Nano Calcium-Aluminum Layered Double Hydroxides for the Conservation of Earthen Immovable Cultural Heritage

Abstract

Earthen immovable cultural relics, such as murals and painted clay sculptures, are prone to deterioration (e.g., efflorescence, flaking, and cracking) under long-term preservation conditions. While conventional restoration materials primarily offer reinforcement, they fail to regulate the migration of soluble salts within the relics, which is the main cause of such damage. Herein, aimed at protecting the painted sculptures and murals of the Yungang Grottoes, nano calcium-aluminum layered double hydroxides (Ca-Al LDHs) were prepared, and their effectiveness in regulating salt crystallization within the earthen ground layer, as well as their reinforcement performance were investigated. Simulated salt crystallization tests revealed that coating the ground layer with Ca-Al LDHs delayed salt-induced damage time by 150%. This can be attributed to the ability of Ca-Al LDHs to adsorb sulfate ions from soluble salts, thereby inhibiting the crystallization of magnesium sulfate on the surface of the ground layer. After curing Ca-Al LDHs-coated samples at 35 °C and 55% relative humidity (RH) for 7 days, their surface Leeb hardness increased by 3.1%, and the weight loss rate (measured via tape peeling test) decreased by 38.3%. These results indicate that the surface bonding strength was enhanced following Ca-Al LDHs coating, with the underlying mechanism being the transformation of part of the LDHs into calcium carbonate under the influence of water and carbon dioxide. This study demonstrates that Ca-Al LDHs not only suppress magnesium sulfate crystallization but also provide effective surface consolidation, showing promising potential for application in conserving painted sculptures and murals at the Yungang Grottoes.

1. Introduction

Earthen immovable cultural heritage, such as the murals and painted clay sculptures in the Yungang Grottoes, constitutes an irreplaceable carrier of human civilization and exquisite craftsmanship. However, these delicate heritage assets are increasingly threatened by severe and prevalent deterioration, including powdering, flaking, and cracking [1,2]. The primary driver of this decay is the cyclic crystallization of soluble salts [3,4] (e.g., sulfates, chlorides) within the porous structure of the earthen ground layer. Fluctuations in environmental humidity cause moisture to transport salts, leading to repeated cycles of dissolution, migration, supersaturation, and crystallization [4,5,6]. The crystallization pressure exerted during salt growth generates significant stress within the pores, mechanically disrupting the microstructure and cohesive bonds of the substrate material. Therefore, developing protective materials that simultaneously suppress salt crystallization and provide structural reinforcement is imperative for the conservation of the Yungang Grottoes’ cultural heritage and similar earthen relics.

Current conservation approaches for salt mitigation and consolidation can be broadly categorized into organic and inorganic materials, each with inherent limitations [7]. Organic polymers, such as acrylics and silanes, offer good adhesive strength and water repellency but raise concerns regarding long-term stability, poor vapor permeability, potential discoloration, and compatibility issues with historic substrates [8,9]. Conversely, traditional inorganic materials like lime-based consolidants exhibit better compatibility and durability but often lack the ability to interact with or control the salts responsible for deterioration [10,11]. Recent advanced strategies have focused on creating functional composites. These include hydrophobic coatings designed to block liquid water ingress [12,13], and innovative organic-inorganic hybrids that aim to modify crystallization habits or transport salts outward [14,15]. While promising, many of these solutions still incorporate organic components or focus solely on creating a barrier, rather than addressing the root cause by immobilizing the damaging ions within the substrate matrix. Consequently, there is a clear research gap for developing a fully inorganic, multifunctional material that can actively inhibit salt crystallization through ion immobilization while concurrently strengthening the weakened substrate, without introducing long-term risks associated with organic compounds.

