Capillary Rise and Salt Weathering in Spain: Impacts on the Degradation of Calcareous Materials in Historic Monuments ^†

Abstract

The crystallization of soluble salts is one of the most significant agents of deterioration affecting porous building materials in historical architecture. This process not only compromises the physical integrity of the materials but also results in considerable aesthetic, structural, and economic consequences. Soluble salts involved in these processes may originate from geogenic sources—including soil leachate, marine aerosols, and the natural weathering of parent rocks—or from anthropogenic factors such as air pollution, wastewater infiltration, and the use of incompatible restoration materials. This study examines the role of capillary rise as a primary mechanism responsible for the vertical migration of saline solutions from the soil profile into historic masonry structures, especially those constructed with calcareous stones. It describes how water retained or sustained within the soil matrix ascends via capillarity, carrying dissolved salts that eventually crystallize within the pore network of the stone. This phenomenon leads to a variety of damage types, ranging from superficial staining and efflorescence to more severe forms such as subflorescence, microfracturing, and progressive mass loss. By adopting a multidisciplinary approach that integrates concepts and methods from soil physics, hydrology, petrophysics, and conservation science, this paper examines the mechanisms that govern saline water movement, salt precipitation patterns, and their cumulative effects on stone durability. It highlights the influence of key variables such as soil texture and structure, matric potential, hydraulic conductivity, climatic conditions, and stone porosity on the severity and progression of deterioration. This paper also addresses regional considerations by focusing on the context of Spain, which holds one of the highest concentrations of World Heritage Sites globally and where many monuments are constructed from vulnerable calcareous materials such as fossiliferous calcarenites and marly limestones. Special attention is given to the types of salts most commonly encountered in Spanish soils—particularly chlorides and sulfates—and their thermodynamic behavior under fluctuating environmental conditions. Ultimately, this study underscores the pressing need for integrated, preventive conservation strategies. These include the implementation of drainage systems, capillary barriers, and the use of compatible materials in restoration, as well as the application of non-destructive diagnostic techniques such as electrical resistivity tomography and hyperspectral imaging. Understanding the interplay between soil moisture dynamics, salt crystallization, and material degradation is essential for safeguarding the cultural and structural value of historic buildings in the face of ongoing environmental challenges and climate variability.

1. Introduction

Historical monuments are tangible testimonies of history, bridging the past and present of a people and endowing their territory with a unique cultural identity. These constructions are an irreplaceable part of a nation’s cultural heritage and must be protected and preserved for future generations [1]. They stand as witnesses to past eras, reflecting the evolution of architecture over time, and often become iconic landmarks and recognizable symbols of their locale. These architectural treasures foster a sense of identity and promote cultural tourism, thereby contributing to the social and economic development of communities. Moreover, they inspire architects in the creation of contemporary designs [2].

It is important to highlight that the significance of historical monuments in contemporary architecture extends beyond their aesthetic and historical value. These structures also offer sustainability advantages, as their construction with durable materials and traditional techniques can reduce the carbon footprint and promote sustainable development [3]. Furthermore, their preservation helps maintain architectural diversity and prevents the homogenization of the built environment, as each historical monument is unique, with its own history and architectural style, contributing to the richness and variety of architecture. The restoration of these monuments also provides opportunities for adaptive reuse and sustainable construction, as it allows for the utilization of existing resources and helps reduce the environmental impact of new construction [4].

After Italy, Spain has the second-highest number of properties inscribed on the World Heritage List. Every autonomous community in Spain has properties on this list, supported by effective protection mechanisms and management projects, as highlighted in reports by the Spanish Committee of the International Council on Monuments and Sites (ICOMOS) [5]. The degradation of irreplaceable historical stone structures made of limestone, primarily due to the combined interactions of substrates, salts, and water, is one of the most critical challenges currently faced in conservation efforts [6,7].

Soil plays a critical role in the hydrological cycle, regulating infiltration, storage, and redistribution of water within watersheds. These processes influence surface and hypodermic runoff, groundwater flow, and aquifer recharge [8], forming the basis for hydrological interactions that sustain ecosystems and influence built environments. Furthermore, soil contributes to atmospheric humidity through both physical evaporation and plant transpiration. When vegetation is present, these processes combine in evapotranspiration, which serves as a significant pathway for water loss to the atmosphere [8,9].

