Materials characterization of historical structures: A review

Abstract

The protection of historical artifacts that hold great significance in the fields of art, architecture, history, and culture ensures the preservation of cultural heritage and safeguards the shared past of humanity. Proper material selection and appropriate application methods are crucial for maintaining the aesthetic and structural integrity of historical structures and ensuring their transmission to future generations. Understanding the composition and properties of these materials is essential for making the right material choices in restoration processes. This study aims to provide a detailed evaluation of analytical methods used in the characterization of historical building materials and to synthesize the existing findings in the literature in a coherent manner. At the same time, it aims to provide a guide for researchers in the field in choosing the methodology by revealing the strengths and limitations of these techniques. Thus, it will contribute to the establishment of a data-based basis for future scientific studies. In this context, the objectives of the methods used to determine the properties of historical building materials, the processes of sampling and preparing materials for testing, the characteristics of the devices used in the tests, as well as the obtained analysis results and evaluations were reviewed.

Graphical abstract

Keywords

historical structures / mortars / physical properties / chemical properties / mechanical properties

1 Introduction

Historical structures serve as a bridge to understanding the lives, cultures, and technological developments of past civilizations. In other words, beyond their aesthetic and architectural features, these structures also form the building blocks of human history [1,2]. The long-term survival of historical structures despite time and adverse environmental effects is directly related to the properties of the building materials used.

Generally, local resources from the region where the structure is located are used as building materials, such as natural stone and mortar [3]. The selection of materials varies depending on their properties. For example, lightweight and easily workable limestone is used to improve thermal insulation and resistance to environmental effects [4]. Volcanic rocks such as basalt and andesite are commonly used for roads and city walls [5,6]. Granite, known for its high hardness and resistance to harsh weather conditions, is typically used in roads and bridges [7]. Travertine, due to its lightweight and porous structure, is generally used for coatings and flooring [8]. Lastly, marble, which is both highly durable and lustrous, is primarily used in columns and monumental structures [3].

In addition to natural stones, mortars play a crucial role in strength and durability and are mostly lime-based materials. For example, Khorasan mortar, which was used in structures such as baths, cisterns, aqueducts, and water wells during the Byzantine, Seljuk, and Ottoman periods, contains brick and tile powder mixed with lime [9,10]. Adobe mortar is composed of a mixture of clay and straw [11]. Ghiara mortar, which has been widely used in structures in the Catania region of Italy, consists of a mixture of lime and a reddish aggregate with pozzolanic properties. This aggregate is made up of various grain sizes that result from the thermal effects of lava flows covering paleo-soils [12]. Similarly, in Azolo mortar, which is used in the same region, ground basalt is used as an aggregate [13].

Determining the existing strength and durability properties of historical structures and applying the appropriate materials in restoration works is crucial for preserving their aesthetic and structural integrity and ensuring their transmission to future generations. In this context, it is essential to accurately determine the chemical composition, as well as the physical, thermal, and mechanical properties of historical building materials [3,14]. Various methods are used to assess the material properties and load-bearing systems of existing structures. These methods are generally classified into two categories: destructive and non-destructive. Destructive methods involve taking samples from the structure or making small-scale interventions, whereas non-destructive methods do not cause any damage to the structure and do not require physical intervention [15–17]. Determining the properties of historical building materials with minimal intervention, without causing any damage, is of vital importance. In the literature, various analyzes are carried out to determine the pore structure, mineral composition, thermal stability and strength of materials. For example, mercury intrusion porosimetry experiment (MIP) [18] to determine the pore structure, size distribution and porosity of materials, thermogravimetric analysis (TGA) [19] to determine the thermal resistance, X-ray diffraction (XRD) [20] to determine the chemical properties, mineral composition and crystal structures, X-ray fluorescence (XRF) analysis [21], Fourier transform infrared spectroscopy (FTIR) [21], Raman spectrometry [21], nuclear magnetic resonance (NMR) analysis [22], and inductively coupled plasma (ICP) analysis [23], ultrasound transmission velocity (UPV) [24], Schmidt hammer [25] and flat-jack [26] tests are used to determine their strength. In addition, image analysis of materials can be performed by optical microscope (OM) [27] scanning electron microscopy (SEM) [28] and infrared thermography [29] methods.

According to the studies in the literature on historical structures, a study by Mu et al. [1] examined the acoustic properties of historical structures, a study by Panakaduwa et al. [2] discussed the difficulties encountered in the restoration of historical structures and energy efficiency, a study by Yan and Wang [3] investigated innovative methods used to prevent deterioration in historical structures over time, a study by Hao et al. [14] focused restoration techniques, a study by İsafça et al. [9] studied Khorasan Mortar, and a study by Kumanan and Sofi [11] studied historical clay-mud brick mortars. In review studies conducted on historical building materials, only a limited number of studies were found on material characterization. The review by Zendri et al. [15] focused solely on non-destructive methods used in historical structures, while the study by Zhao et al. [21] discussed some chemical techniques applied to historical structures.