Layered double hydroxides (LDHs) possess a unique layered structure with interlayer anion-exchange capacity and structural memory effect, making them promising for adsorption and consolidation applications [16,17]. Previous studies have demonstrated LDHs’ efficacy in ion adsorption [18,19] and relic reinforcement—for instance, CaAl-LDH, MgAl-LDH, and CaMgAl-LDH have been shown to reinforce stone tablets [20]. Their ion-exchange capability enables adsorption of harmful salt ions (e.g., SO₄²⁻), while metal cations (e.g., Ca²⁺) released during interaction can potentially enhance matrix mechanical properties. Given these characteristics, LDHs present themselves as an ideal candidate for combating salt damage and consolidation needs in cultural heritage. Motivated by the limitations of existing materials, we propose a novel preventive strategy based on calcium-aluminum layered double hydroxide (CaAl-LDH). The core innovation of this method is its use as an ion-exchange material—specifically applied to trap and stabilize harmful sulfate anions within the substrate. By targeting the primary damaging salt at the Yungang Grottoes (MgSO₄) at its source, this approach seeks to prevent crystallization from occurring, moving beyond conventional methods that only create barriers or remove salts post-damage.

Herein, we synthesize nano-sized CaAl-LDH via a coprecipitation-hydrothermal method and evaluate its performance in protecting simulated samples mimicking the Yungang Grottoes’ painted clay sculptures and mural ground layers. Through water-salt migration and carbonation experiments, the efficacy of CaAl-LDH in inhibiting MgSO₄ crystallization and enhancing matrix consolidation was systematically investigated, and the underlying mechanisms were analyzed.

2. Materials and Methods

2.1. Experimental Materials

Calcium chloride dihydrate (CaCl₂·2H₂O) and aluminum chloride hexahydrate (AlCl₃·6H₂O) were purchased from Tianjin Kermel Chemical Reagent Co., Ltd., Tianjin, China. Sodium hydroxide (NaOH) was obtained from Shanghai Maclin Biochemical Technology Company, Shanghai, China. Magnesium sulfate heptahydrate (MgSO₄·7H₂O) was obtained from Tianjin Tianli Chemical Reagents Ltd., Tianjin, China. All experimental materials were analytically pure, and all solutions were prepared using deionized water.

2.2. Synthesis of CaAl-LDH

Small-sized CaAl-LDH is necessary to enhance its penetration into cultural relic substrates, enabling more effective adjustment and protection. As illustrated in Figure 1, CaAl-LDH was synthesized via a combined coprecipitation-hydrothermal method. At room temperature, 50 mL of solution A (containing 1.98 M NaOH) was dripped at 50 µL/s into 50 mL of solution B (containing 0.66 M CaCl₂·2H₂O and 0.33 M AlCl₃·6H₂O) under vigorous stirring. The resulting mixture was then aged for 15 min. The resulting slurry was equally divided into two 100 mL Teflon-lined autoclaves and subjected to hydrothermal treatment at 85 °C for 5 h. After cooling to room temperature, the white precipitate was collected by centrifugation (8000 rpm, 5 min), washed alternately with deionized water and ethanol three times, and finally vacuum-dried to obtain CaAl-LDH powder.

2.3. Production of Simulated Sample and Addition of LDH

The simulated mural ground layer was formulated with soil, sand, and wheat straw in a mass ratio of 6.5:1:0.1 [22]. Each sample, with a total mass of 152 g, was prepared by dry-mixing the components and then adding deionized water until a wet but non-fluid consistency was achieved. The mixture was compacted into a cylindrical mold (3.5 cm in height × 6 cm in diameter) and dried in a natural environment under shade for one week. The simulated samples were divided into Group A (untreated) and Group B (treated with CaAl-LDH), with three replicates per group. The experimental procedure for Group B (Figure 1) was as follows: after drying, 0.02 g of CaAl-LDH was dispersed in 15 mL of ethanol via 15 min ultrasonication. This resulting dispersion was applied dropwise onto the sample surfaces. The dropwise application was repeated after the evaporation of each dose until the entire 15 mL dispersion had been delivered. The surface coverage is 7.07 g/m².

