Introduction

The selection and innovation of building materials have historically shaped architecture development worldwide. Among these materials, tabia (三合土 in Chinese), an ancient composite of earth, sand, and lime, holds significant value in traditional building technology due to its unique properties and wide applications. Used extensively in ancient China for constructing city walls, tombs, mausoleums and coastal fortifications [1], tabia’s origins date back at least 5000 years to Dadiwan, Gansu Province, China [2]. The Ming Dynasty technical text “Tian Gong Kai Wu” by Song Ying Xing documents its preparation method, prescribing a mixture of river sand, earth, lime, and additives like sticky rice and kiwi vine juice to achieve durability [3]. Later innovations included incorporating sticky rice, brown sugar and egg whites in ancient buildings, such as those in Fujian’s Qing Dynasty earth building complexes. Sticky rice was found to improve the compressive strength of the material, while the addition of an appropriate amount of brown sugar enhanced freeze-thaw resistance. However, the effect of egg white on the material was not significant [4, 5]. Numerous reports have documented the use of tabia mortar in the construction of tombs, such as Xu Jie’s tomb [6], the tomb of Shi Songzhi [1], tomb at Lanjiazhai Site in Wushan of Chongqing [7] and so on.

Traditional tabia materials have been preserved for thousands of years, and their durability are unmatched by modern cement and other new building materials. Research on the mix ratios and strengthening mechanisms of tabia mortars can provide a solid basis for the conservation of ancient buildings. Existing studies on tabia mortar include scientific and technological analyses that replicate original materials and technologies to detect organic additives [1, 3,4,5,6]. Additionally, research have explored the impact of different mix ratios and additives (such as sticky rice soup, brown sugar, Tung oil and proteins) on the structure and mechanical properties of the tabia mortars [7,8,9,10,11,12]. These studies have enhanced the scientific understanding of traditional tabia building materials and provided a stronger foundation for improving their physical properties.

Lin Yimu’s tomb (林一牧墓), located in Baifuyan Village (百福岩村), Yayang Town (雅阳镇), Taishun County (泰顺县), near the Zhejiang-Fujian border (Fig. 1a), is a notable example. This Qing Dynasty tomb complex, suited on the north mountainside of Baifuyan Village, dates back to 1783 AD during the reign of Emperor Qianlong. It is the joint burial site of Lin Yimu, his spouse Mrs. Dong, and Mrs. Zhuang. Lin Yimu was born in 1723 and died in 1785. He was bestowed the honorary title of Instructional Officer (奉训大夫) and posthumously awarded the title of Martial Faith Cavalry Captain (武信骑尉) by imperial edict.

The tomb complex is notable for its expansive size. It features six levels of altars (拜坛) descending from the burial chamber. Key elements within the worship terraces include screen walls (照壁), sacrificial tables (祭桌), and niches (束龛) (Fig. 1b, d). Aside from the wooden-like stone structure of the spirit tablet pavilion (仿木石构神位牌) in front of the tomb chamber, all components, such as the tomb enclosure (墓圈), railings (栏杆), ornamental pillars (望柱), screen walls, sacrificial table, steps (阶墀), are crafted from tabia mortar. This construction method and the overall layout (Fig. 1c) offer insights into Qing Dynasty burial customs and cultural practices.

Despite its historical significance, exposure to natural environment have caused weathering and structural damage over time, evidenced by slight tilting of screen walls, damage to pillar tops, and cracks in railing panels (Fig. 1e). To address these issues, the local cultural heritage conservation department initiated an investigation and planned maintenance to safeguard the tomb, striving to commence the conservation project soon. Given tabia mortar’s unique and pivotal role in the tomb’s structure, a comprehensive multi-analytical study was conducted to assess its composition, physical properties, and mechanical strength, aiming to inform conservation efforts and address key research questions such as:

  1. 1.

    What are the raw materials used and their mix ratio during the preparation of the tabia mortar?

  2. 2.

    What are the physical and mechanical properties of the tabia mortar?

Fig. 1
figure 1

a Geographic location of the Lin Yimu’s tomb; b Top view of the site; c Schematic drawing of the site; d Front elevation of the site; e One crack in the railings’ panels of the altar; f Micrograph of the archaeological tabia mortar sample

Full size image

The analytical results will contribute to our understanding of Qing dynasty construction technologies and the technological knowledge for using tabia mortars in historical China. In addition, those analytical results serve as important references for the future conservation efforts of Lin Yimu’s tomb.

