Introduction

Stone cultural heritage represents a significant aspect of the world’s cultural heritage, carrying valuable information and revealing the development and prosperity of ancient societies. Over time, these constructions have been severely affected by various environmental factors, resulting in substantial damage that urgently requires repair and reinforcement1,2. In recent decades, both inorganic and organic materials have been extensively utilized for the restoration of historical sites, achieving some positive outcomes3,4,5,6,7,8,9,10,11,12,13,14. Among these, inorganic materials, such as Portland cement and traditional lime, account for a large proportion as they usually exhibit similar chemical compositions and physical properties to those of the substrates being repaired15,16,17. However, the repair effect and long-term conservation performance of these materials have proven unsatisfactory recently to suit the project, especially under harsh climatic conditions. They are susceptible to causing secondary damages to the repaired sites, including cracking, peeling, physical salt attacking and loss of integrity18,19,20. Thus, great efforts are still needed to design and develop materials with better workability, compatibility, and durability for the restoration of stone cultural heritage.

Natural hydraulic lime (NHL), an inorganic silicate cementitious material with the property of hardening in both air and water, has the advantages such as moderate mechanical strength, low soluble salt content, and low shrinkage rate compared to conventional building cement and lime. These attributes have garnered significant attention from researchers in the field of heritage conservation and facilitated its application in the protection of historical sites21,22,23,24,25,26,27. Despite these, NHL exhibits certain weaknesses, including a slowly harden rate and limited mechanical strength at the early stage, which are mainly attributed to its special hardening process28,29,30,31,32. During this process, dicalcium silicate (C2S) undergoes a hydration reaction with water, forming calcium silicate hydrate (C-S-H) and Ca(OH)2. Meanwhile, CaCO3 is formed by the reaction of Ca(OH)2 and CO2 in the air. However, CaCO3 usually exerts a minimal influence on the development of NHL’s properties, especially in early stages, due to its lower strength and slower hardening rate33,34. Consequently, the structural strength of NHL is primarily derived from calcium silicate hydrate. However, the low structural activity of C2S, resulting in a lack of reactive oxygen atoms in its crystals, causes a much lower surface dissolution rate compared to C3S, the main hydraulic components of Portland cement35,36,37,38. This means that NHL needs a longer hydration time to achieve the requisite strength for constructions. Additionally, in practice, NHL mortar was highly susceptible to structural breakage, such as cracking and shrinkage during curing, particularly in low humidity environments31,32. All these mentioned factors markedly affect the workability, strength development and durability of NHL, which hinder its effectiveness in achieving satisfactory repair outcomes.

The choice of appropriate functional filler for NHL is a key factor in addressing the above issues. In the past, the minerals with pozzolanic activity, such as fly ash, silica fume, metakaolin, and silicon oxide were most often used. The idea is to increase the hydration product contents of NHL and refine its pore structure, thereby enhancing mechanical strength and resistance to environmental erosion14,39,40,41,42,43. Zhang et al. assessed the effect of different pozzolanic materials (slag powder and silica fume) on the properties of NHL-based mortar, finding that the mechanical and environmental resistance properties of modified NHL are significantly improved44. Similarly, Vavricuk et al. investigated the impact of metakaolin on the efficacy of NHL as a grouting material, demonstrating that the addition of metakaolin enhanced the water retention capacity of the mortar and substantially improved its bonding and compressive strength after hardening45. Nevertheless, these materials are insufficient for fully enhancing NHL performance due to their limited chemical reactivity. Recently, nano-metakaolin (NMK) has gained recognition as an excellent active filler for inorganic cementitious materials because of its nano-size, large specific surface area, and strong reactivity, and has been widely studied in cement and concrete46,47,48,49. However, related research on its application in NHL remains limited, especially as it applies to the restoration and preservation of stone cultural heritage.