2.4. Water and Salt Migration Experiment

For the water and salt migration experiment, MgSO₄ was used as the damaging salt [6,23], with a loading equivalent to 1.3 wt% of each simulated sample’s mass. A salt solution (containing 8.592 wt% MgSO₄) was placed in a Petri dish together with a permeable stone, so that the liquid level of the salt solution was flush with the upper surface of the stone. The samples were then placed on the permeable stone to absorb the salt solution. This setup ensured that each sample absorbed a precise amount of 23 g of the solution. Following absorption, the sides and bottoms were sealed to ensure that evaporation occurred from the top, thereby inducing salt crystallization on the upper surface. Finally, the samples were maintained in a natural environment to monitor the subsequent deterioration processes. To further investigate the reaction mechanism between CaAl-LDH and MgSO₄, an experiment was conducted. First, 0.005 mol of MgSO₄ was dissolved in 100 mL of deionized water. After complete dissolution, 0.5 g of CaAl-LDH was added directly to the solution and allowed to react for a designated period. Subsequently, the treated CaAl-LDH was collected, vacuum-dried, and characterized by X-ray powder diffraction (XRPD) with a Rigaku Corporation, D/MAX 2200PC (Tokyo, Japan), scanning electron microscopy (SEM) with a Hitachi Regulus 8100 (Tokyo, Japan), and elemental analysis to examine its phase composition, morphology, and elemental content.

2.5. Carbonation Experiment

The carbonation test used simulated samples of identical composition and formulation to those in the water and salt migration experiment. However, for carbonation, the mixed soil was evenly spread in Petri dishes (12 cm diameter). The samples were then carbonated for 7 days at a constant 35 °C under three relative humidity (RH) conditions: 55%, 75%, and 95%. Each group comprised three sample types: an untreated control, a CaAl-LDH-treated sample (0.02 g LDH in 15 mL ethanol), and a CaAl-LDH slurry (0.2 g LDH with 0.05 mL water). After the experiment, the samples were tested by X-ray powder diffraction (XRPD) and strength tests to investigate the reaction between CaAl-LDH and CO₂ and to evaluate the consolidation effectiveness of the treatment on the simulated samples.

2.6. Experimental Instruments

The phase composition of the samples was determined using X-ray powder diffraction (XRPD) on a Rigaku Corporation, D/MAX 2200PC (Tokyo, Japan) with Cu Kα radiation (λ = 1.5406 Å), operated at 40 kV and 30 mA. Data were collected in the 2θ range of 5° to 80° with a step size of 0.05° and a scan speed of 10°/min. The surface morphology and microstructure of the samples were characterized by scanning electron microscopy (SEM) equipped with Energy dispersive spectrometer (EDS). A Hitachi Regulus 8100 (Tokyo, Japan) field emission SEM with a 15 kV accelerating voltage was used to obtain high-resolution images. A thin gold coating was applied to the samples to increase conductivity and improve image clarity. FT-IR spectra of LDH were recorded with a Bruker Vector 22 spectrometer (Ettlingen, Germany). The LDH sample was prepared for analysis by compacting 2 mg of the powder with 100 mg of KBr under hydraulic pressure using the KBr disc technique. Data was acquired in the range 4000–400 cm⁻¹, with a resolution of 0.07 cm⁻¹. The transmission electron microscopy (TEM) images were obtained on a FEI Tecnai G2 F20 S-TWIN (Hillsboro, OR, USA) at an acceleration voltage of 200 kV. For TEM sample preparation, LDH nanoparticles were dispersed in ethanol via ultrasonication for 15 min, followed by placing a droplet of the dispersion onto a copper grid coated with an amorphous carbon film. The microscopic morphology of salt crystallization on the samples was observed using a HIROX KH-8700 3D digital microscope (Tokyo, Japan). Under ring illumination, observations were carried out at magnifications of 140× and 350× to examine both the powdering and the surfaces.

2.7. Transparent Tape Test (STT)

The consolidation effect of CaAl-LDH on the simulated samples was evaluated by measuring weight loss. A rectangular strip of transparent tape (2.65 cm × 1 cm) was firmly adhered to the sample surface and then peeled off at a constant speed using tweezers [24]. This process was repeated three times under identical conditions, and the average weight loss was calculated. The weight loss (WL) was determined as follows:

$$\mathit{W}\mathit{L}={\displaystyle \frac{{\mathit{m}}_{\mathbf{2}}-{\mathit{m}}_{\mathbf{1}}}{\mathit{a}\mathit{\ast }\mathit{b}}}$$

In the formula, m₁ is the weight of the square tape, m₂ the weight of the tape removed from the sample, ‘a’ the length, and ‘b’ the width of the transparent tape.