Materials and experimental methods

Archaeological samples

Samples were taken from a fracture in front of altar (Fig. 1e) at the tomb. The irregularly shaped samples initially contained surface debris such as sand, gravel, and moss-like plants, which were meticulously cleaned using small brushes, tweezers, and an air blower prior to characterization and testing.

Characterizations and testing

The mortar samples were analyzed using combination techniques of Polarized Light Microscopy (PLM), Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), Scanning Electron Microscopy-Element Dispersive Spectrometry (SEM-EDX), and Thermogravimetry (TG) and Differential Scanning Calorimetry (DSC).

Polarized light microscopy

Thin sections of the tabia samples were prepared following the SY/T 5913 − 2021 Standard Rock Preparation method [13], and examined using a Leica DM2700P polarizing microscope. Textural parameters were computed using digital image analysis using the ImageJ software (ImageJ 1.48v, National Institutes of Health, USA). Grain size distribution (GSD) determination was achieved by measuring the minimum Feret (mF) values of grains, enabling the calculation of Krumbein phi (Φ) values (Φ= -log2(mF)) [14, 15]. Morphological characterization of the grains was expressed in terms of logit circularity (C; where logit = ln(C/1-C)). The statistical treatment of approximately 300 grain measurements per layer was carried out using the R-4.3.1 software.

Scanning electron microscopy (SEM) and Energy Dispersive X-ray spectroscopy (EDX)

Micro-morphological observations and elemental analyses were conducted using a Zeiss Thermal Field Emission Scanning Electron Microscope (Gemini SEM 300) and an Oxford XMaxN51-XMX1140 Energy Dispersive X-ray Spectrometer (EDX). Prior to testing, the samples were gold-coated to enhance electrical conductivity.

X-ray diffraction (XRD)

Fine powders were obtained by crushing the mortar samples using an agate mortar. X-ray Diffraction (XRD) analysis was performed using a Rigaku SmartLab X-ray diffractometer under the following conditions: a step size of 0.02°, Cu-kα radiation, scanning range from 3° to 90°, and a scanning speed of 10°/min.

Fourier transform infrared spectroscopy (FTIR)

FTIR analyses were conducted using the attenuated total reflection (ATR) module of a Nicolet IS50 FTIR spectrometer on several milligrams of finely ground powdered samples at room temperature. The wavenumber range was 4000 to 400 cm−1, with a spectral resolution of 4 cm−1 and averaging 64 scans.

Thermogravimetry (TG) and differential scanning calorimetry (DSC)

TG-DSC tests were conducted using thermogravimetric-differential scanning calorimetry (TG-DSC) coupled with a Mettler Toledo TGA/DSC 3+. Approximately 40 mg of powdered sample was weighed and placed in a capped 70 µL platinum crucible. The analysis was performed in a nitrogen atmosphere (flow rate of 50 mL/min), with a temperature range of 30–1000 °C and heating rate of 10°/min.

Organic additive analysis

To determine the presence of sticky rice in archaeological tabia sample, we employed the KI-I₂ reagent for identification [16]. Analytical-grade KI from Shanghai McLean Biochemical Technology Co., Ltd., and I₂ from Shanghai Ball Chemical Reagent Co., Ltd., were used to prepare the reagent. Initially, 2 g of KI was dissolved in 5 mL of deionized water, followed by the addition of 1 g of I₂ until completely dissolved. The solution was then diluted to 300 mL with deionized water and stored in a brown reagent bottle. The tabia sample was ground into powder, and 0.1 g of the powder was dispersed in deionized water using ultrasonic dispersion in a test tube. The sample was subsequently heated in a boiling water bath for 3–5 min. After centrifugation at 8000 rpm for 5 min, the pH of the supernatant and precipitate was adjusted to 6–7 using acetic acid. The color change of the samples was observed after adding one drop of the I₂-KI reagent to each of the supernatant and precipitate.

