that of other techniques, and the results of measurements may also depend on other characteristics of the material being tested, such as the type and concentration of salts.Therefore, the results obtained with these instruments should be treated as qualitative rather than quantitative [19].
Optical microscopy is a tool that uses polarised light to study the mineralogical composition of walls and plasters and the spatial relationships between their components.This technique allows the identification of various constituents such as aggregates, binders (including anhydrous clinkers), additives (both organic and inorganic) and pozzolans.Optical microscopy can also be used to identify porosity and physical problems such as cracking or the formation of secondary minerals, including salts.However, it should be remembered that optical microscopy has its limitations -its resolution is approximately 1 micron and the image it provides is two-dimensional [20].
Scanning Electron Microscopy (SEM) combined with Energy Dispersive X-Ray Analysis (EDX) allows detailed examination of the morphology and microstructure of the binder and aggregate components and any reaction products formed between the binder and aggregate.This technique allows the identification of elements such as carbonates and hydrates, as well as the character, form and structure of the binder.SEM/EDX also allows phase changes to be identified.As with optical microscopy, a key challenge is to ensure the representativeness of microstructural observations on a small scale.SEM/EDX allows local qualitative and quantitative analysis of the elemental composition of the observed phases.From this it is possible to calculate stoichiometric mineral formulas [21].
X-ray diffraction (XRD) is another technique that allows the identification and differentiation of binders and types of aggregates in mortars if they are crystalline.For example, cement can be distinguished from natural hydraulic lime only by mineralogical analysis.The same applies to other mineralogical phases if their content in the mortar is high enough (about 3% by weight) [22].
Thermal analysis is a technique that allows the study of plasters using three basic methods: thermogravimetry (TG), differential thermal analysis (DTA) and differential scanning calorimetry (DSC).Each of these techniques has its own peculiarities, but all provide similar information based on the physical transformations that occur in compounds when heated under controlled conditions [23].Thermogravimetry (TG) involves measuring the change in mass of a sample as it is heated.The loss of mass is associated with specific decomposition processes that occur in materials under the influence of heat.For example, gypsum can be identified by a mass loss of approximately 26.5%, which is the result of conversion to anhydrite.
Differential Thermal Analysis (DTA) and Differential Scanning Calorimetry (DSC) are two of the most commonly used methods of thermal analysis.In the case of DTA, a graph is produced showing the temperature difference between the sample and a neutral standard (usually Al2O3), which are heated simultaneously and at the same rate.Endothermic peaks are recorded when the temperature of the standard continues to rise and that of the sample does not.At this point, the sample is absorbing thermal energy which is being used to drive decomposition processes or mineralogical transformations [24].
Differential Scanning Calorimetry (DSC) works on a similar principle to DTA.However, instead of measuring temperature differences, in DSC energy is added to keep the sample and reference material (empty crucible) at the same temperature.The energy consumption is recorded and used as a measure of the heat value of the thermal transformations taking place in the sample.
Despite its usefulness, thermal analysis can sometimes lead to certain ambiguities.These can arise from the fact that some phases may undergo decomposition or phase changes at similar temperatures.For example, the loss of water from calcium silicate hydrates (CSH) can occur at a temperature similar to that at which changes occur in some clays [25].
In addition, there may be a problem in correctly identifying some compounds.For example, portlandite (Ca(OH)2) and magnesite (MgCO3) can confused as both can decompose at a temperature of around 520°C [26].In such cases, thermal analysis may require additional confirmation of results by other investigative techniques.
Fourier Transform Infrared Spectroscopy (FTIR) is a technique that not only identifies the major mineral phases in the binder, but also allows the detection of even small amounts of impurities and additives.Only small amounts of sample are required to perform FTIR measurements.The method is based on the interaction between the applied infrared radiation and the molecules in the compound.
Experimental studies indicate a great potential for the use of infrared spectroscopy (FTIR) for the quantitative determination of carbonates in materials, which is of great importance in the study of mortars.In the research published to date, infrared spectrometry is rarely used as a stand-alone technique.It is usually used in combination with other methods such as X-ray diffraction (XRD), thermogravimetry (TG), differential scanning calorimetry (DSC) or differential thermal analysis (DTA) [25].FTIR allows easy identification of salts such as nitrates, sulphates, oxalates.It is also possible to identify silicates, including calcium silicate hydrates.This method also has the potential to identify hydraulic binders.In addition, Ondruška et al. identified the presence of organic compounds, suggesting that FTIR may be useful in identifying paints, pigments and other organic additives.This technique requires very small samples, with a mass of less than 0.5 mg and an area of less than 0.5 mm 2 , making it an invaluable tool for the study of valuable objects such as murals or architectural textiles [24].
One of the most advanced methods of measuring the salt content of building materials is nuclear magnetic resonance (NMR).The method is based on the principle of nuclear magnetic resonance, in which the nuclei of certain atoms, such as hydrogen, absorb and re-emit electromagnetic radiation when placed in a magnetic field.In the context of salinity measurement, NMR can be used to detect the presence and amount of water in a material.This is because water molecules contain two hydrogen atoms, which are sensitive to the NMR process.If a material is salty, it will contain salt ions dissolved in this water.These ions can affect the behaviour of the water molecules in the NMR process, which can be detected and measured.The NMR method involves placing a sample of the material in a strong magnetic field and then applying a radio frequency pulse.This causes the hydrogen nuclei in the water molecules to resonate.The resonance is detected and analysed to provide information about the water content and its interaction with salts in the material.One of the advantages of using NMR is that it can provide information about the distribution of water and salts within a material, not just their overall levels.This can be important in understanding how salts affect the material and how they might be managed or removed.However, it's important to note that while NMR can provide valuable information about the presence and distribution of salts, it may not provide a direct measure of salinity.Other methods, such as chemical analysis, may be required to accurately quantify salinity [27].

Conclusions
In summary, different methods of investigation have their own characteristics and limitations, but all provide valuable information about the composition and properties of mortars.The choice of method depends on the characteristics of the material under investigation and the purpose of the study.In many cases, the best results can be obtained by combining several techniques, giving a more complete picture of the properties and composition of the mortar.
It's important to remember that none of these techniques is perfect and each has its limitations.For example, some methods require destructive sampling, which may not be possible in the case of valuable or fragile objects.Other methods may require complex equipment and specialised training, which can be costly and time consuming.
Ultimately, the choice of appropriate method depends on many factors, such as the availability of equipment, the skills and experience of the researcher, and the specifics of the material under investigation and the purpose of the study.In any case, it is important to fully understand the limitations of the chosen method and to interpret the results with appropriate caution.

Figure 1 .
Figure 1.Scheme of methods for determining salinity in buildings materials.