2.8. Hardness Test

The surface hardness of the simulated samples was measured using a Linshang Leeb Hardness Tester (Shenzhen, China) [24]. The Leeb hardness of both the untreated and the CaAl-LDH-infiltrated samples was determined after carbonation. Each sample was measured five times, and the average value was calculated.

2.9. Salt Crystallization Monitoring and Damage Assessment

Monitor the crystallization process by acquiring optical images through hourly macroscopic observations. The damage was categorized according to the following criteria: “Edge Crystallization” was defined as the presence of crystals with a height not exceeding 0.3 cm covering more than 75% of the observed edge area. “Edge Powdering” referred to the condition where crystals taller than 0.3 cm covered over 75% of the observed edge area. “Partial Surface Powdering” was identified when the area covered by powdered crystals (height > 0.3 cm) was less than 80% of the total sample surface, leaving the simulated substrate still clearly visible. Finally, “Complete Surface Powdering” was defined as the state where such powdered crystals covered more than 80% of the surface, rendering the simulated substrate nearly invisible.

3. Results and Discussion

3.1. Phase and Morphology of CaAl-LDH

The crystal structure and phase composition of CaAl-LDH were characterized by XRPD. As shown in Figure 2a, the pattern exhibits characteristic (002), (004), and (020) reflections corresponding to a Cl⁻-intercalated CaAl-LDH [25], with sharp peaks indicating high crystallinity. The FT-IR spectrum of CaAl-LDH is presented in Figure 2b. The broad absorption band between 3400 and 3600 cm⁻¹ corresponds to the O–H stretching vibrations of hydroxyl groups and water molecules. The peak at 1619 cm⁻¹ is attributed to the bending vibration of hydrogen-bonded interlayer water. The peak at 1406 cm⁻¹ is assigned to the symmetric stretching vibration of CO₃²⁻, indicating the presence of carbonate as an interlayer anion, likely due to CO₂ incorporation during synthesis. The absorption features below 1000 cm⁻¹ are ascribed to metal–oxygen (M–O) vibrations within the hydrotalcite layers, specifically from Ca–O and Al–O bonds. The TEM and SEM images of the prepared CaAl-LDH are shown in Figure 2c–e. The synthesized CaAl-LDH was oval-shaped, and the particle size was 200 nm–300 nm. The surface of the material was smooth, with slight agglomeration. The elemental composition and distribution of the material were analyzed by EDS mapping. As can be seen in Figure 2f, EDS images of CaAl-LDH demonstrate that the elements Ca, Al and O were uniformly distributed in the material.

3.2. Water and Salt Migration Experiment

The surface morphology of simulated samples, Group A (untreated) and Group B (treated with CaAl-LDH, before the water and salt migration experiment was characterized. As shown in Figure 3a,d, both groups exhibited a flat and dense surface. The surface of Group B appeared slightly lighter in color than that of Group A. However, microscopic and SEM images revealed no significant morphological differences between the two groups. The soil particles in all samples were tightly packed without any signs of powdering, indicating that the incorporation of CaAl-LDH did not alter the soil’s inherent morphology or properties.

The repeated crystallization and dissolution of salts within cultural relics is a primary driver of structural damage and powdering [26,27,28]. The surface morphology of the powdering on the simulated samples serves as a direct indicator of the internal crystallization and damage. As can be seen in Figure 4, within 19 h of salt solution absorption, extensive crystallization had propagated along the entire edge of Group A, while only a small amount of crystallization was observed in Group B. After 47 h, the salt crystallization at the edge of Group A transitioned to powdering, resulting in structural loosening and a soil surface that protruded by 0.3 cm, with a tendency to propagate inward. In contrast, Group B exhibited only moderate loosening with a slight protrusion of 0.1 cm. After 110 h of aging, the surface of Group A was heavily powdered, exhibiting a 0.5 cm height increase, and the powder could be easily dislodged. In contrast, Group B showed partial powdering and retained areas of flat, dense, and intact surface under the same conditions. As shown in

Table 1. Statistics of Experiment Results.