Water movement through the soil profile is governed by gravitational, osmotic, and capillary forces [10]. Capillary rise occurs at matric tensions exceeding 33 kPa, allowing for the upward migration of water from wetter to drier regions [9]. Since surface layers tend to dry more quickly than deeper horizons, an unsaturated saline capillary flow is generated, transporting water and dissolved salts from deeper layers toward the surface [10]. In well-drained soils, this process is slower, but when a shallow water table is present, capillary rise is more pronounced, enabling a sustained vertical flow of water that is not retained by the soil matrix [8,11]. In soils with a balanced texture, this rise can provide 2–3 mm of water per day up to a height of 0.8–1 m above the water table, equivalent to the potential evapotranspiration of a meadow during the summer months [12]. Although this mechanism has been widely studied in agricultural contexts, its relevance extends beyond agronomy. In particular, its implications for the conservation of historical architecture have received far less attention.

This upward movement of saline moisture can have serious consequences for historical constructions. When it reaches calcareous stones—especially those whose porosity has increased due to weathering—it can trigger salt crystallization during evaporation, generating hygroscopic effects that compromise the mechanical integrity of the material [13,14]. The degree of damage depends on pore size and distribution, saturation levels, and the type of salts present. Some salts, such as sodium sulfate, undergo transitions between hydration states, leading to significant volumetric changes. These dissolution and recrystallization cycles generate internal pressures within the pore network, contributing to structural damage [15]. The most aggressive salts often result from ion exchange processes between the solid and liquid phases of the soil, with sulfates, sodium, magnesium, and chlorides being the most prevalent [16].

In semi-arid and Mediterranean climates, such as Southern Europe, the Middle East, or parts of Latin America, the combination of high evaporation rates, shallow water tables, and saline soils exacerbates this type of deterioration. These climatic and edaphic conditions are especially prevalent in historical urban environments, often located near ancient irrigation systems or canals. As climate change alters precipitation patterns and increases the frequency of dry spells, salt accumulation induced by capillary rise is expected to intensify in historic masonry [17].

In addition to climatic factors, human activities also play a significant role. Urbanization and inadequate groundwater management can disrupt the soil–water balance around monuments, promoting the rise or stagnation of saline water beneath structures. In some cases, restoration techniques or landscaping changes—such as replacing permeable surfaces with impermeable materials—have inadvertently intensified capillary flows, aggravating salt crystallization cycles [17,18]. Factoring in these human-induced alterations is essential for implementing effective and lasting conservation measures. Such strategies must also consider the cultural and historical significance of the affected architecture.

The economic and cultural value of historical architecture affected by saline weathering is incalculable. From Roman amphitheaters and Islamic fortresses to colonial churches and medieval monasteries, these structures represent unique cultural identities and artisanal excellence [6]. Their deterioration—often driven by slow, undetected processes originating in the soil—underscores the urgency of integrated conservation approaches that combine soil science, material engineering, hydrology, and heritage conservation [7]. Thus, this review takes a multidisciplinary approach, synthesizing findings from soil physics and stone conservation. By focusing on the interaction between upward saline flows, crystallization processes, and mechanical deterioration of calcareous stone, it aims to contribute to the understanding of salt weathering and its implications for heritage preservation.

The repeated cycles of dissolution, migration, and crystallization of salts are recognized as the primary drivers of calcareous rock weathering, both at the surface and within the substrate of historic structures [17,19]. Addressing this issue requires an integrated understanding of soil hydrodynamics, salt behavior, and material degradation. Accordingly, this review adopts a multidisciplinary perspective—bridging soil science, hydrology, and heritage conservation—to clarify the mechanisms through which saline capillary rise compromises the structural integrity of calcareous stone. In doing so, it aims to fill a critical gap in the literature concerning the links between soil water dynamics, upward salt migration, and the progressive deterioration of calcareous materials in historical buildings.