Herein, we proposed an innovative composite mortar based on natural hydraulic lime (NHL5) and NMK for the restoration and reinforcement of stone cultural heritage. Firstly, we systematically investigated the effect of different proportions of NMK on the properties of composite mortars, including setting time, shrinkage, soluble salt content, surface hardness, and mechanical strength. Then, we assessed its weathering resistance through dry-wet and freeze-thaw cycling. Next, the performance enhancement mechanism of NHL mixed with NMK was explored through various testing methods. Finally, based on its excellent performance, the composite mortar was applied to protect the Yungang Grottoes, a real stone cultural heritage, and the mortar’s protective effect was evaluated. The results of this study expand research on NHL mortar modification and provide a scientific foundation for the promotion and application of NMK-modified NHL mortar in the restoration of stone cultural heritage in challenging environments.

Methods

Materials

Natural hydraulic lime (NHL5) was purchased from Shanghai DESAIBAO Building Materials Co. Ltd. NMK was purchased from China INNER MONGOLIA SUPER New Material Co. Ltd. Tap water was adopted in all experiments. The median particle sizes of the two materials are 1.95 μm (NHL5) and 110 nm (NMK). Mineralogical analyses of NHL5 and NMK obtained by x-ray fluorescence spectrometry (S2 RANGER, BRUKER, Germany) are presented in Table 1.

Table 1 Mineralogical analyses of NHL5 and NMK (at%)
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Sample preparation

The mix proportion design of NMK-modified NHL mortars is shown in Table 2. The preparation procedure involved the following steps: (1) Mix NHL5, NMK and deionized water in proportion to the weighing, and use a mortar mixer to mix for 5 min to ensure homogeneity; (2) Fresh mortar was loaded into 40 mm*40 mm*160 mm moulds, and the moulds were shaken by a mortar vibration table (ZS-20H, China) for 90 s to exclude the air inside the mortar, and then the mortar was placed in a constant temperature and humidity chamber (90% RH, 25 °C) to cure for 24 h. (3) After the mortar had hardened, the moulds were removed and the hardened mortar continued to be cured in a constant temperature and humidity chamber (90% RH, 25 °C). The hardened mortar was tested at specific ages (3, 7, 14, 28 and 56 days). According to the NMK content, the mortar samples were named NMK0, NMK5, NMK10, NMK15, NMK20 and NMK25.

Table 2 The mix proportion design of the NMK-modified NHL mortar (wt%)
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Workability and mechanical properties tests

The setting time of fresh mortar was determined using a Vicat apparatus (WUXI JIANYI EXPERIMENT INSTRUMENT, China) in accordance with GB/T1346-2024.

The shrinkage was calculated by measuring the length of the specimens at different ages using vernier calipers and applying the following formula:

$$S \% =({L}_{0}-{L}_{t})/{L}_{0}\times 100 \%$$

where: S% is the shrinkage of the specimens; L0 is the length after demoulding and Lt is the length at designated ages (3, 7, 14, 28 and 56 days).

The soluble salt content of the modified mortars was determined indirectly by measuring the electrical conductivity of their leachates, following these steps: Firstly, different mortars of identical mass at 56 days were immersed in the identical volume of pure water for 24 h; then, the conductivity of the mortar leachate was determined using a conductivity meter (ERUN-ST3-A4, China); finally, the measured conductivity values were compared against those of standard potassium chloride (KCl) solutions of known concentration. The concentration of the KCl solution exhibiting equivalent conductivity was used to calculate the soluble salt content (expressed as equivalent KCl concentration) of the mortar.

The surface hardness of the hardened mortar was measured using a Leeb hardness tester (Equotip Live Leeb D, Proceq, Switzerland).

The mechanical strength (flexural strength and compressive strength) of hardened mortar at designated ages (3, 7, 14, 28 and 56 days) was determined using a universal testing machine (WDW-300, China) in accordance with GB/T 17671-2021. For each mix proportion and test age, at least three specimens were tested.