There is a need for detailed studies in the literature that provide a holistic and comparative analysis of the physical, thermal, chemical, and mechanical properties of historical building materials. This study aims to comprehensively compare the methods used to determine the properties of historical building materials and systematically compile the results obtained from previous research. Additionally, this study is intended to serve as a reference for future research by highlighting the advantages and limitations of these methods, thereby guiding the selection of appropriate techniques. Moreover, the methodological approaches presented in this study are expected to enhance efficiency in the analysis of historical structures in fields such as engineering and architecture, helping to reduce costs and facilitate the development of scientifically grounded projects. Similarly, this study is expected to contribute to restoration efforts by supporting the selection of suitable materials and the development of proper application techniques, thereby aiding in the preservation of cultural heritage for future generations.

2 Analyze methods for physical and thermal properties

In this section, studies and results obtained on MIP analysis, which is widely used in the literature to determine some physical properties of historical building materials, and TGA analysis, which is widely used in the literature to determine the thermal properties of materials, are mentioned.

2.1 Mercury intrusion porosimetry analysis

The MIP analysis method is used to determine the physical properties of materials, such as porosity, pore structure, size distribution, pore volume, surface area, and average pore diameter. This method is based on the principle that mercury can penetrate the pores of a material due to surface tension, aided by externally applied pressure [13,30,31]. Additionally, the results obtained from this method can be used to indirectly estimate the effects of porosity on the mechanical and durability properties of materials [32,33].

When examining studies in the literature where this method was applied to historical building materials, it was observed that most analyses focused on determining the pore structure, strength, and water permeability of the materials. In the study conducted by Ponce-Antón et al. [34], it was reported that the pore structures of mortars taken from Amaiur Castle directly affected their water absorption behavior through capillarity. As a result of the MIP analysis performed on the samples, two main pore size distributions were identified: 0.01–1 and 1–10 μm. In another study conducted by Ponce-Antón et al. [30], mortar samples taken from the San Martín Arch were examined. It was observed that one of the mortar samples exhibited a homogeneous pore distribution (0.01–1 μm) (Fig.1(a)), while the other had a more heterogeneous structure (Fig.1(b)). This difference was attributed to the presence of quartz particles in the mortar.

In the study conducted by Tripathi et al. [35], MIP analyses applied to Chettinad plasters, a traditional type of mortar, revealed that these plasters possess a well-ordered microstructure that prevents the formation of microcracks and enhances durability. Belfiore et al. [13] examined lime-based Azolo and Ghiara mortars and found that Ghiara mortars had a higher porosity rate (46.6%). This indicates that Ghiara mortar has a greater water absorption capacity. In contrast, Azolo mortars exhibited a denser structure, resulting in higher mechanical strength. In a study conducted by Rispoli et al. [36], the porosity characteristics of coating and bedding mortars used during the Roman period were analyzed. The porosity of coating mortars ranged between 8.6% and 10%, while bedding mortars had a porosity of approximately 19.3%. The low porosity of coating mortars contributed to higher mechanical strength, whereas the high porosity of bedding mortars increased water permeability. In the study by Al-Omari and Khattab [37], the porosity distribution of mortar samples taken from Al-Omariya Mosque was determined to be within the 0.5–5 μm range. These results indicate that the porous structure of the mortars leads to high water absorption rates, which may negatively affect their durability properties.

2.2 Thermogravimetric analysis

TGA, examines the mass loss of a sample as it is subjected to temperature changes and is used to determine the thermal resistance, degradation properties, and thermal stability of the material [27,33,38]. It is also employed to examine the organic and inorganic structural properties that affect the degradation characteristics of the sample [39]. Mass losses in the material occur due to physical and chemical events such as the evaporation of certain components, thermal degradation, dehydration, and pyrolysis [27,40]. In some studies, mass increases due to oxidation in materials have also been observed [41,42]. Mass loss behavior, degradation processes, and thermal stability are analyzed through the thermograms obtained from the measurements. Features such as multi-stage degradation and surface oxidation are determined through the thermogram [42,43].