	0 (h)	20 (h)	50 (h)	110 (h)
Group A		▼	●	▲
Group B			▼	◼

▼ Crystallization at the edges ● Powdering at the edges ◼ Surface part of the powdering ▲ Complete surface powdering.

, the untreated control group (Group A) exhibited crystallization at the edges that met the aforementioned criteria at 20 h (defined as the onset of damage), whereas the CaAl-LDH-treated experimental group (Group B) did not show initial damage of comparable severity until 50 h. This indicates that the treatment delayed the onset of salt-induced damage by 30 h, representing a 150% extension (i.e., a 2.5-fold increase) relative to the control. These results clearly demonstrate the material’s significant efficacy in inhibiting salt migration and delaying crystallization.

The surfaces of the samples from Groups A and B were examined by 3D digital microscopy. The microstructure of Group A is shown in Figure 5b,c. Numerous needle-like and rod-like MgSO₄ crystals were clearly visible, filling the soil pores. This extensive crystallization had destroyed the cementitious structure, leading to particle separation and the formation of a large number of pores, leaving only a small amount of cementitious structure. Group B, as shown in Figure 5e,f, exhibited needle-like MgSO₄ salt crystals that were barely visible. The crystallization appeared predominantly as granular aggregates. This morphology was less destructive to the soil structure [29], which remained largely intact with only limited porosity observable. Compared to untreated samples, the extent of salt crystallization damage in treated samples is less severe. The incorporation of CaAl-LDH significantly affected the crystalline morphology of MgSO₄, substantially reducing the needle-like shape that severely disrupts soil structure. It is inferred that CaAl-LDH influences crystalline morphology by adsorbing a part of SO₄²⁻ ions while simultaneously releasing some ions, thereby altering the salt’s composition and consequently affecting its crystalline morphology.

3.3. Carbonation Experiment

Generally speaking, the surface hardness and consolidation efficiency of simulated samples treated with protective materials are key factors in evaluating the reinforcement effectiveness of the materials [30]. Untreated samples and samples treated with CaAl-LDH were carbonized for 7 days at 35 °C and 55%, 75%, and 95% humidity. Their hardness and solidification efficiency were tested (“STT” experiment). As shown in Figure 6, under conditions of 35 °C and 55% humidity, the weight loss of samples treated with CaAl-LDH was 0.79 mg·cm⁻², while untreated samples exhibited a weight loss of 1.28 mg·cm⁻². By comparing untreated samples with those treated using CaAl-LDH, it was found that the surface hardness of samples treated with CaAl-LDH under all three conditions was higher than that of untreated samples. Additionally, weight loss in the “STT” test is reduced for CaAl-LDH-treated samples, indicating that the addition of CaAl-LDH provides a certain reinforcing effect on the samples. Under three humidity conditions, CaAl-LDH enhances the surface hardness of the samples by approximately 3%. To investigate the carbonation factors of CaAl-LDH, CaAl-LDH slurry was spread onto Petri dishes and placed at 35 °C under humidity levels of 55%, 75%, and 95% for 7 days. XRPD analysis was conducted to detect changes in its composition. The results are illustrated in Figure 7. CaCO₃ diffraction peaks appeared in the XRPD patterns of samples kept at all three humidity levels for 7 days. Under 95% humidity conditions, the CaAl-LDH peak exhibited the most significant weakening, with CaCO₃ becoming the dominant phase and carbonation reaching 87.7%. As shown in Figure 7, under 55% humidity, distinct LDH reflections were observed alongside peaks corresponding to two distinct crystalline phases of CaCO₃. This indicates that high humidity accelerates the transformation of CaAl-LDH into CaCO₃, thereby hastening the carbonation process of CaAl-LDH.