To confirm the presence of oils, proteins and sucrose, chemical analysis was conducted [16]. A method involving sodium periodate and acetyl acetone was used to detect the presence of oil additives in the archaeological sample [17]. To prepare the sodium periodate solution, 1.065 g of sodium periodate was dissolved in deionized water to a final volume of 100 mL, resulting in a 0.05 mol/L sodium periodate solution. For the acetyl acetone solution, 4 mL of acetyl acetone was added to 96 mL of isopropyl alcohol, and thoroughly mixed. A 0.1 g sample of the tabia mortar was placed in a 10 mL test tube, mixed with 3 mL of deionized water, and 0.5 g of sodium hydroxide. The mixture was heated in a water bath for 20 min, then cooled and centrifuged at 3000 r/min for 5 min to obtain the supernatant. Next, 1 mL of the prepared sodium periodate solution was added to the supernatant and allowed to react for 10 min, followed by the addition of 1 mL of the acetyl acetone solution. After 15 min, if the solution turns yellow, it indicates the presence of glycerides.

For proteins, the block mortar was ground into powder, and approximately 0.1 g was placed in a test tube. Next, l mol/L hydrochloric acid was added until no more bubbles formed. The solution was then adjusted to a weakly acidic pH using 0.1 mol/L NaOH solution and 0.1 mol/L hydrochloric acid. Finally, the Coomassie brilliant blue reagent was added. If the solution becomes bright blue, it indicated the presence of proteins in the mortar. In the test for detecting organic additives residues, the control sample used is the whey protein powder with 73% protein content. The presence of protein additives is determined by comparing the color of the reaction solution between the control sample and the tabia mortar sample.

For sucrose, the block of mortar ground into powder, and approximately 0.1 g was placed in a test tube. Next, 1 mol/L hydrochloric acid was added until no more bubbles formed, and the mixture was heated in a water bath for 10 min. After cooling, the solution was then adjusted to a weakly acidic pH using 0.1 mol/L NaOH solution and 0.1 mol/L hydrochloric acid. A saturated Na2CO3 solution was then added until no precipitation occurred, and the mixture was centrifuged. The supernatant was combined with a small amount of Ban’s reagent and a small amount of sugar. If a brick red precipitate formed after heating in a water bath, it indicated the presence of sucrose in the mortar.

In the test for detecting organic additives residues, control samples of the tabia mortar was prepared using a lime-earth-sand ratio of 3:4:8 by mass. A total of 1449 g of raw materials were placed in a mixer and dry-mixed for 3 min at a speed of 600 r/min. Subsequently, 99 mL of water was added during the mixing process, and the mixture was stirred for 7 min at a speed of 1200 r/min. The mixed mortar was then poured into molds. After 7 days of setting, the samples were demolded and placed in a curing box at 20℃, 70% humidity, and around 400 ppm CO2 for 21 days.

Acid digestion test

To determine the relative proportions of the raw ingredients in the tabia sample, chemical methods involving acid digestion were employed. Approximately 5 g of powdered tabia sample was placed in a beaker, and 1 mol/L hydrochloric acid solution was added dropwise until no bubbles formed. The beaker underwent ultrasonic vibration to aid the reaction. The resulting suspension was decanted, and additional hydrochloric acid was added until the solution clarified. After washing with deionized water to a neutral pH, the solution was dried at 100℃ to constant weight in an oven. The dried samples were then sieved through a 200-mesh sieve; particles passing through (less than 75 μm) were classified as earth, while those retained (greater than 75 μm) were classified as sand according to the GB/T 14,684 − 2011 standard [18]. The masses of earth and sand were measured to calculate their respective relative percentages.

Particle size distribution analysis

To further analyze the particle size distribution, the earth and sand fractions obtained after acid digestion were examined using different methods. The earth component was analyzed using a Dandong Biotek BT-2002 laser particle size analyzer (range: 1–2600 μm, shading rate: 15%). The sand component was analyzed using seven standard sieves (10, 20, 40, 60, 120, 150, and 160 mesh) to determine the mass percentages across various particle size ranges.

Porosity and pore size distribution

The porosity and pore size distribution of the tabia samples were analyzed using a Micromeritics AutoPore IV 9500, a fully automated mercury intrusion porosimeter. The test pressure range from 0.2 to 33,000 psi, and the pore size range from 780 μm to 5 nm. In the mercury injection test, the surface tension (γ) of mercury is 485 dynes/cm and the contact angle (Φ) between the pores and mercury is 130°.