Differential thermal analysis (DTA) is used to determine the thermal characteristics of materials by measuring the endothermic and exothermic energy changes that occur during phase transitions, such as melting, evaporation, and chemical reactions [42]. In this method, the sample and the reference material are exposed to the same heat, and the temperature difference between them is continuously measured and analyzed by plotting time-dependent graphs of these temperature differences [43]. TGA and DTA analyses are used together to examine mass losses, oxidation processes, and phase transitions in detail, due to material degradation [27]. Differential scanning calorimetry is used to determine the components and decomposition temperatures of historical building materials, such as mortar and brick, and to analyze their thermal properties [36,43].

During the thermal analyses performed on the materials, the physical and chemical events occurring in each temperature range are listed below.

1) 100–200 °C: Adsorbed water evaporates. As a result, a weight loss of 1% to 3% is observed in the materials, and in samples with high moisture content, this loss can reach up to 5%, as reported by the researchers [27,38,44].

2) 200–600 °C: Dehydration of clay minerals occurs. As a result, a weight loss of 4% to 10% is observed in the materials, with an average weight loss of 8%, as reported by the researchers [45,46].

3) 573–575 °C: The α–β phase transition of quartz is recorded as a low-density endothermic reaction in the DTA analyses, as noted by the researchers [31,47].

4) 600–900 °C: The decomposition of calcite (CaCO₃) results in a high amount of CO₂ emission, leading to a weight loss of 20% to 40%. In materials with high calcite content, this loss is around 30%, as reported by the researchers [39,48,49].

The temperature ranges and weight loss percentages of the minerals identified in the thermal analyses conducted in the literature are shown in Fig.2. According to Fig.2, the calcite mineral shows a weight loss of 20% to 70% in the temperature range of 600–900 °C, the quartz mineral shows a weight loss of 0.5% to 1% in the temperature range of 550–600 °C, portlandite shows a weight loss of 2% to 3% in the temperature range of 400–550 °C, gypsum shows a weight loss of 0.8% to 2.7% in the temperature range of 100–150 °C, illite mineral shows a weight loss of 2.4% to 7% in the temperature range of 100–600 °C, kaolinite mineral shows a weight loss of 1.5% to 2.5% in the temperature range of 400–500 °C, magnesium carbonate shows a weight loss of 3% to 6.5% in the temperature range of 400–550 °C, the vaterite mineral shows a weight loss of 10% to 20% in the temperature range of 600–800 °C, and the mullite mineral shows a weight loss of 0.5% to 1.5% at 900 °C.

3 Analyze methods for chemical properties

Chemical characterization of materials is important to understand their composition, behavior, and durability. Various advanced analytical techniques such as XRD, XRF, FTIR, Raman spectroscopy, NMR, and ICP analysis are widely used to determine basic material properties. In this section, studies conducted in the literature to determine the chemical properties of historical building materials are reviewed and the analyses and results obtained are discussed.

3.1 X-ray diffraction analysis

XRD analysis is a method commonly used for the characterization of historical building materials, particularly for determining the phase contents and mineral compositions of crystalline materials. In this method, X-rays sent to the sample are reflected from atomic planes and diffract at specific angles. These reflections are recorded as diffractograms, and the peaks in the graph provide information about the material’s mineral composition [37,39,44,50,56–64]. The results obtained from XRD analyses in studies related to this topic in the literature are provided in Tab.1. According to the results of the studies in Tab.1, it has been determined that the main minerals in mortars used in historical buildings are mostly calcite and quartz. Additionally, various researchers have reported that phases such as feldspar, hematite, mica, and illite also affect the structural properties of historical buildings [27,32,36,45,54,60,61].

In the studies, calcite, which was identified as the main phase in most of the historical buildings examined, increases the strength due to carbonation reactions and contributes to the durability of the structures by enhancing their resistance to atmospheric conditions. Researchers have also stated that lime-based mortars provide longer-lasting protection to the structure against changes in humidity and temperature. Another main phase, quartz, increases the abrasion resistance of the structure due to its hardness [39,55,56,62].

Minerals such as feldspar, hematite, and illite are important components that increase the mechanical and thermal durability of building materials. Feldspar, with its resistance to high temperatures, protects the structure from sudden thermal changes, while enhancing the structural stability in fired brick and stone buildings. On the other hand, in some ancient buildings, mortars fired at low temperatures, while providing a lighter structure due to their porous nature, somewhat reduce thermal durability. This indicates that in buildings, especially in hot climate regions, the use of mortars fired at low temperatures allows the structure to adapt to the climatic conditions, maintaining a balance between durability and lightness in the structural design [40,60]. Hematite, with its reddish tone, helps to preserve the original appearance of the buildings, particularly in facades, by blending with the historical texture. Minerals like hematite are of great importance for restoration projects in terms of preserving the original colors and textures of buildings. Therefore, the use of hematite in restoration work supports maintaining the appearance of the buildings as they were when originally constructed, preserving the aesthetic integrity of the historical texture [27,63,64].