2. Energy State and Hydraulic Conductivity

Water movement within soil occurs through transmission macropores—interconnected channels with diameters greater than 30 µm [9]. This flow brings water into contact with the surfaces of mineral and organic particles that make up the soil’s solid matrix, exposing it to a complex set of forces. These forces arise primarily from interactions with the solid matrix, gravity, and dissolved ions in the water [20].

Due to the difficulty of precisely measuring the magnitude and direction of these forces, the concept of the soil water energy state is used [21]. The energy state and movement of water are governed by its potential energy at each point in the soil, expressed relative to an arbitrary reference level. The main components contributing to the total potential (Ψ_t) are shown in Equation (1):

Ψ_t = Ψ_g + Ψ_o + Ψ_p

(1)

Ψ_g: gravitational potential, which is associated with the vertical position of water in the soil. It plays a key role in draining excess water from surface horizons after rainfall or irrigation.

Ψ_o: osmotic potential, which is caused by dissolved ions in the soil solution. It is especially relevant in saline soils, where high salt concentrations reduce the total water potential [22].

Ψ_p: pressure potential, which includes other forces acting on soil water, such as air pressure, hydrostatic pressure, and interactions with the soil matrix. These are broken down in Equation (2):

Ψ_p = Ψ_p^a + Ψ_p^u + Ψ_p^m

(2)

Ψ_p^a: pneumatic (air pressure) potential, representing the effect of excess gas pressure at a given soil moisture content.

Ψ_p^u: hydrostatic potential, representing the pressure from a column of water above a given point in saturated soil.

Ψ_p^m: matric or capillary potential, which arises mainly from capillary and adsorption forces that retain water in the pores. This effect is more pronounced in finer pores, meaning that as the soil dries, the remaining water is increasingly confined to these smaller spaces. Matric potential is typically higher in fine-textured soils, such as clayey soils [23].

Commonly used in soil science, matric potential (Ψ_p^m) reflects the influence of the soil matrix. In unsaturated soils, it is negative and can reach values as low as −2000 kPa, while it becomes zero below the water table [21,23]. To simplify the interpretation of these large negative values, the logarithmic scale pF is used. The pF is defined as the base-10 logarithm of the height (in cm) of a water column that corresponds to the suction required to retain water in the soil. In essence, it reflects the pressure needed to extract water from the soil, which corresponds to how strongly it is retained [20].

This scale allows the different constants that characterize soil–water relationships to be expressed within a manageable range of values. For example, the permanent wilting point corresponds—across all soil types—to a capillary potential of 16 atm, or pF = 4.2 (Figure 1). In contrast, the water holding capacity varies by soil texture: pF = 2 for sandy soils, 2.5 for loamy soils, and 3 for clay soils [12,24].

Hydraulic conductivity, the proportionality factor in Darcy’s law as applied to viscous water flow through soil, quantifies a porous medium’s (i.e., soil’s) capacity to transmit water [25,26]. It depends, among other factors, on the soil’s water content and is therefore influenced by the matric potential. Soil resistance to water flow arises from cohesive forces between water molecules and adhesive forces between the water and the pore walls [20,21]. Under saturated flow conditions, hydraulic conductivity remains constant over time, provided that the soil structure remains unaltered [27].

Measuring hydraulic conductivity at different soil depths allows for the identification of variations in water transmission capacity across soil horizons. These vertical variations highlight soil heterogeneity and anisotropy [28]. In environmental and engineering contexts involving water movement, calculations often rely on saturated hydraulic conductivity values (Ks). Representative Ks values vary by soil texture and structure: approximately 10 mm day⁻¹ for fine-textured soils; 10 to 1000 mm day⁻¹ for well-structured loamy soils; and over 1000 mm day⁻¹ for coarse-textured soils, such as sandy soils [25,29].

The response of calcareous materials to water and salt exposure is strongly influenced by their petrophysical properties, such as porosity, pore size range and distribution, and internal connectivity [30]. Stones with finer pore structures tend to retain more water but are also more susceptible to salt crystallization damage, as the pressure exerted by growing salt crystals in confined pores can cause microfractures, weakening the material and making it more prone to fragmentation and visible degradation [31]. Conversely, materials with larger pores often allow greater evaporation rates, promoting surface salt accumulation and associated weathering processes, such as salt crystallization pressure, hydration–dehydration cycles, chemical alteration, and physical disintegration [6]. When soils exhibit high capillary rise combined with petrophysically vulnerable stone, the degradation process can accelerate, particularly under climates characterized by frequent wetting and drying cycles [32].