Mechanical strength

To determine the mechanical properties of the tabia samples, an electronic universal testing machine was employed. Due to the rarity of archaeological samples, the available samples could only be shaped into square blocks measuring approximately 25 × 25 × 25 mm3 for parallel testing. The tabia samples were. Compression tests were conducted using an INSTRON 34TM-50 model universal testing machine at a speed of 1.27 mm/min until the samples were crushed, and the maximum pressure value was recorded. The compressive strength of the tabia samples was calculated by dividing the maximum pressure value by the contact area of 25 × 25 mm2.

A Leeb hardness tester was used to measure the sample’s hardness. Eighteen measurements were taken in parallel and averaged over a relatively flat area on the surface of the tabia sample.

Results

Mineralogical composition of the tabia mortar samples

The XRD analysis (Fig. 2a) reveals that the tabia sample is primarily composed of calcite, quartz, and minor phyllosilicates (illite/mica). This allows us to discern the main components of tabia samples, calcite forms as a stable product of lime undergoing long-term carbonation; quartz (and microcline) is present in both the sandy and earth fractions; and phyllosilicates are commonly found in the earth fraction. The absence of calcium hydroxide in the XRD pattern indicates complete carbonation of lime over time [19]. No hydraulic compounds were identified using XRD.

The FTIR spectra of the tabia sample (Fig. 2b) complement the XRD analysis mentioned earlier. The bands observed include: 3700 cm−1 and 3621 cm−1, represents the stretching vibration of the O-H bond associated to illite [20]; 3400 cm−1 and 1640 cm−1 attributed to surface absorbed water; 1163 cm−1, 1004 cm−1, 798 cm−1, 778 cm−1, 694 cm−1, 518 cm−1 and 452 cm−1, associated with quartz and phyllosilicates; and 1795 cm−1, 1435 cm−1, 874 cm−1 and 713 cm−1, indicative of calcite [21,22,23]. The band at 1004 cm−1 could originate either the C-O absorption of polysaccharides in sticky rice additives [24, 25] or the stretching vibration of the Si-O bond. Further experiments are required to conclusively identify the source of this band due to the overlap in band positions of these functional groups [26].

Fig. 2
figure 2

a XRD pattern of the powder samples of the tabia mortar; b FTIR spectra of the powder samples of the tabia mortar; c TG-DSC curves of the powder samples of the tabia mortar

Full size image

To quantitatively analyze the mix ratio of the raw ingredients in the tabia samples, TG-DSC and chemical analysis were employed. Figure 2c shows the thermogravimetric analysis results of the tabia samples. The mass change in the temperature range of 30–200 °C is primarily attributed to the loss of free water and the removing of H2O molecules located in the interlayers of illite [27], resulting in a mass loss of 2.9%. The further mass-loss between 200–550℃ corresponds to the dehydroxylation process of illite and other minor phyllosilicates [28]. The decomposition temperature of phyllosilicates is around 500 °C. In the 600–800 °C range, the decomposition of calcium carbonate resulted in a mass loss of 11.95%. This allows the calcium carbonate content in the sample to be calculated as approximately 27.2% [29]. Beyond 800 °C, the mass remains largely unchanged. Both TG-DSC and XRD analysis indicate that the lime in the tabia mortar sample has fully carbonated, suggesting that the lime (Ca(OH)2) content in the raw material is about 21.6% [30] .

To determine the ratio of earth to sand in the tabia samples, the carbonate was dissolved using acid dissolution. The mass of the two ingredients was measured after sieving the earth and sand through a 200 mesh. The mass loss due to acid dissolution was found to be about 29%, slightly higher than the thermogravimetric test, indicating that some additional components in the tabia sample could be dissolved in hydrochloric acid solution besides calcium carbonate [31]. The mass ratio of earth to sand was calculated to be approximately 1:2. Combing this with the TG-DSC data, a lime-earth-sand ratio of approximately 3:4:8 was estimated for the raw ingredients in the tabia sample [1, 32]. Song Yingxing of the Ming Dynasty stated in his book Tiangong Kaiwu [3]: that during preparation, one part of lime is added to a mixture of two parts of earth and sand, and stir evenly with sticky rice soup and the kiwi vine juice. Xu Jiagan discusses coastal defense forts constructed primarily from tabia with a lime-earth-sand ratio of 5:3:2 in his book Outline of Coastal Defense, written during the late Qing Dynasty [33]. Using modern analytical techniques, the lime-earth-sand ratio of the “Hai” tabia coastal defense fort was determined to be 2:1:1 [34], while the ratio at the Jiaoshan tabia coastal defense fort in Zhenjiang City, Jiangsu Province, was found to be 3:4:3 [35]. It is evident that these recipes vary from one another, which may be influenced by the specific properties required for different architectural purposes, as well as the traditions of craftsmen in different places and times.