3.2 X-ray fluorescence analysis

XRF analysis is conducted to determine the mineral and chemical composition of historical building materials. This technique is based on the principle of characteristic X-ray emissions emitted by atoms. When atoms are excited by high-energy X-rays, electrons from the inner orbitals are ejected, and to fill this vacancy, electrons from outer orbitals move to the inner ones, resulting in the emission of X-rays specific to each element. Since the wavelengths of X-rays are different for each element, the type of element can be identified [78,79].

The basic components of building materials generally include SiO₂, CaO, Al₂O₃, Fe₂O₃, MgO, and K₂O. These components provide insight into the strength and durability properties of the materials [54,73]. For example, the SiO₂ (quartz) content of the materials increases their durability [52]. In the study conducted by Sağın et al. [54], it was stated that the high SiO₂ content in ancient Cocciopesto mortars contributed to an increase in their strength. The presence of CaO indicates the presence of hydraulic or air lime in the material [78]. In the study conducted by Ponce-Anton et al. [34], it was reported that high CaO levels indicate the presence of calcite or dolomite in the material, which increases the durability and binding of the mortar. The presence of Al₂O₃ indicates the presence of clay minerals, and aluminosilicate formations provide durability during the firing process of the material [68]. Fe₂O₃ ensures that the color of the mortar or brick appears in red tones and can reveal the relationship between color and durability in historical structures [33,80]. Mortars with low CaO and MgO ratios show less cementation. High SO₃ content in some mortars indicates the presence of gypsum and affects the longevity of the mortar [33,48].

The results obtained from XRF analyses in studies related to the topic in the literature are presented in Tab.1. In some studies, XRF analyses were supported by XRD, and comparisons were made between mineral phases. Quartz, calcite, muscovite, feldspar, and various clay minerals were frequently identified in brick and mortar samples. The presence of minerals containing aluminosilicates, such as muscovite, in lime mortars indicates the durability developed during the firing process of the material [32–34,73]. In studies examining the relationship between the color and chemical properties of mortar and brick samples, it was noted that a high Fe content creates red tones, while MgO content leads to lighter shades. While color properties are not always directly correlated with durability and strength, the effect of elemental concentrations on color was observed in these studies [33,64].

3.3 Fourier transform infrared spectroscopy analysis

Another method used to determine the chemical and mineral components of historical building materials such as mortar, plaster, and stone is FTIR analysis. In this method, the amount of infrared radiation absorbed by chemical bonds between molecules is measured. Each chemical bond vibrates at a specific wavelength and exhibits a unique adsorption pattern. The signals obtained are analyzed using Fourier transformation to create a characteristic spectrum for the substance [81,82]. In the literature, studies for FTIR analysis have wavelength ranges of 4000–450, 4000–500, 4000–550, and 4000–600 cm⁻¹, but the wavelength range of 4000–400 cm⁻¹ is most commonly preferred [40,56,62,66,78,83]. Based on the studies, the wavelength ranges for various minerals are shown in Fig.3. As seen in Fig.3, basic minerals such as calcite, quartz, and gypsum have been commonly used in historical buildings. Calcite has been identified as the primary component of lime-based materials in most samples [51,56]. Quartz is frequently used as a filler material in mortars, while gypsum appears as a product of degradation, especially in samples exposed to environmental effects [47,84]. In the study by Qian et al. [56], it was stated that these minerals are used as primary binders or fillers in traditional building mortars, improving strength and durability.

Secondary minerals such as dolomite, feldspar, hematite, kaolinite, aragonite, and vaterite contribute to the physical durability and aesthetic properties of the structure [78,80]. According to the literature, dolomite and feldspar have been used as mineral additives to enhance the durability of mortars [51,66,73]. Hematite causes color changes, providing an aesthetic contribution to the structure, while kaolinite and aragonite facilitate the adaptation of the material to environmental conditions such as moisture and heat [40,60,72]. Furthermore, in some structures, the use of organic additives such as starch and proteins has been observed, which positively contribute to the stability of the mortar, enhance its moisture retention capacity, and support the overall durability of the structure [35,56,78].

3.4 Raman spectrometer

Raman spectrometry is one of the techniques used in the analysis of historical building materials and is employed to identify the presence of various minerals, organic substances, and pigments. Raman spectrometry is based on the scattering of photons from a laser source by molecules or crystal structures in the sample. During this process, changes occur in the energy of the photons. These changes result in an energy shift called the Raman shift, which reflects the vibration modes specific to the sample. Raman spectroscopy holds an important place in the chemical characterization of building materials due to its advantages of non-contact analysis, high resolution, and the ability to cover a wide range of materials [86].

