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Article

Analytical Characterization of the Geomaterials Used in the Construction of the Late Antique Wall in Emerita Augusta (Mérida, Spain)

by
Maria Isabel Mota-López
1,
Juan Miguel Meneses-Rodríguez
1,
Pedro Delgado Molina
2,
Rubén Maderuelo-Sanz
1,* and
Pedro Mateos Cruz
2
1
Instituto Tecnológico de Rocas Ornamentales y Materiales de Construcción, INTROMAC, Campus Universidad de Extremadura, 10071 Caceres, Spain
2
Instituto de Arqueología de Mérida, IAM-CSIC, Plaza de España 15, 06800 Mérida, Spain
*
Author to whom correspondence should be addressed.
Heritage 2026, 9(5), 180; https://doi.org/10.3390/heritage9050180
Submission received: 18 March 2026 / Revised: 27 April 2026 / Accepted: 30 April 2026 / Published: 3 May 2026
(This article belongs to the Special Issue Architectural Heritage and Cultural Landscape)
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Abstract

This work presents the results of an archaeometric research study of the geomaterials used in the construction of the Late Antique wall of Emerita Augusta (currently Mérida, Spain). Dated to the 5th century C.E., this structure belongs to one of the best-preserved historical ensembles in Europe. In-depth knowledge of the geomaterials used in this ancient wall is essential for ensuring reliable restoration strategies and the successful long-term conservation of this monument. To this end, a rigorous sampling procedure was conducted in areas containing original archaeological remains. Samples were characterized using optical microscopy, X-ray diffraction (XRD), X-ray fluorescence (XRF), inductively coupled plasma–mass spectrometry (ICP-MS), thermogravimetry and differential thermal analyses (TGA-DTA), and scanning electron microscopy (SEM). This integrated multi-analytical approach is highly effective for the study of built heritage. The mineralogical, textural, and geochemical properties of the granites allowed for the identification of the granite types used in the wall, while the results obtained for the mortars indicated that lime, fully carbonated and transformed into calcite, was used as the binding agent. Furthermore, the binder/aggregate ratios were found to be consistent with traditional Roman mortar formulations. These findings provide a comprehensive understanding of the material provenance and construction techniques used in this landmark of late antiquity.

1. Introduction

Protecting and safeguarding the world’s cultural and natural heritage, as well as supporting creativity and dynamic cultural sectors, are among the primary objectives of UNESCO, founded in 1945. The World Heritage List, initially adopted in 1975 and formally taking effect in 1978, was created to preserve and protect cultural treasures and natural areas throughout the world. The sites included in this list, which is subject to annual revision, are designated as having outstanding universal value; the list currently encompasses 1248 properties representing the globe’s cultural and natural heritage [1]. At present, Spain has 50 World Heritage Sites, one of the most significant being the Archaeological Ensemble of Mérida. Declared a World Heritage Site by UNESCO in 1993 [2], it is a remarkable example of a Roman city built according to all Roman urban design principles.
The conservation and restoration of historical buildings is an essential task for the history and culture of cities, driven by the necessity to maintain the cultural, religious, artistic, and socio-economic values of their citizens [3]. Interventions in these buildings must respect their cultural significance and the historical evidence they provide. Structural and material requirements should be balanced with their preservation as cultural resources [4]. Therefore, it is necessary to combine scientific and cultural approaches to the study and care of these ancient structures.
These interventions should be minimal, aiming to achieve the least possible impact on heritage values. They are based on a previous study composed of four subsequent phases: anamnesis, diagnosis and assessment of structural performance, therapy, and control [4]. The anamnesis phase, or knowledge acquisition, gathers all the data required for subsequent stages, where construction techniques, materials, and decay processes play a key role. At this stage, a thorough understanding of the original materials is essential to minimize the risk of adverse effects resulting from incompatible materials [5,6]. For instance, it is well known that cement-based mortars are often unsuitable for ancient building restoration due to mechanical and chemical incompatibilities with historic substrates, which can compromise structural integrity over time [7]. Therefore, characterizing historic building materials is of paramount importance to identify similarities and differences with modern alternatives. It must be noted, however, that standard repair materials should generally be avoided on cultural heritage monuments [8,9].
The term geomaterial refers to any material of geological origin that, after undergoing a processing stage, is used in sectors such as civil engineering or construction [10,11]. Thus, both natural stone, rocks extracted from geological outcrops, and mortars, manufactured using raw materials of geological origin, are considered geomaterials [12]. The use of these materials dates back to the dawn of civilization; bricks and stones, for instance, have been in widespread use for at least 12,000 years [10]. As part of the architectural heritage, these geomaterials are exposed to weathering agents (wind, snow, rain, etc.), suffering varying degrees of damage over time. To determine the conservation state of historical buildings, it is necessary to employ multiple complementary techniques. Petrographic analysis and scanning electron microscopy (SEM) can reveal the relationship between these materials and the surrounding geological formations [13] (though materials may sometimes originate from distant sources), as well as the composition, structure, and texture of lime binders or stones [14]. Qualitative and quantitative XRD analyses are used to determine mortar composition, aggregate phases, and the crystalline nature of the materials [15]. Additionally, TGA-DTG is essential for determining the hydraulicity of mortars, identifying low-crystallinity phases not detected by XRD, and calculating the binder/aggregate ratio [16]. Finally, geochemical analyses can establish relationships between different geomaterials or help determine their possible provenance [17].
The present work focuses on the characterization of the geomaterials used in the construction of the Late Antique wall of Emerita Augusta, currently Mérida, which forms part of the Archaeological Ensemble of Mérida, to ensure reliable future restoration and conservation strategies. It serves as an example of the use of locally sourced geomaterials and the reuse of materials from other buildings throughout the centuries [18]. Since these geomaterials had not been previously studied, a multidisciplinary analytical approach was necessary to provide a comprehensive understanding of the building materials used in this ancient wall.