A recent study has demonstrated that in historical settings, especially in urban areas with shallow water tables and high salt content in the subsoil, capillary rise contributes significantly to the upward transport of saline solutions that ultimately crystallize in the stone matrix [33]. These processes are intensified by repeated wetting and drying cycles, which enhance both chemical weathering (e.g., dissolution and ion exchange) and mechanical damage due to crystallization pressure and thermal stress [34]. Moreover, fluctuating relative humidity in proximity to the soil–air interface accelerates salt efflorescence and subflorescence, leading to visible surface decay and internal fracturing of the stone [35,36]. Altogether, the convergence of soil hydrodynamics, salt migration, stone microstructure, and local climatic fluctuations governs the extent and rate of decay in calcareous heritage materials, underscoring the need for integrated conservation approaches.

This mechanistic framework is further supported by empirical observations in Spanish heritage contexts. Soil analyses from historic sites such as the Alhambra in Granada and coastal monuments in Cádiz have revealed electrical conductivity values exceeding 2.5 dS m⁻¹ in the upper 20 cm of soil, indicating moderate to high salinity levels capable of sustaining recurrent salt crystallization cycles [35,37]. In these areas of southern Spain, capillary rise heights of up to 80–90 cm have been reported in clay-rich soils adjacent to historic foundations, allowing direct contact between saline subsoil moisture and porous calcareous stones [35]. Similarly, in the semi-arid region of Bardenas (Aragón), soil electrical conductivity values of 3–4 dS m⁻¹ were documented within the capillary fringe, with water ascending to approximately 70 cm in clay–loam profiles [38]. These quantitative findings provide strong empirical support for the role of capillary saline fluxes in the progressive deterioration of calcareous materials in Spanish historical monuments.

3. Water Movement in Soil

When water movement in soil is primarily driven by the gravitational field—at water potentials lower than −33 kPa—the flow is known as saturated flow, and it follows Darcy’s law. In contrast, unsaturated flow occurs when water is subject to suctions greater than gravitational forces, with other forces such as capillary and osmotic forces becoming predominant. In this regime, flow is governed by the Buckingham–Darcy law [39].

Saturated flow refers to water movement in soils where the pore space is completely filled with water, typically in the presence of a high water table. If the flow is very slow and oxygen renewal is limited, anaerobic conditions may develop due to poor drainage. Saturated flow generally begins with infiltration following precipitation or irrigation. Infiltration is the process by which water enters the soil from the surface and moves into the subsurface. Part of the infiltrated water is retained in the soil against gravity due to interactions with the solid matrix and may be lost later through evaporation or evapotranspiration. The remaining fraction percolates into deeper soil layers and eventually reaches the water table [22]. The continued movement of water through already saturated soil is termed percolation, which contributes to groundwater recharge and transports soluble salts downward from the upper soil layers [40].

Unsaturated flow is predominant in soils undergoing cycles of wetting and drying. When the soil is relatively dry, vapor-phase water transport driven by thermal gradients becomes significant. In contrast, when the soil is moist, capillary flow in any direction plays a greater role. Under unsaturated conditions, pores are only partially filled with water, resulting in a hydraulic conductivity that is lower than in saturated conditions and variable, depending on the matric potential [41].

Water infiltrates soil rapidly through macropores, and infiltration is reduced by any factor that diminishes the number or size of pores. Several factors influence the rate of infiltration [42]:

• Soil texture and structure: Coarse-textured soils, such as sands, facilitate rapid infiltration. Soils with stable aggregates and granular structure also tend to have higher infiltration rates;
• Organic matter content: Increases aggregate stability and protects soil structure by reducing the impact of raindrops;
• Depth to bedrock or impermeable layers: Shallow soils limit the volume of infiltration compared to deeper profiles;
• Initial soil moisture content: Infiltration is slower in already saturated soils;
• Soil temperature: Warmer soils generally allow faster infiltration due to lower viscosity and enhanced biological activity.