Durability tests

In the wet and dry cycle test, the completely dried specimens were immersed in deionized water for 8 h and then dried in an oven set at 60 °C for a further 16 h, with one cycle per 24 h, for a total of three stages of 0, 5 and 10 cycles. At the end of each stage, the specimens were tested for mass, ultrasonic wave velocity, and compressive strength.

In the freeze-thaw cycle test, the water-saturated specimen is placed in an alternating heat and humidity test chamber (GP/TH180-40) and frozen at –30 °C for 16 h, then thawed at 25 °C for 8 h, with one cycle per 24 h, and a total of three phases of 0, 5 and 10 cycles. At the end of each stage, the specimens were tested for mass, ultrasonic wave velocity, and compressive strength.

Compactness and microstructure tests

The ultrasonic speed of hardened mortar at designated ages (3, 7, 14, 28 and 56 days) was determined using a non-metallic ultrasonic tester (ZBL-U5200, China). The surfaces of the probe and the test specimen were coated with petroleum jelly before testing, each proportioning specimen was measured at least five times.

The water absorption of hardened mortar is determined according to the JGJ/T 70-2009 standard. First, the mortar that has been cured for 56 days is placed in the oven to dry completely and weighed, then it is placed in water until its weight stops changing and weighed, and finally, the percentage increase in the mass of the dried mortar is calculated as the water absorption rate.

The mineralogical composition of hardened mortars at different ages has been subjected to X-ray diffraction (D8 ADVANCE, BRUKER, Germany) analysis in the 2θ range of 10° to 70°.

The internal structural changes in the hardened mortar were assessed by scanning electron microscopy (QUANTA-650, FEI, USA).

Stone cultural heritage restoration tests

The bond strength of the mortar and stone substrate was determined by a universal testing machine (WDW-300, China) in accordance with GB/T 22459.4-2022.

In field restoration trials of stone cultural heritage: The surface hardness and ultrasonic speed of the hardened mortar were determined in the same manner as described above. The homogeneity within the hardened mortar was determined using an infrared thermography scanner (TESTO 890, Germany).

This study on the preparation and performance testing of composite mortar was conducted in strict accordance with the above content. For detailed procedures, please refer to Scheme 1.

Scheme 1
Scheme 1
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Preparation and testing methods for composite mortar.

Results and discussion

Workability and mechanical properties

The setting time of mortars is a crucial indicator of workability in stone cultural heritage conservation, as it directly influences the effectiveness of restoration interventions. Generally, NHL exhibit longer initial and final setting times due to the slow dissolution rate of their hydrated components, which can hinder efficient repair processes. However, the incorporation of NMK significantly reduces this limitation, as shown in Fig. 1a. Specifically, the initial and final setting times of the modified mortar were reduced by up to 38.21% and 38.31%, compared to the control mortar. This implies that in practical restoration work, we can adjust the setting time of the mortar within a certain range to meet specific restoration requirements. The accelerated setting can be attributed to the nano-size and high pozzolanic activity of NMK, which accelerates the hydration reaction within the mortar, thus accelerating its hardening rate50,51.

Fig. 1: Workability and mechanical properties of NHL-based mortars.
Fig. 1: Workability and mechanical properties of NHL-based mortars.
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a Setting time; b shrinkage; c soluble salt content; d surface hardness; e flexural strength; f compressive strength.

In terms of mortar compatibility, the drying shrinkage and their soluble salt content changes were particularly emphasized. As shown in Fig. 1b, while NHL-based mortars already exhibit low shrinkage, the shrinkage of NMK-modified mortar can be further reduced. At the age of 56 days, the shrinkage of the control mortar was 0.31%, whereas the NMK20 sample exhibited a shrinkage of only 0.17%, representing a 54.84% reduction. This decrease may be attributed to NMK absorbing some of the free moisture for the hydration reaction, thus slowing down the evaporation of moisture. The soluble salt content of mortar with different ratios at 56 days of age is presented in Fig. 1c. The addition of NMK effectively reduced the soluble salt content of the mortar. It is worth noting that the soluble salt content of the NMK20 mortar was only 20% of that of the control mortar. Lower shrinkage and soluble salt content can effectively prevent the occurrence of shrinkage cracks and salt-induced deterioration, thereby enhancing the compatibility of modified mortars for the restoration of stone cultural heritage.

