Application of nano-pico geotechnics and nano-pico heritage spray techniques for restoring and protecting historical brick buildings

Abstract

This study evaluates nanotechnology’s application in restoring historical brick facades, specifically focusing on how montmorillonite clay nanoparticle spray effectively fills cracks and pores, thereby reducing porosity and decreasing water absorption. The research methodology involves controlled experiments on both historical and purchased brick samples, assessing the impact of nanospray through water absorption and porosity tests. The study seeks to mitigate surface water penetration to the bricks, as well as to fill the capillary cracks and pores that have formed due to various factors. Results indicate that nanospray successfully fills pores and capillary cracks, reducing water penetration and improving brick durability. When nanoparticles fill these pores and cracks, they effectively reduce the overall porosity and also increase the tortuosity that water vapor must take to pass through the material. By reducing the number and size of pores and making the diffusion path more tortuous, the nanoparticle treatment can increase the water vapor diffusion resistance of the brick. While reducing water absorption is beneficial for protecting the brick from water damages, breadthening of historic masonry is necessary, meaning it must be able to absorb or release moisture by changing the humidity level. Using nanoclays as Nano-Pico Getechnics (NaPG) and Nano-Geotechnics (NaG) techniques can consider the balance between reducing water absorption and maintaining sufficient vapor permeability (breathability). clay nanoparticles can fill the pores without completely blocking them, thus allowing vapor to pass through. This study contributes to advancing sustainable restoration practices to assess the long-term effects and environmental adaptability of nanospray applications. This nanotechnology approach is chosen not only for its high efficiency in protecting brick facades but also for its economic viability and environmental compatibility, classifying it as a form of “green restoration” or “sustainable restoration”.

Introduction

Brick has been one of the fundamental materials in the construction of historical buildings, used in various ways across different eras. In Iran, brick has a long-standing history, dating back to the Sassanid era. Notable surviving structures include the Taq Kasra (Arch of Ctesiphon) in Iraq, the hall floor of Jameh Mosque of Isfahan, Chaghazanbil in Shoshtar, and the Dome of Soltaniyeh in Zanjan. The structure of masonry materials is subject to environmental changes, particularly weathering, which often manifests as surface damage due to material inhomogeneity¹. Soil properties can be enhanced through three stabilization methods: mechanical, physical, and chemical. These approaches ranging from soil compaction to modifying physical parameters and incorporating stabilizing materials help increase resistance to environmental erosion².

Given the impracticality of quasi-museum preservation for historical brick buildings due to their exposure to open environments with extreme temperatures, daily temperature fluctuations, environmental pollution, and strong winds carrying suspended particles the most effective solution is nanotechnology. This approach not only offers high protection for brick facades but also proves to be economically viable and environmentally friendly. Nanotechnology represents an innovative method for reducing or eliminating the impact of environmental degradation on historical structures, particularly through the development of multi-purpose surface coatings^3,4,5,6,7,8. Today, the production of nanoparticles is considered one of the most important challenges of new technologies, and one of the reasons for this importance is the creation of new applications due to the high value of the surface-to-volume ratio in nanoparticles⁹. In the construction industry, nanotechnology is regarded as an enabling technology which can be a very effective alternative for developing new materials with unique properties and different characteristics compared to their macroscopic counterparts^10,11. Future buildings should incorporate advanced materials to improve efficiency, enhance security, and prevent energy waste, creating healthier environments. Controlling material properties at the nano scale along with managing related physical and chemical effects facilitates the production of multi-functional building materials with superior performance. These innovations increase durability, quality, and environmental sustainability. Nanotechnology applications include self-cleaning and antibacterial paints, smart glass, thermal and moisture insulation¹²enhanced cement-based materials^13,14,15self-healing concrete¹⁶and architecture aligned with natural environments¹⁷ often referred to as organic architecture¹⁸. At nano-scale dimensions, materials acquire unique properties that are absent at deci- and micromolecular levels¹⁹.