2. Historical and Geological Settings

2.1. The Late Antique Wall: Historical Sequence of Its Construction

The city of Emerita Augusta was founded in the year 25 B.C.E. as a retirement place for war veterans from the V Alaudae and X Gemina legions. It became one of the most important cities of the Roman Empire during the first half of the 1st century B.C.E. [19]. This ancient city contains all the design elements of a typical Roman town, with a very acceptable state of conservation of its elements: theatre, amphitheatre, circus, aqueduct, baths, forum or a defensive wall [20]. The Late Antique wall was constructed at the end of the 5th century C.E. [21,22] as an architectural reinforcement of the original Roman defensive wall (Figure 1). This structure reused ashlars, and architectural and funerary decorative elements from various Roman monuments of the city (Figure 2). These elements are laid in headers and stretchers, and in some cases, they appear bonded with lime mortar. The structure measures between 2.40 and 2.60 m in width, which, combined with the original Augustan enclosure, gives the resulting wall a width of 5 m and a height of about 10 m [23].

2.2. Geological Setting

Emerita Augusta is located in the transition zone between the Ossa Morena Zone (OMZ) and the Central Iberian Zone (CIZ) [24,25]. The OMZ comprises a significant volume of igneous rocks, including calc-alkaline intrusives and extrusives, whereas the CIZ is characterized by a great abundance of granitic batholiths intruded during and after the Variscan Orogeny, primarily derived from the melting of Serie Negra metasediments. One such intrusion is the Mérida batholith, which covers an area limited by Mirandilla, Proserpina, and Esparragalejo, and where more than 50% of the outcrops are granitic rocks [26,27] (Figure 3). This batholith is composed of three elongated plutons with distinct facies: Sierra Bermeja pluton (two facies), Proserpina pluton (four facies), and Valdetorres pluton (two facies). The facies for the first pluton include biotite porphyritic granite and an equigranular, medium-grained two-mica monzogranite with cordierite. The second pluton consists of biotite-bearing cordierite leucogranite, two-mica cordieritic granite, megacrystic granite, and fine-grained muscovite granite. Finally, the facies for the third pluton are medium-grained cordierite monzogranite and biotite porphyritic granite [26].

3. Materials and Methods

3.1. Sampling

The sampling of the geomaterials was restricted and only possible with the legal authorization of the competent institution in charge, the Consortium of the Monumental City of Mérida. Sampling was only carried out in areas where original remains of the wall, dated to the 5th century C.E., are preserved, as demonstrated by previous archaeological studies [16]. Many sections of the wall are currently buried beneath modern buildings, making archaeological investigation and sample collection impossible in those locations. Therefore, samples have only been collected from accessible areas dating from the 5th century, strictly adhering to the sampling limitations imposed on UNESCO World Heritage sites. These sites are located near the Guadiana River (Anas and Morería Streets, and within the Alcazaba), as well as near the Roman amphitheatre (Figure 4). The extraction of geomaterials was performed manually using a hammer and a chisel. Due to legal restrictions, both the number and the size of the samples were strictly limited (Figure 5). Stone samples were obtained from fragments of granite ashlars, while mortar samples were collected from the binding material used between them. Various tests were subsequently carried out to assess their mineralogical and microstructural properties (Table 1). The sample nomenclature follows the format: Emerita Augusta (EA)—Mortar Sample (MS) or Stone Sample (SS)—sample number (1–9 for stone samples and 1–4 for mortar samples).

3.2. Optical Microscopy

Optical microscopy was employed to determine the mineralogical and textural characteristics of the geomaterials [27,28,29]. For this purpose, a petrographic microscope, Leica Leitz Laborlux 12 POL S (Ernst Leitz Wetzlar GmbH, Wetzlar, Germany), in transmission and using crossed polarizers, was used. Eleven thin sections—nine for the stone samples and two for the mortar samples—were prepared and examined, and the final images were recorded with a JVC Digital Colour Camera, model TK-C1480E (Victor Company of Japan, Limited, Yokohama, Japan). Thin sections were prepared as follows: a representative specimen of each sample was cut and flattened, then bonded to a glass slide using epoxy resin. The mounted sample was subsequently ground and polished using progressively finer silicon carbide abrasives until a uniform thickness of approximately 30 µm was achieved. Finally, the section was protected with a glass coverslip for optical observation.

3.3. X-Ray Diffraction

The XRD technique was used to determine the crystalline phases present in the geomaterials [30,31,32]. The crystalline phases of the samples were determined by a Bruker D8 ADVANCE diffractometer (Bruker Corporation, Billerica, MA, USA) in Bragg–Brentano geometry, using Cu-Kα radiation (k = 1.54178 Å), operating at 30 kV and 40 mA, at a speed with a step width of 0.02° and a counting time of 1 s/step, from 10° to 70° 2θ. The DIFFRACplus EVA software, by Bruker AXS (Karlsruhe, Germany), and the International Centre for Diffraction Data—Powder Diffraction Files (ICDD-PDF) were used to identify the crystalline phases present in the samples. Initially, the samples were dried at 50 °C for 72 h, crushed in an agate mortar, sieved, and mounted on a silicon support to minimize background.

3.4. TGA-DTA Analysis

TGA-DTA was used to characterize the mortar samples [33,34,35]. The equipment used was a Netzsch STA 449 F3 Jupiter thermal analyzer (NETZSCH-Gerätebau GmbH, Selb, Germany), and the data analysis was carried out with Netzsch Proteus 6.1.0 and Opus 7.0 software. The analysis was performed on powdered samples (sieved at 0.063 mm). Samples were placed in alumina crucibles and initially subjected to isothermal stabilization at 40 °C for 30 min. Subsequently, a dynamic heating ramp from 40 to 1000 °C at 10 °C min−1 was carried out under an inert argon atmosphere. Based on the resulting thermograms, mass losses were determined within specific temperature ranges, corresponding to the release of physically adsorbed water (30–120 °C), hydrated salts (120–200 °C), and compounds of hydraulic nature (200–600 °C), as well as the CO2 released from the decomposition of carbonate compounds (600–900 °C). These mass losses enabled the identification and classification of the nature of the binder used in the manufacture of each of the mortars analyzed.

3.5. XRF Analysis

XRF analyses were used to determine the major element concentrations in the geomaterials [36,37]. The analyses were conducted with a PHILIPS MagiX Pro (PW-2440) spectrometer with a 4 kW X-ray generator (PHILIPS, Amsterdam, The Netherlands) at the Centre of Scientific Instrumentation of the University of Granada. The estimated detection limit for major elements was 0.01 wt.%. Primary crushing, performed on approximately 200 g per sample, was carried out using a steel jaw crusher, yielding fragments smaller than 1 cm. After homogenization, about 50 g aliquots per sample were subjected to a second crushing stage in a tungsten carbide ring mill, reducing the material to a grain size of less than 25 µm. It should be noted that this procedure introduces contamination by W, Co, and Ta [38].