Composite mortars also exhibit enhanced mechanical properties, as reflected by improvements in surface hardness, flexural strength and compressive strength. As shown in Fig. 1d, the hardness of the mortar initially increases and then stabilizes with the increase of NMK dosage. Compared to the control mortar, the hardness of the modified mortar can be increased by up to 97.12%. More pronounced enhancements were observed in both flexural and compressive strength, as illustrated in Fig. 1e, f. As can be seen from the figure, the control mortar (NMK0) exhibited low strengths: flexural strength of 0.79 MPa at 7 days and 1.16 MPa at 14 days; compressive strength of 1.80 MPa at 7 days and 2.89 MPa at 14 days. In contrast, the addition of NMK significantly improves this deficiency. The NMK-modified mortars achieved significantly higher strengths: flexural strength reached 2.53 MPa at 7 days and 3.31 MPa at 14 days, while compressive strength reached 14.88 MPa at 7 days and 24.38 MPa at 14 days. This represents a maximum increase of 220.25% in 7-day flexural strength, 185.34% in 14-day flexural strength, 726.67% in 7-day compressive strength, and 743.60% in 14-day compressive strength compared to the control mortar (NMK0). However, it is noteworthy that excessive NMK dosage can slightly reduce the strength of hardened mortars, likely due to particle agglomeration. Overall, the mortar exhibits relatively better performance when the replacement rate of NMK is 20%(NMK20).

Durability

The above results indicate that NMK modified NHL mortars exhibit enhanced workability, compatibility and mechanical properties. Yet it is well known that the application environment for restoration materials in stone cultural heritage is highly complex, often being subjected to various adverse factors, such as humidity fluctuations, temperature changes, soluble salts, and biological infestations. Among these factors, variations in humidity (wet-dry) and temperature (freeze-thaw) are the most common and critical. Thus, the durability of composite mortar under wet-dry cycles and freeze-thaw cycles was further investigated52,53,54. Given the comparable performance between NMK5 and NMK10, and similarly for NMK15, NMK20 and NMK25, the representative samples NMK10 and NMK20 were selected for comparative analysis with the control mortar (NMK0).

Figure 2 shows the wet and dry cycling resistance data for the control mortar and the modified mortar. Figure 2a, b illustrates the changes in the appearance and morphology of the mortar after different numbers of wet and dry cycles. The control mortar exhibited varying degrees of edge damage after 5 and 10 cycles, which may be attributed to differential shrinkage caused by intense dry-wet variations. In contrast, the NMK20 mortar retained its original appearance and morphology without any visible changes.

Fig. 2: Dry and wet cycle resistance of NHL-based mortars.
Fig. 2: Dry and wet cycle resistance of NHL-based mortars.
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a Appearance and morphology changes of NMK0; b appearance and morphology changes of NMK20; c compressive strength; d ultrasonic speed; e mass change.

The variations in compressive strength, ultrasonic velocity, and mass of the mortar after different numbers of wet and dry cycles are depicted in Fig. 2c–e. As the number of cycles increases, the performance indicators of the mortar exhibit varying degrees of decline. This is attributed to the fact that water entering the mortar dissolved some soluble substances, thereby increasing the number of pores. Moreover, the repeated ingress of water caused the mortar to soften. Upon completion of 10 cycles, the compressive strength, ultrasonic velocity and mass of the control mortar, NMK10 and NMK20 decreased by 11.22%/13.11%/2.8%, 29.47%/27.14%/12.68% and 1.23%/0.94%/0.38%, respectively, compared to their initial condition. It is obvious that NMK20 has better resistance to wet and dry cycles.