3.4. Mechanism Research

The preceding section demonstrates that incorporating CaAl-LDH delays salt damage in simulated samples and alters the salt’s crystallization morphology. To investigate the underlying mechanism, the morphology and elemental composition of the samples after salt-damage treatment were characterized. In both Group A and B, as illustrated in Figure 8 and Figure S1, the contents of Mg and S elements representing salt in the powdered areas were 30% higher than on the surfaces. This significant enrichment within the powdered zones provides direct empirical evidence that salt crystallization is not merely a surface phenomenon but a deeply seated process. The higher ionic concentration in these areas leads to increased crystallization pressure upon drying, which mechanically disrupts the substrate matrix, directly linking salt accumulation to the physical manifestation of powdering. This indicates that salt enrichment is a primary driver of powdering, and variations in salt composition also influence its development. Elemental analysis of the powdered areas showed distinct differences: in Group B (Figure 8d), the contents of Mg and S (representing salt) were 4% lower than in Group A (Figure S1d). The contents of Ca and Al (indicative of soil structure) were also approximately 15% lower in Group B, while its C content was markedly higher. It is supposed that CaAl-LDH reacts to immobilize Mg²⁺ and SO₄²⁻ ions from MgSO₄ [31,32], thereby inhibiting the formation of destructive MgSO₄ crystals. This ion-exchange and immobilization process directly explains the lower Mg/S content measured in Group B’s powder. The captured ions are stabilized within the LDH structure or transformed into less harmful phases, reducing the amount of free salt capable of forming destructive crystals. Simultaneously, CaAl-LDH reacts with CO₂ to form reinforcing compounds. This carbonation reaction is likely responsible for the observed increase in carbon content. The formation of CaCO₃-CaAl-LDH phases or related compounds could create a cementing network that enhances cohesion between soil particles. This process helps mitigate the disruption caused by MgSO₄ crystallization to the simulated soil structure, which is consistent with the powdering phenomenon observed in simulated samples A and B during the water and salt migration experiment.

The XRPD pattern of the solid product (Figure 9) shows a new peak appearing to the left of the main CaAl-LDH peak, along with a characteristic CaCO₃ peak at approximately 29°. The FT-IR spectrum showed a stretching vibration peak of the S=O bond near 1120 cm⁻¹, corresponding to the presence of SO₄²⁻ ions and confirming the adsorption capability of CaAl-LDH for SO₄²⁻. Concurrently, the microstructure of CaAl-LDH transitioned from individual flakes to agglomerated, flower-like clusters composed of multiple flakes. According to the EDS, SO₄²⁻ intercalated into the interlayer of CaAl-LDH, while Mg²⁺ ions substituted for Ca²⁺ in the host layers. The released Ca²⁺ subsequently reacted with CO₂ to precipitate as CaCO₃, which is consistent with the corresponding XRPD peak. This ion exchange phenomenon can be attributed to the instability of the original CaAl-LDH structure, primarily due to the significant mismatch between the ionic radii of Ca²⁺ (0.098 nm) and Al³⁺ (0.05 nm). In contrast, Mg²⁺ (0.065 nm) has a radius closer to that of Al³⁺, leading to a more stable layered structure after substitution. Reducing CaAl-LDH to the nanoscale endows it with high surface energy, which drives its interaction with Mg²⁺ ions to achieve a more stable configuration. In this process, Mg²⁺ ions substitute for Ca²⁺ in the host layers of the LDH, while SO₄²⁻ ions are accommodated within the interlayers. This ion exchange alters the crystallization behavior of MgSO₄ and leads to the formation of CaCO₃. Consequently, CaAl-LDH inhibits the powdering of the simulated samples and enhances their mechanical strength.

4. Conclusions

This study demonstrates that CaAl-LDH, synthesized via a coprecipitation-hydrothermal method with a controlled particle size of 200–300 nm, effectively consolidates simulated soil samples by mitigating the dual challenges of powdering damage and structural weakness. The key finding is that CaAl-LDH not only delays and mitigates powdering caused by destructive MgSO₄ crystallization but also enhances the mechanical strength of the samples. This dual functionality is explained mechanistically: CaAl-LDH immobilizes Mg²⁺ and SO₄²⁻ ions from the environment, while the concomitant release of Ca²⁺ ions reacts with atmospheric CO₂ (a process accelerated under high humidity) to in situ precipitate reinforcing CaCO₃. This ion-exchange and carbonation transformation is central to its protective role. Consequently, the research confirms the potential of CaAl-LDH as a reactive consolidant in the conservation of immovable cultural relics, proposing a novel material strategy that moves beyond passive protection towards active chemical stabilization of vulnerable substrates.