In the studies conducted, the crystal structures of material compositions and the characterization of organic molecules were determined using different devices and laser wavelengths. Minerals such as calcite, quartz, dolomite, aragonite, and vaterite were generally detected, while organic compounds were found in some samples. The characteristic bands include 711 and 1086 cm⁻¹ for calcite, 1091 cm⁻¹ for vaterite, and 217 and 287 cm⁻¹ for aragonite [62,86]. In the study conducted by Singh et al. [62], lime samples taken from the Alampur temples in India were subjected to Raman spectroscopy analysis. Bands such as 1342, 1450, and 1527 cm⁻¹, belonging to the phthalocyanine blue pigment, were detected in the analysis of organic compounds. It was reported that these pigments originated from the paints used in festivals. For carbonaceous materials, in the Raman spectrum shown in Fig.4, the band at 1348 cm⁻¹ and the band at 1590 cm⁻¹ were observed in the study by Ponce-Antón et al. [30].

3.5 Nuclear magnetic resonance analysis

NMR spectroscopy is an effective method used for the detailed analysis of the physical, chemical, and mechanical properties of building materials. Various academic studies highlight the comprehensive data collection capability of this method in both laboratory and field applications. In the study conducted by Luzar et al. [87], the hardening process of air lime and white cement-based injection mortars used to reinforce historical masonry structures was investigated using NMR. Limestone powder and superplasticizer were used to improve their mechanical properties. NMR analysis revealed that the hydration time of the produced mortars progressed slower than that of cement pastes, and this period increased as the amount of cement increased. In the study conducted by Sharma et al. [88], the analysis of historical porous building materials was carried out. The pore structure of sandstone and bricks was examined using NMR. The internal structure of the material was analyzed by measuring proton relaxation times and compared with MIP analysis. According to the MIP results, it was stated that NMR provided better results. In the study conducted by del Hierro et al. [89], ²⁸Si and ²⁷Al-MAS-NMR (Magic Angle Spinning-Nuclear Magnetic Resonance) methods were used to analyze the mineralogical and thermal properties of lime mortars in a historical building located in the village of Magán, Spain. Amorphous phases and pozzolanic reactions were determined, and this was supported by FTIR and Raman spectroscopy methods. It was observed that the mortars hardened through pozzolanic reactions and contained an aluminosilicate structure. In the study conducted by Poli et al. [90], the water content in porous building materials was analyzed with a portable NMR device. In the analyses carried out on noto stone and commercial bricks, a linear relationship was found between the NMR signal and the amount of moisture in the samples. In the study conducted by Singh et al. [91], phosphate groups were detected in the calcite structures using 31P NMR analyses of the lime mortar from Janjira Sea Castle. It was determined that phosphate groups were bound to calcium ions and were regularly located in the mortar. The NMR results were confirmed by FTIR and XRD analyses, and the molecular interactions of phosphate compounds were explained in detail. In the study conducted by Shi et al. [92], low-field NMR (LF-NMR) techniques were used to analyze the pore structures of samples obtained from an ancient city in China, particularly to examine the pore size distributions and porosity levels of historical city wall bricks in detail. The results showed that there were different pore structures between white and blue bricks. In the LF-NMR measurement of blue bricks, it was determined that the porosity was lower than in white bricks due to paramagnetic iron minerals.

3.6 Inductively coupled plasma analysis

ICP analyses are methods used to examine the chemical components and durability properties of historical building materials. With this method, the ratios of elements such as Ca, Mg, Si, Al, and Pb in the material can be determined. Atomic Emission Spectrometry (ICP-AES) is based on the principle of converting samples into a solution, ionizing them at high temperatures, and determining the elements according to the wavelengths of the light emitted by the atoms. Laser Ablation Mass Spectrometry (ICP-MS) allows the material to be evaporated from the surface by the laser ablation method, and the ionized atoms are measured in the mass spectrometer, revealing the accumulation of heavy metals that cause surface wear in detail [49,93].

In the study conducted by Paama et al. [49], the results of ICP-AES analysis performed on mortars from St. John’s Church, collected from different periods, showed that the main elements soluble in acid were Ca, Mg, Si, Al, Fe, Mn, and Pb. The calcium content of the mortars was observed to be between 11.82% and 14.86%, while the magnesium content ranged from 2.65% to 4.32%. In the study conducted by Ruffolo et al. [93], the results of ICP-MS analysis performed on black crust samples taken from different facades of Seville Cathedral showed significant accumulation of heavy metals such as As, Pb, Zn, Cu, Sb, Cr, Ni, Sn, and V. In particular, it was determined that the concentrations of elements such as Pb (606 ppm) and Zn (436 ppm) were higher in regions exposed to more traffic emissions compared to other areas. These findings indicate that black crusts reflect the effect of atmospheric pollution sources and can be used as a passive sampler in pollution analysis.