Numerous nanoparticles are used in the construction industry: titanium dioxide, carbon nanotubes, nanosilica, nanocellulose, nanoalumina, and nanoclay^10,11. These nanoparticles provide special properties including compressive strength, density, porosity, and improved performance in building components^20,21. Most studies conducted in the field of construction are related to the use of nanoclays due to their biocompatibility properties with cement-based products, unique shape, high surface-to-volume ratio, and loading potential, which seeks to develop new materials with superior properties^22,23. Among natural clays, nanomontmorillonite nanoclay is the most widely used, due to its superior suspension, diffusion, and dispersion capabilities. Montmorillonite is a hydrated sodium calcium aluminum magnesium silicate hydroxide (Na_0.2Ca_0.1Al₂Si₄O₁₀(OH)₂(H₂O)₁₀)²⁴.

Evidence shows that the addition of nanoparticles significantly reduces water capillary absorption and water vapor permeability²⁵. The effect of nanosilica particles on the permeability of concrete through the use of nanosilica spraying technique shows a significant increase in the compression strength and resistance of the samples to water penetration²⁶. Also, direct spraying of nano-silica gel on concrete results in the creation of a super hydrophobic concrete surface. The advantages of using this material include ease of construction, high compatibility with the cement substrate, cost-effectiveness, self-breathability, and wide-area applicability²⁷. Other results show that using a nanospray to create a hydrophobic layer on the surface of solar systems significantly reduces dust accumulation on the transparent surface²⁸. In low-fired porous clay building materials, especially bricks, the use of silica nanoparticles reduces water capillary absorption, resulting in little or no disruption to the building materials’ ability to breathe²⁹. Another study shows that the use of nanoclay can significantly increase the adhesion of earthen materials and protect earthen buildings from water erosion³⁰. Research also shows that adding nanoclays to the concrete improves durability, resistance to chloride and solvent penetration, reduces the shrinkage, increase the mechanical strength, and reduces resistance to water and oxygen permeability³¹. The composition of montmorillonite nanoclay serves as an effective coating for reinforcing brick structures^32,33. Additionally, studies in nanotechnology have explored applications in historical monument preservation, including the reinforcement of historical adobe with Montmorillonite clay nanoparticles³⁴. Many studies have investigated the role of these nanoparticles in filling pores, increasing density, and ultimately increasing the compressive and tensile strength of cement for different amounts of nanoparticles^35,36,37. In addition, previous studies show that soil performance in terms of hydraulic conductivity can be improved by adding clay nanoparticles to the soil^38,39,40,41.

The aim of this research is to present a restoration plan for the aforementioned monument, particularly addresses the Mehmandoost Tower, focusing on the application of nanotechnology for the restoration of historical brick facades. The study seeks to mitigate surface water penetration to the bricks, as well as to fill the capillary cracks and pores that have formed due to various factors such as diurnal and seasonal temperature fluctuations, structural stresses, and sandblasting and local/regional winds carrying abrasive particles. Montmorillonite nanoclay was selected here due to its good diffusion for filling the pores of historical brick, resulting water absorption reduction. Furthermore, its ability to reduce the vapor resistance and breathability of brick due to the layered structure of nanoclay, and also superior dispersion of it to prevent agglomeration and reduce the shrinking.

Materials and methods

Materials

This research utilizes two types of bricks for experimentation. The first type consists of historical bricks sourced from Mehmandoost Tower, while the second category comprises handmade bricks which is selected according to chemical analysis of historical building with dimensions of 10 cm × 10 cm × 3 cm, sold under the brand name Sepand Brick.

Mehmandoost Tower, located south of Mehmandoost village in Semnan Province, Iran, dates back to the Seljuk period (490 AD) an era renowned for architectural innovation. The tower consists of three distinct structural sections: its circular base, a dodecagonal main body featuring triangular notches at each corner, and a final decorative circular section at the top (Fig. 1).

The historical bricks used in this research were carefully extracted from designated sections of the Mehmandoost Tower. The samples were collected primarily from the façade at varying heights to assess how environmental exposure such as moisture, erosion, and seasonal temperature changes affects their structural integrity. The dimensions of the bricks were recorded to maintain consistency throughout the comparative analysis.