In addition to the wet and dry cycles, the freeze-thaw action of water is another significant factor contributing to the deterioration of stone cultural heritage. Water within the pores freezes and expands at low temperatures, causing structural cracking. As temperatures rise, the melting water seeps into the new cracks and expands again when the temperature drops. The freeze-thaw cycles lead to the continuous expansion of cracks, which seriously affects the lifespan of stone cultural heritage. Therefore, restoration materials for stone cultural heritage need to have good freeze-thaw resistance. Figure 3 presents the freeze-thaw cycle resistance data for control mortar and modified mortar. Figure 3a, b demonstrates the changes in the appearance and morphology of the mortar after 5 and 10 freeze-thaw cycles. At the conclusion of the test, the control mortar showed significant crack expansion and edge breakage due to repeated ice expansion of the pore water. On the contrary, the NMK20 mortar maintained its morphological integrity.

Fig. 3: Freeze-thaw cycle resistance of NHL-based mortars.
Fig. 3: Freeze-thaw cycle resistance of NHL-based mortars.
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a Appearance and morphology changes of NMK0; b appearance and morphology changes of NMK20; c compressive strength; d ultrasonic speed; e mass change.

The changes in compressive strength, ultrasonic velocity, and mass of the mortar after different numbers of freeze-thaw cycles are shown in Fig. 3c–e. It is evident that the influence of freeze-thaw action on the mortar properties is more significant than that of wet-dry cycles. Following the completion of 10 freeze-thaw cycles, the compressive strength, ultrasonic velocity and mass decreased by (30.36%/15.37%/13.29%) for the control mortar, (32.42%/37.17%/33.57%) for NMK10, and (21.65%/23.75%/9.99%) for NMK20, respectively, compared to the initial condition. The reason lies in the fact that the repeated cycle of freezing and thawing leads to the continuous expansion and interconnection of cracks, resulting in a decline in the performance of the mortar. Despite the rigorous testing conditions, it is noteworthy that the strength of NMK20 remained significantly higher than that of the control mortar in its normal state after 10 freeze-thaw cycles. Apparently, NMK20’s ability to resist freeze-thaw cycles is much higher than that of other mortars, which indicates its potential for application in low-temperature environments.

Compactness and microstructure

Previous research has indicated that the degree of compactness and structure within the cementitious material is a key factor in determining its physical, mechanical, and durability properties55,56. Consequently, we investigated the alteration in compactness, hydration reaction rate, and product morphology of NMK-modified mortar is essential for elucidating the mechanisms by which NMK enhances the performance of NHL mortars.

Figure 4 demonstrates compactness and microstructure of NHL-based mortars. The changes in the compactness and water absorption of the mortar before and after modification are shown in Fig. 4a, b. The ultrasonic velocity of the modified mortar increases rapidly, significantly higher than that of the control mortar, with this phenomenon being particularly pronounced at 7 and 14 days of curing. This indicates that the addition of NMK is highly beneficial for improving the early density of the mortar. Correspondingly, as the dosage of NMK increases, the water absorption rate of the mortar continuously decreases. However, when NMK is added in excess, the water absorption rate increases slightly. This is attributed to the strong water absorption capacity of the layered structure of NMK. Overall, after the curing process is completed, the mortar with 20% NMK (NMK20) exhibits the highest ultrasonic velocity and a significantly lower water absorption rate compared to other mortar mixtures, indicating that NMK20 achieves the highest density.

Fig. 4: Compactness and microstructure of NHL-based mortars.
Fig. 4: Compactness and microstructure of NHL-based mortars.
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a Ultrasonic speed; b water absorption; c XRD patterns of mortar at 14 days of age; d XRD patterns of mortar at 56 days of age; e SEM images of the 14-days NMK0 mortar; f SEM images of the 14-days NMK20 mortar.