3.6. ICP-MS Analysis

This technique was used to determine the trace element contents in the geomaterials [27]. It was performed using a NexION 300D plasma mass spectrometer (PerkinElmer, Inc, Waltham, MA, USA) at the Centre of Scientific Instrumentation of the University of Granada. The estimated detection limit for trace elements was 1–5 ppm. Harker diagrams [39] and chondrite-normalized rare earth element (REE) abundance patterns, according to Sun and McDonough [40], were used to analyze the trace element data across the different samples.

3.7. Scanning Electron Microscopy

SEM was used to determine the morphology and the elemental composition of the mortar samples, as well as to detect the presence of neoformation products [41,42]. SEM was carried out using a FEI QUANTA 3D FEG, equipped with an EDS detector for microanalysis (Thermo Fisher Scientific, Hillsboro, OR, USA), at the Analysis and Characterization of Solids and Surfaces Service (SACSS) of the University of Extremadura (Spain). The accelerating voltage was set at 15 kV, with a working distance (WD) ranging from 9.7 to 10.3 mm across three continuous scans. A secondary electron detector (SED) was used to obtain high-resolution topographical images. For the analysis, unaltered specimens of 4 mm thickness were prepared and gold-coated by sputtering to ensure conductivity.

4. Results

The macroscopic examination of the granitic samples from the Late Antique wall revealed yellowish beige to grey rocks with occasional reddish and pinkish tones, fine to coarse-grained and showing phaneritic and inequigranular texture. It was possible to observe a porphyritic trend due to K-feldspar megacrysts, with crystal sizes varying up to 6 cm. In the case of the sample EA-SS-1, the rock colour was white, fine to medium-grained, displaying phaneritic, and inequigranular texture. Mineralogically, the granitic samples were mostly composed of quartz, potassium feldspar (mainly microcline and orthoclase), plagioclase (mainly albite and anorthite), and accessory minerals such as biotite, muscovite, and zircon (Figure 6). The dark-coloured biotite crystals, being abundant, give the rocks a mottled appearance. Quartz, both monocrystalline and polycrystalline, occurs as fine to coarse-grained and anhedral to subhedral, with undulatory extinction. K-feldspar occurs as fine to coarse-grained and perthitic due to the presence of irregular albite veins. Euhedral plagioclase crystals occur as fine to coarse-grained and are usually zoned. Occasionally, oxidation is observed with the presence of ferruginous cement at grain boundaries, alongside widespread alteration of feldspars into clay minerals.
The granitic samples displayed a consistent modal composition with quartz (26–32%), potassium feldspar (29–34%), plagioclase (26–29%), biotite (4–5%), and accessory minerals (6–16%). On the QAP diagram [43], all stone samples fall within the monzogranite field (Figure 7).
The macroscopic examination of the mortar samples showed a white-to-beige binder containing primarily granite and quartzite fragments, along with quartz and feldspar minerals, with sizes ranging from 0.5 to 1.2 cm. Petrographic analysis of the mortar samples (Figure 6g,h) showed a high degree of similarity between them, with a lime-based binder and siliceous aggregates, mainly composed of monocrystalline and polycrystalline quartz grains, and K-feldspars and plagioclases. The fine-grained sizes varied up to 1 mm, with grain roundness varying from angular to subangular, and low sphericity. The binder, turned into calcite as a result of carbonation of the lime used, displayed a micritic texture and, due to weathering in some cases and/or sample preparation, exhibited voids which were coated with re-precipitated minerals of calcitic composition [44].
Figure 8 presents the diffractograms obtained for the stone samples. Table 2 shows the qualitative mineralogical compositions. The analysis mainly revealed the presence of quartz (26.67°2θ; 3.34 Å), with feldspars (K-feldspar as microcline (27.52°2θ; 3.24 Å) and orthoclase (8.83°2θ; 10.00 Å), and plagioclase as albite (27.88°2θ; 3.20 Å) and anorthite (26.75°2θ; 3.31 Å)), muscovite (8.86°2θ; 9.97 Å), biotite (8.74°2θ; 10.12 Å), and ilmenite (32.58°2θ; 2.75 Å). In addition to the aforementioned minerals, trace amounts of chlorite (12.46°2θ; 7.10 Å) were identified.
In the case of mortar samples, the XRD analyses of their aggregate fraction (>63 µm) revealed that these aggregates were primarily composed of quartz, with trace amounts of other minerals, such as microcline and albite (Figure 9a). The XRD analyses of the binder fraction (<63 µm) revealed the presence of quartz and calcite (29.45°2θ; 3.03 Å) as the predominant mineral phases, as well as trace amounts of other minerals, such as microcline, albite and anorthite, although in minor proportions, for samples EA-MS-2, EA-MS-3 and EA-MS-4 (Figure 9b). In the latter two, the CaCO3 polymorph aragonite (26.21°2θ; 3.39 Å) is present. In the case of sample EA-MS-1, the predominant mineral phase is quartz, while calcium carbonate is not observed in the diffractogram; instead, mineral phases in the form of hydroxyapatite are detected. The diffractograms also show the presence of other mineral phases, such as quartz and albite, similar to those identified in the aggregate fraction. These appear to represent the finest fractions of the aggregate, with a particle size comparable to that of the binder.