Fig. 1

Mehmandoost Tower (Photo by the Authors).

Most restoration techniques for historical buildings have traditionally relied on powder and liquid materials, which can cause damage to these structures. Additionally, existing methods use various additives and materials without a standardized scaling system for particle size in restoration processes. Addressing these gaps, this research has the potential to revolutionize historical building restoration by incorporating both these considerations and utilizing the spray technique for enhanced protection.

Naturally, building destruction directly depends on climatic factors⁴². On the exterior facade of brick buildings, erosion and degradation typically begin with hairline cracks. Factors such as daily and annual temperature fluctuations, compressive forces, and facade stresses contribute to this deterioration. Additional causes of erosion in historical brick structures include atmospheric influences like seasonal and regional winds. These winds, varying in intensity and carrying suspended particles, lead to surface erosion through impact often resulting in a sandblasting effect.

Moreover, water and especially water penetration, is another effective and harmful natural factor in the decay of building materials⁴³. High humidity levels combined with temperature shifts near freezing cause water to infiltrate the brick. As water freezes, it expands, exerting pressure on the pore walls and ultimately weakening the structure. This process leads to peeling and cracking⁴⁴.

Based on the observations and investigations conducted on Mehmandoost Tower, the primary environmental factors affecting brick material erosion are as follows:

• 1. Thermal gradient: Temperature differences between day and night, as well as seasonal variations, contribute to contraction and expansion due to thermal stress. This process leads to the formation of hairline cracks on brick surfaces. A sample of such cracks is illustrated in Fig. 2.

• 2. Water infiltration: Moisture from rainfall and rising dampness penetrates brick materials, reacting with metal elements within the brick and causing discoloration and landscape damage. Furthermore, excessive water absorption during colder seasons leads to freezing, flaking, and cracking on the exterior surfaces of the tower. Examples of these effects on buildings are shown in Fig. 3.

• 3. Mechanical wear: Seasonal and regional winds carrying particulate matter cause sandblasting-like erosion, damaging brick surfaces and forming voids. An example of these structural voids is depicted in Fig. 4.

Fig. 2

Sheet fractures and hair cracks below (Photo by the Authors).

Fig. 3

Change the color of the facade 0.5 to 3 mm on different surfaces of the brick (Photo by the Authors).

Fig. 4

Abrasion by seasonal winds (Photo by the Authors).

According to IRPATENT 102,499 ⁴⁵, nanomontmorillonite clay was selected for this study. The clay, produced by Sigma Aldrich (USA) under the brand name Montmorillonite K10, has a density ranging from 0.5 to 0.7 g/cm³, with particle sizes between 1 and 2 nm. Additionally, this nanoclay consists of 50.95% SiO₂ and 19.6% Al₂O₃, with the remaining composition comprising Fe₂O₃, MgO, CaO, Na₂O, K₂O, and TiO₂.

Preparing samples

The preparation of samples involves several steps: surface cleaning, numbering, classification, and identification of each sample alongside the nanoclay solvent (96% ethanol). The next stage is determining the concentration of nanomontmorillonite clay in the solution and subsequently preparing the mixture. Based on prior studies^8,34,45,46different concentrations of nanoclay were examined, specifically weight-to-volume ratios of 8% and 10% w/v nanomontmorillonite clay dissolved in ethanol. Since ethanol alcohol (96% purity) has no chemical or physical impact on historical bricks, it was chosen as the preferred solvent for spray application in heritage preservation. Following preparation, the solutions were placed in an ultrasonic bath for 30 min to disperse nanoclay for further processing (Fig. 5). After well dispersing the nanoclays using an ultrasonic device, 100 ml of suspension of nanoclay in ethanol was sprayed directly onto all brick surfaces three times. Each brick samples has 10 cm × 10 cm × 3 cm dimensions. The coated brick samples were placed at room temperature and under natural conditions for 24 h to dry and stabilize the nanoclay on the surface of bricks.

Fig. 5

Dispersing the nanoclay in ethanol in the ultrasonic device (Photo by the Authors).