In Fig. 4c, d, X-ray diffraction analyses provide valuable insights into the changes in the mineral composition of mortar samples cured for 14 and 56 days. The main mineral phases in the mortar samples are Quartz (SiO₂), Calcite (CaCO₃), Portlandite (Ca(OH)₂), Larnite (Ca₂SiO₄), and Albite (NaAlSi₃O₈). It is noteworthy that Fig. 4c shows a clear gradient difference in the content of Ca(OH)₂ (CH) in the mortar before and after modification, as evidenced by the intensity of the diffraction peaks near 18° and 34°, and C-S-H cannot be observed because of poor crystallinity57. As a key participant in the pozzolanic reaction, the CH content decreases progressively with increasing NMK dosage, especially for NMK20 at 14 days of curing, where the CH diffraction peak is almost undetectable. In contrast, the internal reactions in other mortars continue to progress after 14 days of curing, as indicated by the reduction in CH content, as shown in Fig. 4d. These phenomena suggest that the addition of NMK significantly accelerates the rate of the pozzolanic reaction within the mortar samples, consuming a large amount of CH. Specifically, NMK20 completes most of the pozzolanic reactions within the first 14 days of curing, thereby providing excellent early-age performance for the mortar. Figure 4e, f shows SEM images of the internal structure of the control mortar and the NMK20 mortar at the age of 14 days. As depicted in Fig. 4e, the internal structure of the control mortar appears loose, characterized by the presence of distinct pores and a small number of needle-like structures interspersed between the particles. These needle-like structures are calcium silicate hydrate (C-S-H), which forms as a reaction product during the mortar hardening process. Surprisingly, the information presented in Fig. 4f is quite different, as the particle profiles are hardly visible inside the NMK20 mortar and are replaced by densely structured agglomerates with a large amount of reticulated C-S-H on the surface of the agglomerates. In comparison, it is evident that the incorporation of NMK has profoundly altered the microstructure of the hydration products58,59,60. Specifically, the C-S-H has transitioned from a loose, needle-like morphology to a dense reticulated structure. This transformation has significantly enhanced the densification of the modified mortar, thereby endowing it with superior performance characteristics.

Based on the experimental results of NHL-based mortars, the mechanism of NMK-modified NHL mortar is illustrated using a schematic diagram (Fig. 5). The nanoscale dimensions of NMK allow for uniform dispersion within the NHL matrix, promoting homogeneity and reducing pore connectivity. This contributes to reduced porosity, lower drying shrinkage, and enhanced resistance to water penetration. Furthermore, benefiting from its high pozzolanic activity, NMK reacts with the original and reaction-generated calcium hydroxide (CH), forming additional calcium silicate hydrate (C-S-H) gel. This secondary hydration product effectively fills capillary pores and refines the microstructure, resulting in improved matrix density and mechanical strength. Most importantly, the denser and more integrated microstructure imparted by NMK significantly improves the durability of NHL composites, as evidenced by increased resistance to freeze-thaw cycles and wet-dry alternation. This enhanced microstructure enables the composite to effectively resist water-induced dissolution and damage from cyclic water-ice phase transitions.

Fig. 5
Fig. 5
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Schematic diagram of structural changes of NMK-modified NHL mortar.

Restoration of sandstone grottoes

Overall, NMK20 mortar demonstrates excellent physical, mechanical and durability properties, making it highly suitable for the restoration of stone cultural heritage. Nevertheless, the bond strength between the mortar and the substrate cannot be overlooked, as a robust interfacial bond is essential for ensuring the effective restoration of stone cultural heritage. Therefore, in subsequent research, intact sandstone samples were collected from the stratigraphic layers of the Yungang Grottoes. The bond strength of the mortar was then tested to evaluate this critical performance metric. Figure 6a, b shows the specimens after bonding of the mortar to the sandstone and the testing process. Figure 6c demonstrates the bond strength for both control and NMK-modified mortars. Apparently, the addition of NMK improves the bond strength. The bond strength of the control mortar is only 0.62 MPa, whereas that of NMK20 mortar reaches 1.68 MPa, representing a 170.97% improvement. This substantial increase confirms that NMK20 forms a strong bond with sandstone, further validating its applicability in stone cultural heritage conservation.