4 Analyze methods for mechanical properties

Destructive methods should not be used in determining the mechanical properties of historical structures. Instead, non-destructive or semi-destructive methods such as UPV, Schmidt hammer and flat-jack are used. In this section, studies conducted in the literature to determine the mechanical properties of historical building materials are examined and the analyses and results obtained are discussed.

4.1 Ultrasonic pulse velocity

The working principle of the UPV test, which is used to analyze the mechanical properties of historical building materials non-destructively, is based on measuring the transit time of ultrasonic waves sent through the material by a transmitting transducer to estimate its strength [24,58,94,95]. A visual representation of the UPV test is shown in Fig.5. High impact velocity readings are generally an indicator of high-quality concrete. The general relationship between concrete quality and impact velocity is given in Tab.2. A strong correlation can be established between cube compressive strength and impact velocity [94]. This relationship allows the prediction of the strength of structural concrete with an accuracy of ±20%, provided the aggregate type and mix proportions are kept constant [96].

Parameters such as the porosity of the material, unit volume weight, and micro and macro cracks directly affect the wave speed [58,95]. In the study by Al-Omari et al. [37], on Amaiur Castle and in the study by Özmen and Sayın [58], on Malatya Taşhoran Church, a strong relationship was found between UPV values and the density and strength of the material. It was determined that dense and solid structures provide higher UPV values, while porous and weak materials yield lower UPV values. In the study by Uygunoğlu et al. [94], UPV tests were performed on many different materials. It was found that properties such as porosity directly affect material performance. Wave speed was observed to be low in materials with high porosity, which negatively impacted mechanical strength.

4.2 Schmidt hammer

Another method used to analyze the mechanical properties of historical building materials non-destructively is the Schmidt hammer test method. In this method, the surface hardness of the material is measured, allowing for the indirect determination of compressive strength and elasticity modulus. During the test, ten measurements are taken from each surface, and the average of these measurements is used to calculate the rebound index. These values are directly correlated with mechanical properties such as compressive strength, density, and elasticity modulus. This relationship is typically expressed using statistical models or regression equations [58,64]. Fig.5 presents an image of the Schmidt hammer test.

The Schmidt hammer test, conducted in compliance with standards such as EN 12504-2:2012 or ASTM C805/C805M, is affected by various factors, including the moisture content of the material surface, cracks, and the type of device used (L-type or N-type) [64,95]. In a study by Borosnyoi-Crawley [64], a relationship between compressive strength (f_cm), rebound index (R), and density (ρ) was established as follows (Eq. (1)):

(1)f_{\mathrm{c}\mathrm{m}}=3.37\times {10}^{-5}\times {\mathit{R}}^{0.88}\times {\rho }^{1.28}.

In their study at Malatya Taşhoran Church, Özmen and Sayın [58] applied the Schmidt test at six different points to measure the rebound values of both original and restoration stones. The average rebound value of the original stones was 42, while that of the restoration stones was 28. Based on these data, the uniaxial compressive strength of the original stones was calculated as 57.83 MPa, while that of the restoration stones was 23.13 MPa. The same study also observed a relationship between rebound values and dry density: the density of the original stones was measured at 2.69 g/cm³, while that of the restoration stones was 1.52 g/cm³. Baeza et al. [97], highlighted the accuracy limitations of standard test methods for low-density, high-porosity stones and emphasized the need for special prediction models for such materials. Pinto et al. [38], used a device with a larger plunger surface to obtain more accurate results for soft materials. In mortar samples, the relationship between rebound values, aggregate content, and microstructure was examined. It was found that higher aggregate content and larger pores (10–100 μm) reduced the rebound values, whereas mortars with fewer macro voids exhibited higher rebound values. Additionally, the CO₂/H₂O ratio was observed to have an impact on rebound values.

4.3 Flat-jack

The flat-jack test is a semi-destructive method used to evaluate the mechanical properties of historical stone and brick structures in situ [32,38]. This method aims to measure the current stress state and deformation properties of the material by applying axial pressure to the structural elements [98]. Flat-jack tests are generally carried out in two stages: single and double. While single flat-jack tests are performed to determine the current stress state of the structure, double flat-jack tests allow for the calculation of mechanical properties such as bearing capacity, compressive strength, and Young’s modulus. In single flat-jack tests, the recovery stress is calculated using Eq. (2) [38,99].