Tests. The experiments were conducted in accordance with the following Iranian industrial standards, each of which governs specific aspects of clay brick manufacturing and evaluation:

• Industrial Standard No. 2943 – Defines the properties and quality criteria for clay used in brick production, including composition, particle size distribution, and suitability for construction applications.

• National Standard No. 7 – Establishes guidelines for clay bricks, addressing key properties such as compressive strength, water absorption, dimensional accuracy, and durability requirements.

• Industrial Standard No. 5771 – Covers regulations for industrial minerals and stones, specifying material characteristics, extraction methods, and processing techniques relevant to brick manufacturing.

Water absorption test. According to Iran’s industrial standard and national standard No. 7 for clay bricks, if water absorption exceeds 30%, the brick will gradually disintegrate when used in facades.

To determine the moisture percentage, the following procedure was performed:

• 1. The brick was dried in an oven at 110 °C for 24 h and then carefully weighed (Fig. 6a).

• 2. The brick was gently placed into a water pan and left submerged for 24 h (Fig. 6b).

• 3. After 24 h, the brick was removed, excess water was removed using a dry cloth, and the brick was weighed again (Fig. 6c).

The same process was conducted for the sprayed samples.

Fig. 6

Three steps of water absorption test (Photo by the Authors).

The weight of the brick in its saturated state was used to calculate the percentage of water absorption$\:\left(Gi\right)$, as follows:

$$\:\left(\frac{{Wi}_{2}-{Wi}_{\:1}}{{Wi}_{1}}\right)\times\:100=Gi\%\:\text{W}\text{i}\text{t}\text{h}\:\text{c}\text{o}\text{n}\text{d}\text{i}\text{t}\text{i}\text{o}\text{n}:\:{G}_{i}<30\%$$

(1)

Where $\:{\text{W}\text{i}}_{0}$(g) is the Initial weight of bricks with moisture, $\:{\text{W}\text{i}}_{1}$ (g) the Weight of dried brick, and $\:{\text{W}\text{i}}_{2}$ (g) the Weight of brick saturated with water.

To ensure greater precision in the experiments, eight samples were tested for each brick type. For these eight samples (n = 8), the average water absorption $\:\left({G}_{M}\right)$ of each brick can be calculated using the following equation:

$$\:\sum\:_{i=1}^{8}\frac{{G}_{i\:}}{8}={G}_{M}\%\:\text{W}\text{i}\text{t}\text{h}\:\text{c}\text{o}\text{n}\text{d}\text{i}\text{t}\text{i}\text{o}\text{n}:\:{G}_{M}<30\%$$

(2)

In the second stage, the sprayed samples were dried in an oven at 110 °C for 24 h (or at 60 °C for 48 h) before being weighed.

Next, the sprayed bricks were carefully placed in a water pan and left submerged for 24 h. After this period, the bricks were removed, excess water was wiped off using a cloth, and they were weighed again.

The percentage of water absorption for the sprayed bricks was then calculated using the following equation.

$$\:\left(\frac{{Wi}_{2}^{{\prime\:}}-{Wi}_{1}^{{\prime\:}}}{{Wi}_{1}^{{\prime\:}}}\right)\times\:100={\rm Gi}'\%$$

(3)

Where $\:{\text{W}\text{i}}_{0}^{{\prime\:}}$(g) is the Initial weight of sprayed bricks with moisture, $\:{\text{W}\text{i}}_{1}^{{\prime\:}}$ (g) the Weight of sprayed dried brick, and $\:{\text{W}\text{i}}_{1}^{{\prime\:}}$ (g) the Weight of sprayed brick saturated with water.