By applying geochemical techniques, it was possible to achieve improved differentiation and definitive discrimination among the granitic samples. These analyses, including major oxide concentrations, trace element abundances, and especially chondrite-normalized REE patterns, supported the petrographic study in distinguishing among the different materials used, providing important feedback for the identification of the granitic samples (Table S1). Granitic samples were characterized by high contents of SiO2 (72.10–74.42%), Al2O3 (13.43–15.25%), and low contents of CaO (0.53–0.90%), TiO2 (0.08–0.23%), Fe2O3 (0.75–2.03%) and P2O5 (0.14–0.37%), low mafic components (Fe2O3 + MgO + TiO2 = 1.03–2.52%), with high Fe2O3/(Fe2O3 + MgO) ratios (0.79–0.89%). They were also enriched in MgO relative to TiO2 (1.00–2.50%), subalkaline (K2O + Na2O), with high contents of total alkalis (8.02–8.80%) and high K2O contents relative to Na2O (K2O/Na2O = 1.13–1.47). All samples were strongly peraluminous, with a Shand index A/CNK (molar Al2O3/[CaO + Na2O + K2O]) ranging from 1.09 to 1.23, without significant variation between units. The calculated ASI (molar Al2O3/[(CaO − 3.33·P2O5) + Na2O + K2O]) according to Zen [46] shows values slightly higher, ranging from 1.24 to 1.38, due to the combination of the P2O5 with CaO in apatite [47]. The peralkalinity index, AI (molar ratio of [(Na2O + K2O)/Al2O3]), ranged from 0.73 to 0.81. The A/NK was high and had relatively higher values within a short range (A/NK = 1.23–1.37) (Table S1). The relationship between TiO2 and Fe2O3 (Figure 10a) follows a well-aligned band, suggesting that biotite and Fe-bearing minerals are involved in the evolution of granite. In the Na2O vs. CaO (Figure 10b) diagram, the trend is for a parallel increase in both oxides, indicating a decrease with acidity. In the Na2O vs. K2O diagram (Figure 10c), it can also be seen that as the K2O content increases, the Na2O content remains constant.
All the samples showed high contents of the alkali metals (Rb, Cs, and Li), low contents of some high field strength elements (HFSE) (Y, Nb, Sn, Ta, Th, U, Sc, and Pb), and low contents of Ba, Sr, Eu, and Hf. The samples are characterized by moderate Rb/Sr (5.82–25.74), except for the sample EA-SS-4, which shows a high ratio (Rb/Sr = 107.11) (extreme plagioclase fractionation), and Rb/Ba (1.06–4.34) ratios, showing high values of Ga (17.94–25.99), with 10,000 Ga/Al values ranging from 2.42 to 3.22. Almost all samples had a low abundance of light rare-earth elements (LREE = 38.60–106.15 ppm), and heavy rare-earth elements (HREE = 3.64–8.57 ppm), except for sample EA-SS-4, which had lower abundance (6.17 and 1.47 ppm respectively) (Table S1).
Primitive mantle-normalized spider diagrams showed marked negative Ba, Nb and Sr, and significant enrichment in Cs, Rb, U and Pb, being typical characteristics of A-type granites [48] (Figure 11). The chondrite-normalized REE patterns (Figure 12) showed similarities in terms of pattern and negative slope, with patterns moderately downwards, (La/Sm)N ranging from 2.27 to 2.01 and (Gd/Yb)N ranging from 1.15 to 1.98, visible fractionation of LREE and HREE, (La/Lu)N values ranging from 5.71 to 9.30, and low negative Eu anomalies (Eu/Eu* = 0.34–0.75) indicating feldspar fractionation, along with small positive Ce anomalies ranging from 0.99 to 1.14.
The mass loss at different temperature ranges was determined from the thermograms obtained for the mortar samples. The weight losses observed at different temperature intervals are shown in Table 3, displaying the temperature ranges corresponding to the elimination of absorbed water or moisture inherent to the sample (30–120 °C), the loss of water of crystallization associated with the presence of hydrated salts (120–200 °C), dehydroxylation of hydraulic compounds and clays (200–600 °C), and decomposition of carbonates (600–900 °C). Additionally, the CO2/H2O ratio is included as an indicator of the degree of hydraulicity of the binder. In the temperature range between 30 and 120 °C, the samples EA-MS-2, EA-MS-3, and EA-MS-4 showed adsorbed moisture percentages between 2 and 4%, while the sample EA-MS-1 showed an adsorbed moisture percentage equal to 11.10%. Between 120 and 200 °C, the obtained percentages for the samples EA-MS-2, EA-MS-3, and EA-MS-4 were below 1%, while for the sample EA-MS-1 this value was higher than 1%. Within the range of 200–600 °C, significant weight losses of between 4 and 6% were obtained for all the samples. In the range of 600–900 °C, high percentages of loss were found for the samples EA-MS-2, EA-MS-3, and EA-MS-4. Once again, the sample EA-MS-1 had a percentage of less than 2%. The CO2/H2O ratio was higher than 3 for samples EA-MS-2, EA-MS-3, and EA-MS-4, and lower than 1 for sample EA-MS-1 (Figure 13). Finally, the acid dissolution method by wet chemical separation was used to determine the binder/aggregate ratio due to the sole presence of siliceous aggregates in the mortar samples. The obtained binder/aggregate ratio by weight ranged from 1/13 to 1/3 (Table 3).
Figure 14 shows the SEM analysis of the mortar samples. Figure 14a shows a scattered network of calcite crystals with relatively well-defined geometric morphologies (polyhedral and angular), ranging from 5 to 10 μm in size. The larger angular crystals (such as the central grains, ranging from 10 to 15 μm) are identified as quartz grains. Surrounding these larger crystals, a mass of much smaller (sub-micrometric) particles characterized by an irregular and porous morphology is present. Moreover, small black voids are present too. Figure 14b shows calcite crystals, with an average size ranging between 2 µm and 5 µm, exhibiting a well-defined granular and euhedral texture. In Figure 14c the carbonation matrix displays a granular appearance, traversed by a network of filamentous structures. Finally, Figure 14d reveals acicular and tabular crystals (lamellae). These are characterized by elongated, needle-like, or “leaf-like” interlocking habits, with a finer amorphous mass observed between the larger crystals.