(2)SS=Ke\times p\times Ka,

where SS represents the recovery stress; Ke denotes the flat-jack constant; p refers to the hydraulic pressure; and Ka is the ratio between the shear area and the effective area of the flat-jack. In double flat-jack tests, maximum compressive strength and Young’s modulus are calculated using various mathematical models [38].

According to ASTM and RILEM standards, certain assumptions must be made for the flat-jack test to be applied. These include the homogeneity of the material, the even transmission of pressure by the flat-jack device, and the negligible effect of lateral walls. Initially used in the field of rock mechanics, this test was later adapted for historical structures through the research. Today, the flat-jack test is widely employed in historical building conservation projects due to its ability to quickly determine the existing stress state of a wall and its advantage of allowing structural repairs after testing. However, due to the significant scatter in results, the test parameters must be adjusted specifically for each type of structure [98]. In a study conducted by Murano et al. [98], Elasticity modulus and compressive strength values were determined for different wall types using flat-jack tests. Similarly, in the study by Pinho et al. [99], flat-jack tests were applied to laboratory-prepared samples to analyze the mechanical structure of stone walls.

5 Visualization techniques

In this section, studies and results obtained with SEM analysis, OM, and infrared thermography (IRT), which are widely used in the literature to display the components and humidity status of historical building materials, are discussed.

5.1 Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) analysis

SEM-EDS is an advanced method used to analyze the microscopic structures and chemical compositions of mortar and plaster materials in historical buildings [27,100]. The electron beam sent to the surface of the sample with SEM produces backscattered electrons and characteristic X-rays [60,70]. The EDS system determines the presence and amount of each element by measuring the energy of these X-rays [56]. Thus, the SEM-EDS technique provides rapid and detailed information on both surface morphology and elemental distribution. The SEM-EDS technique provides critical information in determining the durability of building materials, binder-aggregate relationships, and components contributing to the longevity of building materials by examining the distribution of main elements and minerals in samples [27]. It is observed that Ca, Al, Si, and oxygen elements are frequently detected in SEM-EDS analyses [46,51,70,77]. Quartz, feldspar, and calcite stand out among the main minerals analyzed in the studies. In the studies, it has been observed that the parameters used in SEM-EDS analyses generally include accelerating voltage in the range of 15–20 kV, low vacuum conditions, and different magnification levels [52,68,70,80,85,100].

With the SEM-EDS technique, the density and distribution of basic elements such as calcium, silicon, aluminum, and iron are determined. Calcium-rich binders and silica-containing aggregates stand out as the main components that increase durability [84,94]. Studies support the accuracy of these analyses and detail the chemical composition of different mortar types. In the study conducted by Tian et al. [101], the ratios of minerals such as calcite, quartz, and feldspar were determined in mortars analyzed with SEM-EDS, and the durability of the mortars was improved with the addition of nanosilica.

These analyses, supported by the high-resolution imaging and detailed elemental analysis provided by the SEM-EDS method, offer important information for the protection of historical structures and the development of appropriate restoration techniques. This comprehensive data set obtained from SEM-EDS analysis guides the selection of materials to be used in preserving the original properties of building materials and in modern restoration projects [80,102].

5.2 Optical microscope

Examinations conducted with an OM analyze the mineral compositions and microstructures of mortar samples taken from historical buildings. This method is effective in describing the distribution and shape characteristics of common minerals, such as calcite, quartz, and feldspar, by examining thin sections under polarized light [27,30,55]. In the studies conducted, the micrographs obtained using the OM show that calcite and quartz minerals are dominant and reveal the distribution of porosity within the mortar [100].

The working principle of the OM is based on observing the optical properties of these components through the interaction of light with mineral surfaces. Under polarized light, the internal and crystal structure of the mineral is analyzed, and the unique optical properties of each mineral can be determined. With this method, the interference colors of minerals are identified, making the mineral identity more clearly defined [66,100,103]. This technique plays an important role in analyzing the shape, size, and distribution of grains, as well as determining the mineral compositions in thin sections. Among the minerals identified under the OM are calcite, quartz, feldspar, and, more rarely, biotite. These minerals provide critical data in understanding the mineralogical character and durability of structures [67,68]. This information serves as a guide in identifying the mineral components that can be used in restoration processes and in evaluating the material quality [103]. OM and SEM images of calcite, quartz, and feldspar obtained from some studies are shown in Fig.6.