This process was repeated on eight sprayed brick samples. The average water absorption of the sprayed bricks was calculated using the following equation:

$$\:\sum\:_{i=1}^{8}\frac{{G}_{i}^{{\prime\:}}}{8}={G}_{M}^{{\prime\:}}\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:n=8$$

(4)

As a result, three possible scenarios can be considered:

• First: If the moisture absorption remains unchanged before and after applying the nano spray, it indicates that the spray has had no effect.

$$\:{G}_{M}^{{\prime\:}}{=G}_{M}$$

(5)

• Second: If moisture absorption after spraying is lower than before, the nanospray is effective, reducing the brick’s water absorption percentage and preventing deterioration.

$$\:{\:G}_{M}^{{\prime\:}}{<G}_{M}$$

(6)

• Third: If moisture absorption before spraying is lower than after, the nanospray is ineffective, increasing the brick’s water absorption percentage and accelerating deterioration.

$$\:{\:G}_{M}^{{\prime\:}}{>G}_{M}$$

(7)

Determination of porosity percentages test. Porosity is defined as the ratio of cavity volume to total volume and is directly linked to mechanical properties such as compressive strength and water absorption. It can be calculated using the following equation:

$$\:{\alpha\:}_{i}=\frac{{W}_{i2}-{W}_{i1}}{V}\times\:100$$

(8)

In the above relation $\:{\alpha\:}_{i}$ is the volume of empty space, $\:{W}_{i2}$ the weight of the brick in saturation state, $\:{W}_{i1}$the weight of the dry brick and V the volume of the brick$\:{(cm}^{3})$.

Experiments were carried out in two stages, before and after spraying, and in each stage, 8 samples of bricks with dimensions 10 cm×10 cm ×3 cm were performed (Fig. 7a-c).

The average porosity volume for the samples is obtained from the following relationship.

$$\:\sum\:_{i=1}^{n}\frac{1}{n}\:{\alpha\:}_{i}={\alpha\:}_{M}$$

(9)

The average porosity volume was calculated in two stages: for the unsprayed $\:{(\alpha\:}_{M})$ and the sprayed sample$\:{(\alpha\:}_{M}^{{\prime\:}})$.

To conclude:

• If$\:{\:\alpha\:}_{M}={\alpha\:}_{M}^{{\prime\:}}$, the nano spray has no effect.

• If$\:{\:\alpha\:}_{M}<{\alpha\:}_{M}^{{\prime\:}}$, the nano spray was ineffective and negatively impacted the samples.

• If$\:\:{\alpha\:}_{M}>{\alpha\:}_{M}^{{\prime\:}}$, the nanospray was successful, effectively reducing the empty spaces in the brick.

Fig. 7

Steps of porosity percentages test (Photo by the Authors).

XRD (X-ray diffraction)

Chemical composition analysis using X-ray diffraction is a non-destructive method that provides comprehensive information about chemical compounds, elemental types, and their percentages in a sample. This test was conducted using the X.PER-PRO-MPD model to identify and qualitatively compare the minerals present in both the control and purchased brick samples.

SEM (Scanning electron microscopy)

The SEM test was conducted to determine the optimal amount of nanoclay in the solution by analyzing surface coverage in the images obtained using the MIRA3 TESCAN-XMU device. Additionally, to assess the effect of nanospray on brick surface reinforcement, all samples were examined with SEM after undergoing the water absorption test.

EDX (Energy dispersive X-ray spectroscopy)

EDX analysis is an add-on to SEM devices for determining the percentage of elements in solid samples. This analysis was used in samples 1 and 2 to determine the type of elements and their weight or atomic percentage.

Results and discussion

Identification of the brick used in mehmandoost tower (historic brick) and comparison with the purchased brick

The EDX analysis of both the historical bricks from Mehmandoost Tower and the purchased bricks is presented in Figs. 8 and 9. According to the EDX analysis, both brick samples contain the elements oxygen, sodium, magnesium, aluminum, silicon, calcium, chlorine, potassium, and iron. Among these, oxygen, calcium, and silicon are present in the highest concentrations. Based on the weight percentages shown in Table 1 and the atomic composition detailed in Table 2, a similarity between the historic brick and the purchased brick samples can be concluded.

Fig. 8

EDX Analysis of Historical Brick Samples.

Fig. 9

EDX Analysis of Purchased Brick Sample.

Table 1 Comparison of the weight% of elements in the historical brick sample and the purchased brick.

Table 2 Comparison of the atomic percentage of elements in the historical brick sample and the purchased brick.