Based on these characteristics, infiltration rates can be classified as follows [43,44]:

-

Very low (<0.25 cm h⁻¹): Typical of clay-rich, compacted soils;

-

Low (0.25–1.25 cm h⁻¹): Found in shallow or degraded soils with low organic matter;

-

Medium (1.25–2.5 cm h⁻¹): Observed in loamy soils with moderate structure;

-

High (>2.5 cm h⁻¹): Associated with sandy or silty loam soils with deep profiles.

During hot, dry periods, high evaporation (or evapotranspiration if vegetation is present) reduces soil water content below field capacity. Even under prolonged drought, the soil retains a minimal amount of water, known as hygroscopic water, which is tightly bound to soil particles at a potential of approximately pF 4.7 (around −5000 kPa or 50 atmospheres). This residual moisture does not participate in water transport processes and therefore does not contribute to capillary rise or salt mobilization, but it reflects the strong affinity of dry soils for retaining moisture under extreme conditions [22].

The dynamics of water movement in soils—particularly under unsaturated conditions—play a central role in the capillary rise of saline solutions, a key process in the deterioration of historic calcareous materials. In fine-textured soils, water can ascend from deeper layers by capillary action, transporting dissolved salts toward the surface or into porous building materials lacking damp-proof courses. Once near the surface, evaporation induces salt precipitation within the pore structure of stone or mortar, triggering crystallization and hydration–dehydration cycles. These cycles, intensified by fluctuations in temperature and humidity, generate mechanical stresses that lead to scaling, granular disintegration, or contour flaking of calcareous stones [45]. Thus, understanding water flow and capillary behavior in soils is essential for assessing the risk of salt-induced weathering in historic constructions exposed to rising damp.

4. Soluble Salts and Capillary Rise

The accumulation of soluble salts in soils primarily originates from the weathering of rocks, which release their constituent elements. Two principal salinization cycles can be distinguished [46]: (1) continental cycles, where chlorides, sulfates, sodium carbonate, and bicarbonates are mobilized, redistributed, and accumulated in low-lying areas with poor drainage as a result of rock weathering, and (2) marine cycles, in which soils in coastal plains and marshes accumulate marine salts, primarily sodium chloride, due to shallow saline water tables, tidal flooding, or the deposition of salts from maritime air masses. These salts may be deposited either as aerosols containing suspended salt crystals or as saline droplets [37].

In Spain, the continental salinization cycle is the most prevalent, particularly associated with calcilutites (marl) from the Keuper facies, Oligocene, or Miocene periods. These formations serve as sources for the redistribution of salts across multiple drainage basins [47].

The use of electrical conductivity (EC) and the exchangeable sodium percentage (ESP) has become standard practice for evaluating soil behavior in relation to salinity and sodicity. These two parameters allow for the classification of soils based on their soluble salt content and ESP [48] (

Table 1. Indicative values of EC, determined using a soil: water ratio of 1:5, and sodicity index (ESP), calculated by the following formula: ESP = (Exchangeable Na/Cation Exchange Capacity) × 100.

	Non-Saline	Slightly Saline	Saline	Highly Saline
EC (dS m−1)	<0.35	0.35–0.65	0.65–1.15	>1.15
	Non-sodic	Slightly sodic	Sodic	Highly sodic
ESP (%)	<7	7–15	15–30	>30

).

The salt mineralogy is highly complex and exhibits significant spatial and temporal variability, depending on the temperature and humidity of the environment in which they crystallize. As a general reference, the most representative types of salts are listed in

Table 2. The most representative soluble salts in soils.

Class	Presence in Soils	Solubility (g L−1)
Chlorides
Sodium	Common	264
Magnesium	Common	353
Calcium	Very low	400–500
Potassium	Low	344
Sulfates
Sodium	Common	f(t) 1
Magnesium	Common	262
Potassium	Low	f(t) 2
Sodium carbonate	Sodic soils	178
Sodium bicarbonate	Sodic soils	262
Nitrates	Very low	High

¹ Solubility depends on temperature. At low temperatures, it dissolves slowly (50 g L⁻¹ at 0 °C). ² Solubility increases to 111 g L⁻¹ at 20 °C.