5. Discussion

Based on the macroscopic examination and mineralogical characterization of the different stone samples—notably their grey colour, the abundance of biotite, muscovite, and orthoclase, and crystal sizes up to 6 cm—these rocks were classified as two-mica granites with a porphyritic texture, with the exception of sample 1, which was defined as a muscovite leucogranite. Samples EA-SS-2, EA-SS-3, EA-SS-5, EA-SS-6, and EA-SS-8 displayed pinkish hues, which may be attributed to various oxidation processes [27]. X-ray diffraction results showed that the diffraction patterns for all stone samples were remarkably similar (Figure 8). The relative intensities of the peaks may indicate that quartz is the predominant phase in all samples, followed by K-feldspar and plagioclase. Furthermore, the presence of orthoclase, biotite, muscovite, and chlorite, the latter likely resulting from the alteration of biotite, was confirmed [28]. Regarding the geochemical analysis, a clear correlation between the major element concentrations of the samples was evident, with the exception of sample EA-SS-4, which exhibited lower SiO2 and higher contents of other major elements (Table S1). In terms of trace elements, the concentrations of HREE and LREE in sample EA-SS-4 differed significantly from those in the other samples, showing notably lower values (Table S1). This distinction is visible in the chondrite-normalized REE patterns (Figure 12) and the primitive mantle-normalized spider diagrams (Figure 11). All samples, except for EA-SS-4, exhibited a consistent geochemical trend characterized by a prominent negative slope due to LREE enrichment relative to HREE. These samples also displayed moderate-to-strong negative Eu anomalies, suggesting significant plagioclase fractionation during their evolution. In contrast, sample EA-SS-4 followed a distinct U-shaped pattern with significantly lower total REE concentrations. Notably, it presented a strong positive Eu anomaly, which points toward plagioclase accumulation or a more primitive mineralogical character. Consequently, it can be affirmed that the remaining samples belong to A-type granites, characterized by high SiO2, K2O, Na2O, (K2O + Na2O)/CaO, and REE contents, along with low CaO and Sr levels and relatively high Ga/Al ratios [48]. The results suggest that the provenance of sample EA-SS-4 differs from that of the other samples.
A comparison between the results of this study and previous research characterizing the granitic rocks of the Emerita Augusta monuments [27,28,50]—aimed at identifying potential quarry sources—reveals that our findings are consistent with those earlier observations, especially considering the similar analytical techniques used. Such studies require a multidisciplinary approach, integrating optical microscopy and geochemical analysis, among other techniques, to accurately characterize both the stone used in the monuments and the nearby rock outcrops [51,52]. A comparison of the petrographic results from the stone samples in this study with those from previous research [27,28] reveals remarkable consistency. Williams-Thorpe and Potts [50] defined the granitic rocks in the Cuarto de la Charca area as two-mica granites (biotite and muscovite) composed of quartz, orthoclase, and plagioclase, with accessory minerals such as ilmenite and zircon. In contrast, they noted that granites in the Proserpina area appeared more altered and exhibited a coarse-grained matrix. Similar observations were reported by Mota et al. [27,28], who identified the various granites used in the construction of the theatre and amphitheatre as two-mica granites with a porphyritic texture. Regarding the concentrations of major and trace elements, as well as the primitive mantle-normalized spider diagrams and chondrite-normalized REE patterns reported by Mota et al. [27,28], the results are highly consistent with our findings. For instance, SiO2 concentrations in the Roman theatre samples ranged from 71.72% to 76.12%, aligning closely with the 72.93–74.42% range observed in the present study. Similarly, SiO2 values in quarry samples near Emerita Augusta ranged from 72.10% to 75.74%. Comparison of the REE patterns reveals remarkable geochemical homogeneity between the samples in this study and those analyzed by Mota et al. [27,28]. All profiles, with the exception of sample EA-SS-4, exhibit a steep negative slope with a clear LREE enrichment and a pronounced negative Eu anomaly, characteristic of the granites from the Mérida batholith. This matching signature between the building materials and the Proserpina or Cuarto de la Charca quarries confirms the use of local supply sources. The anomalous behaviour of EA-SS-4, with its U-shaped pattern and positive anomaly, suggests a distinct petrogenesis. Therefore, it can be affirmed that the granitic stones used in the construction of the Late Antique wall likely originated from historical local quarries or were reused from earlier Roman monuments (spolia) [51,53] dismantled prior to the wall’s construction. This practice of spoliation was frequent between the 3rd and 5th centuries to avoid the transport of heavy materials over long distances [54], a well-documented process in both urban and rural Late Antique contexts [18].
Mineralogical and petrographic examinations of the mortars revealed consistent morphological characteristics across all samples, which consist of a fine-grained lime binder with siliceous aggregates embedded within the matrix. Furthermore, XRD analysis of these aggregates—which were virtually identical across all specimens (Figure 9a)—confirmed their siliceous nature. This suggests that the aggregates consist of granite or quartzite fragments, both of which are highly abundant in the region due to the geological setting of Mérida [27,28]. Regarding the XRD analysis of the binder (Figure 9b), calcite was identified as the predominant phase in samples EA-MS-2, EA-MS-3, and EA-MS-4, confirming they are lime-based mortars [35]. The occurrence of aragonite in samples EA-MS-3 and EA-MS-4 is noteworthy and may be attributed to the presence of impurities that can not only inhibit or enhance the dissolution and precipitation of calcium compounds but may also favour the precipitation of aragonite over calcite [35]. This phenomenon may serve as an indicator of material erosion and aging resulting from prolonged exposure to varying environmental conditions over time. Another important finding is the presence of hydroxyapatite in sample EA-MS-1. This may indicate the use of powdered or calcined bone as an additive to improve the mechanical properties of the