We measured the chemical composition, porosity percentage, water absorbance, and obtained SEM images of both historical and various man-made bricks; these results are reported in the following sections. Due to limitations in using historical bricks, man-made bricks with matching chemical and physical properties were acquired and utilized for all subsequent tests.

Effect of Nanomontmorillonite clay spray on brick samples

This study conducted various experiments to evaluate the protective role of nanospray on historical bricks. The following sections present the results:

Scanning electron microscopy (SEM)

To examine the effectiveness of montmorillonite nanoclay in filling brick samples, both historical and purchased brick samples were tested at the designated concentrations.

Figure 10 presents SEM images of a historical brick sample (control) before spraying which is presented in different magnifications to show the various pores cavities within the brick. Figure 10a shows the porous structure of brick, and the Figs. 10b-d indicates the pores with the size of 6 μm to 1500 nm.

In Fig. 11, a historical brick sample is displayed after nano spraying with a 10% concentration of montmorillonite nanoclay at various magnifications. Figure 11a clearly shows the filling of pores with nanoclay and the smoother surface was observed after spraying the nanoclay. A comparison of Figs. 10b-d with Figs. 11b-d indicates that nanospray successfully fills the pores and cracks on the brick surface. The pores and cracks clearly filled with nanoclays and lower size of pores were observed in Figs. 11c, d. The larger pores in brick reduces from 6 μm to 1 μm (Figs. 10b and 11b), as well as smaller pores decreases from 1 μm to 250 nm (Fig. 11d and d).

Fig. 10

SEM image of the Historical sample at different magnification (a)1 mm, (b)10 μm, (c)2 μm, (d)500 nm magnification.

Fig. 11

SEM images of the historical sample after spraying with 10% montmorillonite nanoclay at different magnification (a)1 mm, (b)10 μm, (c)2 μm, (d)500 nm magnification.

To compare the purchased brick with the historical brick and evaluate the effect of nanospray filling, SEM images were taken of the purchased brick samples before and after spraying.

Figure 12 presents SEM images of the purchased bricks before spraying which is presented in different magnifications to show the various pores within the brick. Figure 12a shows the porous structure of brick, and the Figs. 12c indicates the pores with the size of 500 nm which indicates the lower pore size than historical bricks.

Fig. 12

SEM image of the purchased brick samples before spaying at different magnification (a)1 mm, (b)2 μm, (c) 500 nm magnification.

Figure 13 displays the SEM images of purchased brick sample after nano spraying with a 10% concentration of montmorillonite nanoclay at various magnifications. Figure 13a clearly shows the filling of pores with nanoclay and the smoother surface was observed after spraying the nanoclay. A comparison of Figs. 12c with Figs. 13c indicates that nanospray successfully fills the pores and cracks on the brick surface. The pores and cracks clearly filled with nanoclays and lower size of pores were observed in Figs. 13. The pores in brick reduces from 500 nm to 160 nm, demonstrating its success in the process. (Figures 12c and 13c).

Fig. 13

SEM image of the purchased brick samples after spaying at different magnification (a)1 mm, (b)2 μm, (c) 500 nm magnification.

According to the obtained data, a 0.1% solution (10 g of nanomontmorillonite clay in 100 ml of solvent) is optimal compared to a 0.08% solution (8 g of nanomontmorillonite clay in 100 ml of solvent). Observations indicate that concentrations below 0.08% do not provide complete coverage. The results show that applying sprays to a brick surface three times achieves acceptable coverage. Based on the obtained images, the nano-clay spray effectively fills hairline cracks and cavities on the brick surface. To further evaluate the nanospray’s properties, additional tests were conducted on the purchased brick samples using 8% and 10% nanomontmorillonite clay solutions.

Determining the percentage of water absorption of bricks

Table 3 presents the brick’s water absorption levels before nanospray application. Additionally, the average water absorption percentage for samples prior to spraying was calculated as 18.25% ($\:{G}_{M}=18.25\%<30\%$).

The results from the water absorption percentage test for five samples after nanomontmorillonite clay spraying are presented in Table 4.