[12].

Rainfall causes a downward movement of water in the soil, resulting in the leaching of salts. During dry months, when precipitation (P) is lower than potential evapotranspiration (PET) (P-PET < 0), evapotranspiration will cause the flow to reverse, leading to the upward movement of salts. These processes occur cyclically in a non-percolating moisture regime. The temporal variability of soil salinity must be studied across different periods. For example, it has been observed that the top 10 cm of soil can reach a salinity approximately ten times higher in May compared to late October, after the autumn rains [49].

The crystallization of salts near the evaporation front—where moisture transported by capillarity is lost to the atmosphere—generates mechanical stress that often exceeds the tensile strength of calcareous stones. Over time, this process leads to the formation of efflorescence and the deterioration of both the structural and aesthetic integrity of heritage buildings [7]. In heritage contexts, these effects are intensified by anthropogenic and environmental changes such as altered drainage conditions, rising groundwater levels, or urban sealing, all of which promote moisture accumulation around the base of monuments. Additionally, certain calcareous stones commonly used in historic constructions—such as marly limestones and fossiliferous calcarenites—are particularly vulnerable due to their mineralogical composition and high capillary absorption capacity [50,51]. Effective conservation, therefore, requires not only identifying the nature and source of soluble salts but also implementing strategies to manage the environmental conditions that control moisture transport and salt crystallization, including ventilation, evaporation dynamics, and material porosity.

In parallel with these conservation-oriented measures, ongoing research has increasingly focused on predictive modeling approaches that integrate environmental variables and material properties to assess salt-related deterioration risks under diverse climatic scenarios. For instance, Cappai et al. [52] implemented a Mamdani-type fuzzy inference model that integrates environmental variables (e.g., temperature, relative humidity, solar radiation, and rainfall) with stone properties to simulate monthly probabilities of salt crystallization cycles. This model showed a strong correlation with observed efflorescence patterns at a Mediterranean archaeological site and provides a non-invasive framework for assessing crystallization risk over time. In addition, thermodynamic formulations—such as those described by Flatt et al. [53]—allow the estimation of crystallization pressures based on salt supersaturation and environmental conditions, facilitating the evaluation of mechanical stress within the stone pore network during humidity fluctuations. However, Godts et al. [54] highlight that equilibrium-based models alone are insufficient, as actual deterioration risk also depends on the kinetics of salt crystallization and dissolution, as well as on the behavior of mixed salts frequently present in built heritage. Laboratory experiments by Benavente et al. [55] further support this approach by quantifying deterioration through mass loss in porous stones subjected to controlled wetting–drying cycles with saline solutions. Adapting these predictive models to regional climatic datasets would enable simulations of future degradation rates in calcareous monuments under various IPCC climate scenarios—standardized projections developed by the Intergovernmental Panel on Climate Change to represent alternative greenhouse gas emission pathways.

5. Efflorescence and Subflorescence

The UNE 41805-5:2009 standard, Building Diagnosis. Part 5: Pathological Diagnosis of Building Structures [56] defines mechanical, physical, and chemical pathological processes that can affect buildings constructed with load-bearing walls. Among these, the most complex are those caused by rising capillary moisture, whose effects range from simple surface staining (efflorescence) to more severe deterioration that may even compromise the structural stability of the building.

Efflorescence refers to the formation of crystalline salt deposits on the surface of porous building materials, resulting from the evaporation of water from a salt-laden solution. These deposits usually form as whitish, coherent outer layers due to the migration and subsequent crystallization of dissolved salts on the surface of the material (Figure 2). The origin of these salts may lie in the constituent building materials, such as bricks or stones, the mortar used in masonry joints, or in groundwater that rises through capillary action and interacts with the foundation [6,57].

Several research has emphasized that efflorescence is not solely related to rising damp but can also be triggered by external environmental factors, such as marine aerosols in coastal regions, de-icing salts in colder climates, or atmospheric pollutants, all of which can introduce soluble salts into the masonry through direct deposition or rainwater infiltration [6,51,58].