Fig. 6: Bond tests for NHL-based mortars.
Fig. 6: Bond tests for NHL-based mortars.
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a, b Mortar bonded to rock; c bond strength.

Under the premise of ensuring the excellent performance of the repair mortar and the safety of the restoration process for the stone cultural heritage, NMK20 mortar was employed for the restoration of damaged sandstone in the Yungang Grottoes. Subsequently, following the completion of the repair work, essential monitoring evaluation of the mortar’s performance development were conducted. Figure 7 shows the effects of sandstone restoration and monitoring data. The restoration test area was located on a severely damaged rock mass adjacent to Cave 4 of the Yungang Grottoes, as shown in Fig. 7a. A visual comparison of the sandstone before and after restoration is provided in Fig. 7a, b. Upon completion of the restoration, the NMK20 mortar was cured by covering with water-saturated dust-free cloth and polythene film. The infrared thermography photographs, surface hardness and ultrasonic speed of NMK20 mortar at different ages are shown in Fig. 7c–e. At 14 days, NMK20 mortar retained internal moisture and exhibited a lower surface temperature than the surrounding rock. By 28 days, the surface temperature of NMK20 mortar became uniform and consistent with the surrounding environment, indicating dense internal structure without hollowing or cracking61. The surface hardness and ultrasonic speed of NMK20 mortar increased with age, reaching stability after 28 days of age, slightly higher than that of the surrounding sandstone. Collectively, these results demonstrate NMK20 mortar’s favorable performance development after application, mitigating common issues associated with traditional cementitious materials such as cement and lime, including soluble salt efflorescence, excessive rigidity, and insufficient strength. This highlights its superior compatibility with stone cultural heritage.

Fig. 7: Sandstone restoration and data monitoring at the Yungang Grottoes.
Fig. 7: Sandstone restoration and data monitoring at the Yungang Grottoes.
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a Field test area location b comparison of materials before and after curing; c infrared thermography; d surface hardness; e ultrasonic speed.

Discussion

This study targets the limitations of NHL mortar in the restoration of stone cultural heritage, such as low early strength and inadequate durability. It employed NMK to modify the mortar, investigated the comprehensive properties of the modified mortar, elucidated the mechanism of NMK action, and applied the composite mortar to on-site restoration projects, thereby addressing these critical challenges in stone cultural heritage conservation. The main findings of the current study are as follows:

  1. (1)

    The incorporation of NMK effectively shortens the setting time of fresh mortar, thereby enhancing the efficiency of repair operations. Additionally, it substantially reduces the soluble salt content, shrinkage rate, and water absorption of the hardened mortar. This mitigates potential issues such as salt crystallization damage and shrinkage cracking that could arise when using modified mortar for the restoration of stone cultural heritage. Moreover, the addition of NMK significantly improves the early strength deficiency of mortar, and the superior mechanical strength and density provide the mortar with a greater ability to resist external forces. When the NMK content reaches 20%, the modified mortar exhibited relatively better performance. The modified mortar also exhibits excellent resistance to dry-wet and freeze-thaw cycles and is well-suited for the restoration of stone cultural heritage in damp environments.

  2. (2)

    The reason for the excellent properties of the modified mortar is that the NMK, which has high pozzolanic activity, is dispersed in the mortar and significantly accelerates the hydration rate within the NHL mortar. A large amount of calcium silicate hydrate (C-S-H) is formed in a short time. The network-like C-S-H fills the pores between particles, thereby enhancing the mortar’s density.

  3. (3)

    In the restoration of stone cultural heritage using NMK20 mortar, the mortar achieves a robust bond with sandstone, ensuring excellent restoration performance. Furthermore, under simplified outdoor curing conditions, the modified mortar demonstrated excellent performance development, effectively addressing deterioration and demonstrating its applicability for the repair and bonding reinforcement of stone cultural heritage.