mortar, a technique widely employed in ancient construction [55,56,57,58,59]. This is evidenced in the diffraction pattern by a broad peak in the 31.77–32.90°2θ range (the characteristic hydroxyapatite triplet), along with a well-defined peak at 25.88°2θ, which may indicate the growth of acicular hydroxyapatite crystals [60]. However, caution should be exercised regarding the proposed use of hydroxyapatite as an additive in the mortar. Since hydroxyapatite has long been employed in the conservation of cultural heritage [61], its presence could lead to the interpretation that this is a restoration mortar; nonetheless, previous archaeological studies have already ruled out this possibility [18]. Furthermore, the presence of trace amounts of sodium and magnesium silicates, iron oxides, and anorthite is noteworthy. The low proportion of these phases suggests they may be mineral impurities inherent to the limestone source or result from contamination with ceramic residues within the lime kilns.
Regarding the thermal analysis of the mortars, the weight loss percentages obtained for samples EA-MS-2, EA-MS-3, and EA-MS-4 during the first step (30–120 °C) are within the typical range for conventional lime mortars. Their relatively low moisture content may suggest the absence, or a very low proportion, of ceramic materials or pozzolans as supplementary cementitious materials [62]. In contrast, sample EA-MS-1 exhibited a high moisture content exceeding 10%. This unusual value, compared to the other mortars, can be explained by its distinct composition and may be related to the presence of pozzolanic or hydraulic phases, such as hydrated calcium aluminosilicates [63]. In the 120–200 °C range, weight losses below 1% for samples EA-MS-2, EA-MS-3, and EA-MS-4 indicate a negligible presence of hydrated salts associated with efflorescence. However, sample EA-MS-1, which contains hydroxyapatite, showed a weight loss close to 2%. This is attributed to the ability of hydroxyapatite to retain water molecules in two distinct ways: while most molecules are weakly adsorbed onto the surface, others are held within the crystalline lattice, requiring higher temperatures (120–200 °C) for complete removal. Furthermore, due to their high adsorption capacity, these minerals can retain significant amounts of moisture, explaining the 11.10% weight loss observed between 30 and 120 °C [63]. These results are consistent with the diffraction data, which showed an absence of hydrated salts. In the 200–600 °C range, the weight loss values suggest the presence of hydraulic compounds in the binder. This mass loss is attributed to dehydroxylation (the elimination of hydroxyl groups as water molecules) from clay minerals and/or hydraulic phases, such as calcium silicates or aluminosilicates [64]. The weight loss in this range can be categorized into two groups: samples EA-MS-2, EA-MS-3, and EA-MS-4, with a hydration water content of approximately 6%; and sample EA-MS-1, with a lower percentage of 4.2%. This difference is primarily due to the specific hydroxyapatite content in the binder of the latter. For temperatures exceeding 600 °C, the high mass loss recorded for samples EA-MS-2, EA-MS-3, and EA-MS-4 confirms the carbonatic nature of the binder. In contrast, the value obtained for sample EA-MS-1 (less than 2%) is not associated with carbonates but rather with the decomposition of phosphates and the further dehydroxylation of hydroxyapatite minerals.
To classify the hydraulicity of the mortars, the CO2/H2O ratio must be evaluated in relation to the percentage of CO2, as this parameter is inversely proportional to the degree of hydraulicity (Figure 13). With this relationship, it is possible to categorize lime binders as pozzolanic (<3), hydraulic (3–9), and aerial (>9) [65]. However, because hydraulicity is a complex property to estimate, this technique is not sufficient to determine the nature of the mortars, and it is desirable to obtain the Cementation Index (CI), or the chemical, and mineralogical composition of the binder [66]. Furthermore, the hydraulic properties of the mortars cannot be addressed without the so-called C-(A)-S-H phases, whose detection can be complicated. It was not possible to identify these phases in the diffractograms of the mortar samples, possibly due to their very low crystalline structure; furthermore, the main X-ray peaks tend to overlap, making their differentiation complicated. However, this does not imply that these mortars lack hydraulic character [67]. The amorphous content, also related to hydraulicity, was not identified either, although this can depend on other factors such as the presence of clays. Samples EA-MS-2 and EA-MS-3 exhibited CO2/H2O ratios of approximately 3 and relatively high contents of CO2 (~27%), which could be associated with the use of hydraulic binders featuring a certain pozzolanic character [65]. In the case of sample EA-MS-4, its CO2 content (31.51%) and hydraulicity index (4.24), would be compatible with the use of conventional hydraulic lime. Conversely, sample EA-MS-1 cannot be classified as a lime mortar. Its hardening is believed to occur through the precipitation of calcium in the form of apatite-derived minerals, the exact origin of which cannot be determined with the current data [68]. Finally, the binder/aggregate ratio was in agreement with those obtained for other historic mortars [34,35,37,68,69].
SEM analysis of the mortar samples revealed a marked angularity of the grains (Figure 14a), suggesting that the aggregates are not of fluvial origin—which would typically be more rounded—but may instead be sourced from quarries or produced by the crushing of granitic rock [70]. The small black voids present in the sample may indicate material loss resulting from aging and moisture exposure, or potentially from the sample preparation process. In Figure 14b, the well-defined granular and euhedral texture of the calcite crystals suggests that the calcite experienced adequate spatial and temporal conditions to recrystallize into larger, more defined individual grains. These grains can generate a dense crystalline matrix, providing high resistance and durability [16]. In Figure 14d, the acicular and tabular crystals likely represent the crystallization of hydroxyapatite (attributed to the use of bone ash additives [55,58,59,71]), consistent with the XRD analysis of sample EA-MS-1. The observed amorphous mass may represent the remaining silicate/aluminosilicate binder (such as the albite or microcline identified by XRD), which acts as the base cementitious phase from which calcite may have been removed by leaching.