The texture and cohesion of efflorescence can vary significantly depending on the type and concentration of salts, ambient humidity, and the porosity of the substrate. In many cases, the deposits are powdery, friable, and easily removed by dry brushing or light mechanical cleaning. However, in other instances, particularly when calcium or magnesium salts are involved—such as gypsum (CaSO₄·2H₂O) or epsomite (MgSO₄·7H₂O)—crystallization often leads to the formation of denser, more adherent crusts. These crusts can bond strongly to the surface of the substrate, making them considerably more difficult to remove without risking damage to the underlying material. This phenomenon is especially pronounced under fluctuating humidity conditions, which promote repeated dissolution and recrystallization cycles within the pore network, thereby increasing the crystallization pressure and the degree of adhesion [59,60].

In general, the damage caused by efflorescence on vertical surfaces is not structurally significant, as it usually manifests as light-colored surface stains. These typically do not compromise the integrity of the construction materials. In the worst-case scenario, some types of paints or thin-layer coatings may degrade or detach [61].

In contrast, subflorescence represents a much greater threat to material durability. In this process, salts crystallize within the pore structure of the stone or mortar, exerting crystallization pressure on the pore walls. This internal pressure can exceed the tensile strength of the material, leading to microcracking, pore expansion, and loss of cohesion [62]. Because this form of crystallization occurs beneath the surface, it often evolves without any early visible signs, complicating early diagnosis. Recent research using X-ray computed tomography (XCT), digital image correlation (DIC), and scanning electron microscopy (SEM) has revealed that subflorescence induces microstructural weakening even at early stages of salt crystallization [63]. In more advanced stages, subflorescence leads to alveolar fracturing, a reduction in the effective structural cross-section, and degradation of the material’s mechanical and elastic properties (Figure 3). In its most severe form, this process may cause creep, irreversible deformation, and local collapses that threaten the stability of the structure (Figure 4 and Figure 5) [64].

These salt-related decay phenomena—ultimately driven by capillary rise—pose significant challenges not only for the preservation of the material’s appearance but also for long-term conservation and structural safety. The cumulative and often concealed nature of damage caused by salt crystallization, particularly subflorescence, necessitates proactive approaches to diagnosis and maintenance. Without timely intervention, these processes can lead to irreversible material loss and compromise mechanical integrity, resulting in extensive and costly restoration efforts.

Comprehensive conservation strategies should encompass preventive measures such as the implementation of effective drainage systems, the application of barrier layers to mitigate moisture ingress, and the use of compatible materials in repair works. For example, clay-based barriers have demonstrated efficacy in limiting capillary moisture intrusion in historic structures [65], representing a sustainable solution to reduce salt mobilization.

These strategies have proven effective in practice, as demonstrated by recent conservation interventions in historic Spanish monuments. Field experiments conducted on a segment of the western wall of the medieval Muralla de Ávila showed that structural interventions aimed at limiting moisture ingress—such as waterproofing the adarve and improving surface and subsoil drainage—significantly reduced indicators of rising damp. Capacitance-based moisture probes (FDR) recorded a 30–40% reduction in volumetric water content over a six-month monitoring period, with treated sections consistently maintaining lower moisture levels than untreated controls [66]. The intervention materials, including membranes and drainage channels, were fully compatible with the original masonry and required only a moderate investment, estimated by the IPCE at approximately EUR 150/m², which is substantially lower than typical consolidation treatments (EUR 1500–2000/m²). Similarly, a comprehensive study of several historic buildings in the old town of Seville reported that chemical capillary barriers, particularly silane-based injections combined with moisture-resistant coatings, achieved reductions in rising damp levels exceeding 70% over a five-year period [67]. These treatments were found to be minimally invasive, cost-effective, and highly compatible with limestone and lime-based mortars, while maintaining significantly lower moisture levels in treated areas. Together, these findings highlight the long-term efficacy, material compatibility, and economic feasibility of both physical and chemical capillary barrier systems as effective conservation strategies for calcareous heritage structures across diverse climatic regions in Spain.