5.3 Infrared thermography

IRT is a non-destructive analysis method used to detect thermal differences in building materials and identify deterioration, moisture problems, and other subsurface defects. Studies have shown that IRT is an effective tool, especially in the protection and evaluation of historical structures [58,107]. Inactive thermography was generally preferred in the studies. This approach analyzes material properties and moisture distribution using ambient temperature differences. In particular, measurements taken in closed areas or under shade provided more accurate results. IRT studies were generally carried out with high-resolution and high-sensitivity thermal cameras. Alexakis et al. [108] used a device with a 200 × 150 pixel resolution and 80 mK temperature sensitivity in their study. Solla et al. [107] used a device with a 640 × 480 pixel resolution, a spectral range of 8–14 µm, and sensitivity of ±2 °C or ±2% in their study. Özmen and Sayın [58] used a device with a 320 × 240 pixel resolution and sensitivity below 80 mK in their study.

In the study conducted by Alexakis et al. [108], in the Holy Aedicule and Msma’a historical building, moisture rising by capillary action was detected. Moisture areas on the surface, which were not noticeable to the naked eye, were successfully identified with IRT. The temperature in the lower areas of the Holy Aedicule structure ranged from 25.5 to 27.5 °C, while in the upper areas, it ranged from 27.5 to 29.5 °C. In the Msma’a historical building, 19 °C was measured on the lower walls, and 20–20.5 °C was recorded in the upper areas. In the study by Özmen and Sayın [58], in Malatya Taşhoran Church, cracks that were invisible to the naked eye were detected on the surface through thermographic analysis. The detection of heat accumulation in the cracked areas indicated the decomposition process of the stones. In the study by Solla et al. [107], in San Segundo Hermitage, IRT was shown to be an effective method for detecting moisture, cracks, and thermal bridges. It was observed that moisture originating from capillarity in the wall decreased upwards, with high-temperature values near the ground. In the study by Apostolopoulou et al. [44], in the Holy Aedicule, the thermal compatibility of the filling mortars was evaluated. While the temperature difference in the lower regions was measured as 23.5–25.5 °C before the restoration, the difference narrowed to 0.7 °C after the restoration. This demonstrated that compatibility between the materials was achieved. The analysis of IRT data was strengthened using modern image processing and deep learning methods. Alexakis et al. [108], achieved high accuracy rates using the pyramid scene parsing network (PSPNet) based segmentation model. IRT views of bell tower of Santa Barbara Church are shown in Fig.7.

6 Conclusions

In studies on the protection of historical structures, the correct determination and definition of building materials is of great importance not only in terms of structural strength but also in terms of the protection and sustainability of cultural heritage. In addition, it is very important to determine the vulnerabilities of different historical structures in case of exposure to natural disasters such as earthquakes and floods. In this context, material characterization is also essential for numerical modeling [110]. In this study, the analysis methods used in determining the characterization of building materials such as stone, brick, mortar and plaster, which are widely used in historical structures, were examined. Techniques for determining the physical, chemical, thermal, mechanical and visual properties of building materials were evaluated with a comparative approach. The results obtained are as follows.

1) Analysis of physical and thermal properties helps in evaluating the long-term performance of historical building materials. With the MIP method, properties such as pore structure, porosity level and water permeability of materials can be understood in detail. With the TGA and DTA methods, the behavior of the material against temperature, mass loss rates and thermal stability properties can be determined.

2) The components of historical building materials can be determined qualitatively and quantitatively with methods such as XRD, XRF, FTIR, Raman Spectrometry, NMR and ICP used in the determination of chemical and mineralogical properties. It provides important information about the effects of minerals such as calcite, quartz, and feldspar on the performance of historical structures.

3) In determining mechanical properties, the strength levels and elasticity properties of material types in historical structures can be compared with non-destructive or semi-destructive methods such as UPV, Schmidt hammer and flat-jack.

4) Among the visual and microstructural analysis methods, such as SEM, OM, and IRT are effective in determining the morphology of materials, element distribution and deterioration that will occur due to humidity. The IRT method allows the detection of internal structural deterioration without damaging the structure. In addition, it stands out as an effective method in the conservation studies of historical structures.

5) All these analysis methods allow us to examine historical building materials not only technically but also historically and culturally. The examined studies are also important in terms of showing how local material types used in different geographies adapt to climatic conditions and building needs. For example, the addition of volcanic material to mortars in the Catania region of Italy or the microstructural order in traditional Chettinad plasters used in India reveal the material knowledge of past civilizations. In this context, this compilation study systematically examines existing analysis methods and emphasizes the importance of these methods in developing holistic approaches to the preservation of historical structures. It is clearly seen in the literature that the combined use of different analysis methods provides more consistent and reliable results.

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