6. Conclusions

World Heritage Sites can suffer detrimental effects on their structural integrity due to their continuous exposure to weathering agents. To preserve buildings that form part of these World Heritage Sites it is necessary to accomplish restoration and conservation interventions. In many cases, the materials used in these interventions are not compatible with the original materials employed in their construction and may cause detrimental effects opposite to those desired, so it is imperative to use materials with petrophysical properties similar to the original ones. Therefore, prior to any intervention in historical buildings, a scientific study of the original geomaterials is both necessary and essential, with the aim of achieving a respectful restoration of the building.
This work aims to provide a new contribution to the characterization of the geomaterials used in this monument, which have never been previously studied. This will enable future comparative research with other monuments from a similar period. This knowledge, achievable through complementary analytical techniques, represents an essential tool for establishing a suitable conservation plan. Furthermore, it can help clarify mechanisms and interactions that remain unclear, allowing for the planning of optimal interventions.
Hence, in this work, a comprehensive characterization of the geomaterials used in the construction of the Late Antique wall of Emerita Augusta was carried out, and the results obtained have enabled the following conclusions to be drawn:
  • Mineralogical and geochemical analyses revealed that most of the granitic stones are similar to those used in older structures, such as the Roman theatre and amphitheatre. This suggests that the Late Antique wall was constructed by incorporating stones from local historical quarries—matching the lithologies outcropping in areas adjacent to Emerita Augusta—or stones repurposed from abandoned buildings in the surrounding area, the latter being a common practice during Late Antiquity.
  • The mortars were mainly composed of calcite and siliceous aggregates, coming from quarries near the city, or from river sands near the city (Guadiana and Albarregas Rivers), or produced through crushing granitic stones spoliated from other nearby monuments. The occurrence of hydroxyapatite in one of the samples is noteworthy, as it suggests the potential use of bone meal or calcined bone as an additive to enhance the mortar’s properties. However, the presence of hydroxyapatite could also be attributed to the use of modern restoration mortars; nevertheless, this possibility is less likely given that previous archaeological studies have identified these mortars as historic. The binder/aggregate ratios of the studied mortars were consistent with those obtained for other ancient mortars.
  • The use of incompatible materials to restore historic buildings can cause premature deterioration of heritage structures. For this reason, a careful approach to the application of compatible restoration geomaterials with original materials is essential through a prior study of the latter, suggesting compatible raw materials that perfectly match the original materials, avoiding the need for recurrent repairs.
  • The characterization of the geomaterials used in this monument is the starting point for accurately determining the provenance of the raw materials, which will be the subject of future work. Furthermore, knowledge of the original materials is essential for the formulation of compatible mortars, ensuring proper restoration and the long-term conservation of this ancient wall.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/heritage9050180/s1: Table S1: Whole-rock chemical compositions for stone samples (major elements in wt.% and trace elements in ppm). LOI: loss on ignition (LLD < 0.01%).