Recent advances in non-destructive testing (NDT) methods have significantly enhanced the ability to monitor salt-related damage. Electrical resistivity tomography (ERT) has been successfully applied to assess moisture and salt distribution within masonry elements [68], while hyperspectral imaging (HSI) allows the surface mapping of salt efflorescence and material alteration without physical contact [69]. Moreover, the integration of ground-based remote sensing with machine learning algorithms is emerging as a powerful tool for the in situ, real-time identification of salts and moisture in historic buildings, enabling continuous monitoring and early intervention [70].

While non-destructive techniques are essential tools for diagnosing and monitoring salt weathering in heritage structures, it is crucial to acknowledge their methodological boundaries and limitations. Techniques such as ground-penetrating radar (GPR), infrared thermography (IRT), and electrical resistivity often face resolution constraints—particularly when applied to stones with high salt content. Elevated salinity increases ionic conductivity, which can reduce the penetration depth of electromagnetic signals in GPR and distort resistivity readings, potentially leading to inaccurate interpretations [71,72]. Likewise, surface-based methods such as hyperspectral imaging and thermography may have difficulty distinguishing superficial salt crusts from deeper accumulations within heterogeneous substrates, thereby limiting their diagnostic precision [73,74]. In addition, environmental factors, calibration errors, and ambient noise introduce further uncertainties that complicate data interpretation and reduce reproducibility [75]. For this reason, while non-destructive approaches remain valuable for early detection and risk mapping, their results should ideally be complemented by targeted destructive sampling or long-term monitoring where high diagnostic accuracy is required for conservation planning.

Recent studies in materials science and environmental engineering have contributed mechanistic insights that help to deepen our understanding of salt behavior in porous substrates. For instance, Zheng et al. [76] investigated the adsorption mechanisms of gaseous elemental mercury using a Fe-UiO-66@BC composite, demonstrating how pore structure, surface functionalization, and physicochemical interactions affect adsorption efficiency and stability under dynamic conditions. Although primarily developed for pollutant removal, this research highlights the critical roles of internal porosity, ion mobility, and crystallization dynamics—factors that similarly govern subflorescence-related deterioration in calcareous materials. These parallels suggest that emerging composite materials and advanced characterization techniques may offer promising pathways for developing innovative protective strategies against salt-induced decay in built heritage.

Understanding the complex interactions between environmental conditions, salt dynamics, and material properties is crucial to designing sustainable preservation strategies. The implementation of preventive conservation, supported by advanced diagnostic technologies, can significantly enhance the resilience of calcareous heritage materials to salt weathering processes induced by capillary rise.

6. Conclusions

Capillary moisture is inherent in virtually all buildings constructed with load-bearing walls. Its effects range from superficial deterioration and insalubrity to long-term structural damage that may compromise the mechanical stability of the building. The phenomenon of capillary rise is inherently complex, involving multiple interacting variables such as pore size distribution, soil texture, electrical charges, migratory gases, environmental conditions, and the mineralogy of the salts involved.

In the context of historic constructions built with calcareous materials, capillary rise facilitates the upward migration of saline solutions from the subsoil into the masonry. As these solutions evaporate, salts crystallize either on the surface (efflorescence) or within the pore structure (subflorescence), leading to internal stresses that exceed the tensile strength of the material and contribute to its physical and chemical degradation.

Particularly vulnerable are certain calcareous stones—such as fossiliferous calcarenites and marly limestones—which exhibit high porosity and strong capillary absorption, accelerating the cycles of salt dissolution and recrystallization.

In Spain, the most harmful salts present in soils are chlorides and sulfates, with the latter being particularly damaging due to the greater volumetric expansion of their crystals. Therefore, identifying the type and origin of these salts is essential for understanding the underlying causes of deterioration and for selecting effective conservation strategies.

Preserving calcareous heritage buildings requires a multidisciplinary approach that integrates soil physics, material science, hydrology, and conservation engineering. The implementation of preventive measures—such as improved drainage, capillary barriers, and the use of compatible repair materials—must be informed by a thorough diagnostic process, including the use of advanced non-destructive testing techniques. In light of climate change and increasing environmental pressures, developing sustainable and proactive conservation methodologies will be essential to protect the cultural and historical value of these irreplaceable structures.