The average amount of water absorption for three samples of 10 g is equal to $\:{G}_{MI}^{{\prime\:}}=14.6$ and for two samples of 8 g, $\:{G}_{MII}^{{\prime\:}}=16.$

As a result, because $\:{G}_{MI}^{{\prime\:}},{{G}_{MII}^{{\prime\:}}<G}_{M}$, so nanospray works successfully and lowers the amount of water absorption of bricks and prevents destruction.

If we get the difference between $\:{G}_{MI}^{{\prime\:}}$ and $\:{G}_{MII}^{{\prime\:}}\:$and call it $\:{\Delta\:}{G}_{M.}^{{\prime\:}}$

$$\:\varDelta\:{G}_{M}^{{\prime\:}}=16-14.6=1.4\:\%$$

(10)

Since the difference in nano material consumption between the two sprayed samples is 2 g, it can be concluded that increasing the nano material by 2 g reduces the water absorption percentage by a factor of 1.4.

It can also be said if ∆ is the percentage difference of water absorption in samples before and after spraying, for 10% nanospray samples:

$$\:{G}_{M}-{G}_{MI}^{{\prime\:}}={{\Delta\:}}_{\text{{\rm\:I}}}=3.65\%\:\:\downarrow\:$$

(11)

$$\:{\varDelta\:}_{\text{{\rm\:I}}}^{{\prime\:}}=\frac{{{\Delta\:}}_{\text{{\rm\:I}}}}{{G}_{M}}\times\:100=\frac{3.65\times\:100}{18.25}=20\%\:\downarrow\:$$

(12)

$\:{\varDelta\:}_{\text{{\rm\:I}}}^{{\prime\:}}$ represents the difference in the average water absorption percentage between samples sprayed with 10% nanoclay and those left unsprayed. Therefore, water absorption is significantly reduced in the samples treated with the highest nanoclay concentration (10% nanoclay) compared to the unsprayed samples, demonstrating its effectiveness: $\:{(G}_{M})$ $\:{\varDelta\:}_{\text{{\rm\:I}}}^{{\prime\:}}>{G}_{M}$

Table 3 Results of brick water adsorption before Spraying.

Table 4 Results of brick water adsorption after Spraying.

Determination of porosity percentages test

Table 5 shows the results of the porosity test for the samples before spraying with an average porosity: $\:{{\upalpha\:}}_{\text{M}}=35.12\text{\%}$.

The results of the porosity test after spraying the samples are shown in Table 6.

The average porosity for the samples after nanospray is $\:{{\upalpha\:}}_{\text{M}\text{{\rm\:I}}}^{{\prime\:}}=29$ (this value corresponds to two samples containing 10 g of nanoclay, with sample 1 excluded).

The difference in porosity between unsprayed and sprayed samples indicates that the empty spaces in the sprayed samples have decreased by 6%. $\:{{\upalpha\:}}_{\text{M}}-{{\upalpha\:}}_{\text{M}\text{{\rm\:I}}}^{{\prime\:}}=6.12\text{\%}$

Additionally, the average porosity percentage for the two 8% nanoclay samples is $\:{\alpha\:}_{M\text{I}\text{I}}^{{\prime\:}}=31.5\%$. For the 8% nanoclay samples, porosity decreased by 3.62% following the nanospray application.

Based on the defined relationships $\:{\alpha\:}_{M}>{\alpha\:}_{M\text{{\rm\:I}}}^{{\prime\:}}$ and$\:\:{\alpha\:}_{M}>{\alpha\:}_{M\text{I}\text{I}}^{{\prime\:}}$, it can be concluded that the nanospray was successful in reducing the empty spaces in bricks.

Furthermore, according to the porosity percentages obtained for the sprayed samples, a 2% increase in the nanoclay concentration led to an improvement in whole filling and a reduction in voids within the brick structure.

Table 5 Results of porosity percentages before Spraying.

Table 6 Results of porosity percentages after Spraying.