Author Contributions

Conceptualization, M.I.M.-L. and P.M.C.; methodology, M.I.M.-L. and P.M.C.; validation, M.I.M.-L. and P.M.C.; formal analysis, J.M.M.-R. and R.M.-S.; investigation, P.M.C. and P.D.M.; resources, P.M.C.; data curation, R.M.-S.; writing—original draft preparation, R.M.-S.; writing—review and editing, M.I.M.-L.; visualization, M.I.M.-L. and P.M.C.; supervision, P.M.C.; project administration, P.M.C.; funding acquisition, P.M.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research is funded by the Regional Government of Extremadura (Comunidad Autónoma de Extremadura, Consejería de Economía, Ciencia y Agenda Digital) to aid the financing of Research Projects (“VI Plan Regional de I + D + I, 2017–2020”, Decreto 91/2017). The European Regional Development Fund accounts for 80%. Grant number: IB20046, Project: “La muralla de Augusta Emerita”.

Data Availability Statement

The original contributions presented in the study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Image of the Late Antique wall of Emerita Augusta (a) inside the Arab citadel (from which samples EA-SS-1, EA-SS-2, EA-MS-1 and EA-MS-2 were taken), and (b) near the Roman amphitheatre (from which samples EA-MS-3 and EA-MS-4 were taken).
Figure 1. Image of the Late Antique wall of Emerita Augusta (a) inside the Arab citadel (from which samples EA-SS-1, EA-SS-2, EA-MS-1 and EA-MS-2 were taken), and (b) near the Roman amphitheatre (from which samples EA-MS-3 and EA-MS-4 were taken).
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Figure 2. Image of ashlars (a), and architectural (b,c) and funerary decorative elements (d) from different Roman monuments of the city used to build the Late Antique wall of Emerita Augusta.
Figure 2. Image of ashlars (a), and architectural (b,c) and funerary decorative elements (d) from different Roman monuments of the city used to build the Late Antique wall of Emerita Augusta.
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Heritage 09 00180 g003
Figure 3. Geological scheme of the city of Emérita Augusta (obtained from GEODE, the continuous digital geological map of Spain, scale 1:50,000; http://info.igme.es/visorweb, accessed on 22 January 2026).
Figure 3. Geological scheme of the city of Emérita Augusta (obtained from GEODE, the continuous digital geological map of Spain, scale 1:50,000; http://info.igme.es/visorweb, accessed on 22 January 2026).
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Figure 4. Schematic plant of the Late Antique wall (in red) of Emerita Augusta and sampling areas.
Figure 4. Schematic plant of the Late Antique wall (in red) of Emerita Augusta and sampling areas.
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Figure 5. Photographs of the geomaterial samples, stones and mortars.
Figure 5. Photographs of the geomaterial samples, stones and mortars.
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Figure 6. Thin section of mortar and stone samples in cross (NX) polarized light; (a) quartz grains and fine-grained muscovite and plagioclase, (b) fine to medium-grained muscovite and biotite, zircon, fine to medium-grained plagioclase and medium-grained K-feldspar with poikilitic texture, (c) medium-grained K-feldspars with perthitic texture due to the presence of irregular albite veins, (d) medium-grained polycrystalline quartz with undulatory extinction, (e) fine to medium-grained biotite, fine-grained muscovite, and monocrystalline and polycrystalline quartz, (f) fine-grained monocrystalline quartz and muscovite, and fine to medium-grained biotite displaying metamict halos produced by zircon inclusions, (g) monocrystalline and polycrystalline quartz, and plagioclase grains embedded in a fine-grained cohesive lime binder with pores coated with secondary calcite crystals, (h) angular quartz grains and polycrystalline quartz in a carbonated matrix with different pores.
Figure 6. Thin section of mortar and stone samples in cross (NX) polarized light; (a) quartz grains and fine-grained muscovite and plagioclase, (b) fine to medium-grained muscovite and biotite, zircon, fine to medium-grained plagioclase and medium-grained K-feldspar with poikilitic texture, (c) medium-grained K-feldspars with perthitic texture due to the presence of irregular albite veins, (d) medium-grained polycrystalline quartz with undulatory extinction, (e) fine to medium-grained biotite, fine-grained muscovite, and monocrystalline and polycrystalline quartz, (f) fine-grained monocrystalline quartz and muscovite, and fine to medium-grained biotite displaying metamict halos produced by zircon inclusions, (g) monocrystalline and polycrystalline quartz, and plagioclase grains embedded in a fine-grained cohesive lime binder with pores coated with secondary calcite crystals, (h) angular quartz grains and polycrystalline quartz in a carbonated matrix with different pores.
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Figure 7. Quartz–alkali feldspar–plagioclase ternary classification diagram [43] for stone samples.
Figure 7. Quartz–alkali feldspar–plagioclase ternary classification diagram [43] for stone samples.
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Figure 8. XRD patterns from granitic samples (Qz: quartz; Cal: calcite; Mc: microcline; Or: orthoclase; Ab: albite; An: anorthite; Ms: muscovite; Bt: biotite; Arg: aragonite; Chl: chlorite; Ilm: ilmenite; Hap: hydroxyapatite) [45].
Figure 8. XRD patterns from granitic samples (Qz: quartz; Cal: calcite; Mc: microcline; Or: orthoclase; Ab: albite; An: anorthite; Ms: muscovite; Bt: biotite; Arg: aragonite; Chl: chlorite; Ilm: ilmenite; Hap: hydroxyapatite) [45].
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Figure 9. XRD patterns from mortar samples: (a) aggregates and (b) binder.
Figure 9. XRD patterns from mortar samples: (a) aggregates and (b) binder.
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Figure 10. Major element oxide variation diagrams of the stone samples: (a) TiO2 vs. Fe2O3, (b) Na2O vs. CaO, and (c) Na2O vs. K2O.
Figure 10. Major element oxide variation diagrams of the stone samples: (a) TiO2 vs. Fe2O3, (b) Na2O vs. CaO, and (c) Na2O vs. K2O.
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Figure 11. Primitive mantle-normalized spider diagrams [40] of stone samples.
Figure 11. Primitive mantle-normalized spider diagrams [40] of stone samples.
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Figure 12. Chondrite-normalized REE patterns [49] of stone samples.
Figure 12. Chondrite-normalized REE patterns [49] of stone samples.
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Heritage 09 00180 g013
Figure 13. Binary CO2/H2O vs. CO2 (%) diagram of mortar samples.
Figure 13. Binary CO2/H2O vs. CO2 (%) diagram of mortar samples.
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Figure 14. SEM micrographs of samples EA-MS-2 (a), EA-MS-3 (b), EA-MS-4 (c), and EA-MS-1 (d).
Figure 14. SEM micrographs of samples EA-MS-2 (a), EA-MS-3 (b), EA-MS-4 (c), and EA-MS-1 (d).
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Table 1. Location and characterization tests performed to assess mineralogical and microstructural properties of the geomaterial samples.
Table 1. Location and characterization tests performed to assess mineralogical and microstructural properties of the geomaterial samples.
Sample
Identification		LocationOptical
Microscopy		XRDXRFICP-MSSEMTGA-DTA
Stones
EA-SS-1Arab Citadelxxxx--
EA-SS-2Arab Citadelxxxx--
EA-SS-3Morería Streetxxxx--
EA-SS-4Morería Streetxxxx--
EA-SS-5José Ramón Mélida Streetxxxx--
EA-SS-6José Ramón Mélida Streetxxxx--
EA-SS-7Anas Streetxxxx--
EA-SS-8Anas Streetxxxx--
EA-SS-9Anas Streetxxxx--
Mortars
EA-MS-1Arab Citadel-x--xx
EA-MS-2Arab Citadelxx--xx
EA-MS-3Roman amphitheatre wallxx--xx
EA-MS-4Roman amphitheatre wall-x--xx
Table 2. Qualitative mineralogical composition of the geomaterial samples obtained by XRD (Notation used: (+++) high proportion; (++) medium proportion; (+) low proportion; (±) traces; (-) not detected).
Table 2. Qualitative mineralogical composition of the geomaterial samples obtained by XRD (Notation used: (+++) high proportion; (++) medium proportion; (+) low proportion; (±) traces; (-) not detected).
Sample
Identification		QuartzCalciteK-FeldsparsPlagioclasesMuscoviteBiotiteChloriteAragoniteHydroxyapatite
Stones
EA-SS-1+++-++++±----
EA-SS-2+++-++++±±±--
EA-SS-3+++-++++±±---
EA-SS-4+++-++++±±---
EA-SS-5+++-++++±±±--
EA-SS-6+++-++++-±±--
EA-SS-7+++-++++±±---
EA-SS-8+++-++++--±--
EA-SS-9+++-++++±±---
Mortars
EA-MS-1+++-±±----+
EA-MS-2++++++±±-----
EA-MS-3++++++±±---±-
EA-MS-4++++++±±---±-
Table 3. Sample mass losses and calcium carbonate contents (%) obtained by TGA-DTG analysis, and binder/aggregate ratio.
Table 3. Sample mass losses and calcium carbonate contents (%) obtained by TGA-DTG analysis, and binder/aggregate ratio.
Sample
Identification		30–120 °C120–200 °C200–600 °C>600 °CLoss
of
Ignition		CO2/H2OBinder/Aggregate
Ratio
EA-MS-111.101.594.181.8418.710.441/13
EA-MS-22.130.825.8418.3127.103.141/5
EA-MS-33.930.585.5616.3926.462.951/3
EA-MS-42.990.615.3322.5831.514.241/3
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Mota-López, M.I.; Meneses-Rodríguez, J.M.; Delgado Molina, P.; Maderuelo-Sanz, R.; Mateos Cruz, P. Analytical Characterization of the Geomaterials Used in the Construction of the Late Antique Wall in Emerita Augusta (Mérida, Spain). Heritage 2026, 9, 180. https://doi.org/10.3390/heritage9050180

AMA Style

Mota-López MI, Meneses-Rodríguez JM, Delgado Molina P, Maderuelo-Sanz R, Mateos Cruz P. Analytical Characterization of the Geomaterials Used in the Construction of the Late Antique Wall in Emerita Augusta (Mérida, Spain). Heritage. 2026; 9(5):180. https://doi.org/10.3390/heritage9050180

Chicago/Turabian Style

Mota-López, Maria Isabel, Juan Miguel Meneses-Rodríguez, Pedro Delgado Molina, Rubén Maderuelo-Sanz, and Pedro Mateos Cruz. 2026. "Analytical Characterization of the Geomaterials Used in the Construction of the Late Antique Wall in Emerita Augusta (Mérida, Spain)" Heritage 9, no. 5: 180. https://doi.org/10.3390/heritage9050180

APA Style

Mota-López, M. I., Meneses-Rodríguez, J. M., Delgado Molina, P., Maderuelo-Sanz, R., & Mateos Cruz, P. (2026). Analytical Characterization of the Geomaterials Used in the Construction of the Late Antique Wall in Emerita Augusta (Mérida, Spain). Heritage, 9(5), 180. https://doi.org/10.3390/heritage9050180

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