Effect of nanospray on water adsorption by SEM test

Figure 14 presents SEM images of the sprayed brick after 24 h of submersion in water, captured at different magnifications. A comparison of Figs. 11 and 14 indicates that the nanospray has effectively acted as a filler, sealing the brick’s pores and preventing water absorption. Furthermore, as observed, the nano coating remains unchanged even after 24 h of exposure to water.

Fig. 14

SEM image of the sprayed brick samples after 24 h of floating in the water at different magnification (a) 1 mm, (b) 2 μm, (c) 10 μm and, (d) 1 μm magnification.

One of the significant challenges in using nanoparticles in historical building is maintaining breathability (Water Vapor Permeability) while simultaneously reducing water absorption and increasing mechanical strength. Nanoclays can be inherently hydrophilic, although their modified forms with organic polymers used as the hydrophobic agent³¹. The type of nanoclay and its modification method are crucial in determining its effect on breathability. Also, montmorillonite nanoclays, due to their layered structure and ability to disperse at the nanoscale, can affect porosity and, consequently, breathability.

The primary mechanism by which nanoclay enhances mechanical strength and reduces water absorption is by penetrating and filling tiny pores and cracks. This leads to a reduction in the overall porosity. By filling these pores, pathways for liquid water penetration are blocked, thereby reducing surface and deep water absorption. If nanoparticles completely block the pores, the pathways for water vapor passage are also partially obstructed. Montmorlionite nanoclay has a layered structure with good dispersion which help to prevent complete blocking of pores, thus allowing vapor to pass through. It is the reason that nanoclays can consider the balance between reducing water absorption and maintaining sufficient vapor permeability (breathability). Furthermore, if nanoclays are well dispersed, nanoparticle aggregation is decreased and the degree of shrinkage is reduced.

Conclusions

This study explored the effectiveness of montmorillonite clay nanospray in reducing brick porosity and water absorption for the restoration of historical facades. Through controlled experiments, comparisons between unsprayed and sprayed bricks revealed the nanospray’s ability to fill pores and hairline cracks, improving resistance to moisture penetration. While the difference in porosity and surface behavior between the 8% and 10% concentrations appears modest (e.g., 2% variation), prior studies and preliminary tests suggest that even small differences in concentration can influence the distribution and penetration of nanoclays within porous substrates. However, it is important to note that these differences were not statistically significant at the 95% confidence level, indicating that the 8% and 10% treatments may have comparable effectiveness in practical applications. Thus, recommendations based on these concentrations should be made cautiously, considering both technical performance and material conservation priorities. Although no mechanical strength tests were conducted, the findings support the nanospray’s role in enhancing durability and preservation of historical brick structures.

Key Findings:

• Porosity Reduction: The nanospray application led to a 6.12% decrease in porosity for 10% nanoclay-treated samples.

• Water Absorption Improvement: Increasing nanoclay concentration by 2% resulted in 1.4 times lower water absorption.

• Optimal Coverage: Three sprays on a 10 cm × 10 cm × 3 cm brick surface achieved effective distribution and filling of voids.

• Nanoclay Concentration Impact: A 0.1% solution provided optimal coverage compared to 0.08%, ensuring better pore-filling.

• Environmental Compatibility: Montmorillonite, as a natural material, supports sustainable and eco-friendly restoration practices.

In terms of scalability and field application, the use of montmorillonite nanospray shows promising potential for large-scale restoration projects; however, practical challenges must be considered. On-site application over expansive or intricate surfaces, such as large historical monuments, may require specialized spray equipment and consistent environmental conditions to ensure uniform coverage and nanoparticle dispersion. Nonetheless, the use of natural, non-toxic materials like montmorillonite and ethanol enhances the feasibility of this method for heritage sites, supporting its classification as a safe and sustainable restoration approach.

This study focused on the application of nanosprays on historical bricks and demonstrated their effective performance in the protection of heritage structures. In contrast, previous researchers³³ have primarily investigated traditional coatings on bricks and their positive effects on certain types of deterioration, but only at a non-nano scale and in limited studies. The enhanced performance of nanomaterials highlighted in this research may represent a significant advancement, offering a novel and innovative approach for the future of historical brick conservation.