Particle size distribution analyzes of the earth and sand were conducted. The results revealed that over 90% of the sand particles in the tabia samples fell within the medium to coarse sand range (≥ 250 μm), with the remaining sand particles ranging from fine sand to fine silt (Fig. 3a) [11]. For the earth component, the median particle size was determined to be 11.48 μm, and 90% of the earth particles had sizes less than 45.86 μm (Fig. 3b). This indicates that the earth materials used as raw ingredients ranged from fine silt to clays, with a predominant particle size around 17 μm.

Fig. 3
figure 3

a particle size distribution of sand analyzed using a laser particle size analyzer b particle size distribution of the earth treated by acid digestion method

Full size image

Organic additives within the tabia mortar samples

Tung oil, sticky rice, animal blood, brown sugar, egg white, and other organic substances with specific viscosities were commonly added to enhance the properties of tabia mortar in ancient China [36,37,38]. These additives could be categorized based on their chemical compositions into fatty acid esters, starches, proteins and polysaccharides.

To further investigate the presence of sticky rice additives in the archaeological sample, a laboratory-prepared KI-I2 reagent test was conducted, as shown in Fig. 4a. No purplish-red color was observed in either the supernatant or the precipitate (Fig. 4a); only the inherent color of the diluted KI-I2 reagent was visible. This suggests two possible explanations: (1) sticky rice soup may not have been incorporated into the preparation of the archaeological tabia samples; (2) if added, the quantity of sticky rice soup was insufficient, or it may have degraded to undetectable levels due to prolonged environmental exposure. Consequently, starch was not detected in the tabia samples, confirming the 1004 cm−1 band observed in the infrared spectra (Fig. 2b) corresponds to the stretching vibration of the Si-O bond.

A method involving sodium periodate and acetyl acetone was used to investigate the presence of oil additives in the archaeological sample, as shown in Fig. 4b. Compared with the control sample (no.1 in Fig. 4b), the supernatant solution obtained from archaeological tabia mortar sample turned yellow, indicating the presence of glycerides in the tabia mortar sample (no.2 in Fig. 4b).

The Coomassie Brilliant Blue test was performed to investigate the presence of protein additives in the archaeological sample, as shown in Fig. 4c. Coomassie brilliant blue staining method, developed by Bradford in 1976, detects proteins based on the change in color of Coomassie Brilliant Blue G250 dye [38]. Archaeological tabia mortar samples exhibited a different color reaction compared to protein powder (commercial whey protein powder with 73% protein content), indicating the absence of added protein in the tabia mortar.

Sucrose is a common non-reducing disaccharide in nature that hydrolyzes upon dilute acid treatment to yield one molecule each of D-glucose and D-fructose. Both glucose and fructose are reducing sugars and can be oxidized by heavy metal ions (Cu2+、Ag+、Hg2+ or Bi3+) in alkaline solutions. Ban’s reagent uses Cu2+ ions as an oxidizing agent, resulting in the formation of brick-red Cu2O precipitate upon oxidation of reducing sugars. Ban’s reagent, initially light blue in color, facilitates observation of the phenomenon. The absence of a brick-red precipitate in both the lab-made tabia without sugar additives and archaeological tabia samples indicates the absence of sucrose in the tabia sample (Fig. 4d).

Fig. 4
figure 4

a KI-I2 detection: from left to the right, the samples are I-I2 reagent (no.1), KI-I2 reagent added to sticky rice soup (no.2), KI-I2 reagent added to the supernatant of tabia mortar samples (no.3) and KI-I2 reagent added to tabia mortar samples precipitates (no.4). b Glycerides detection: from left to the right, the samples are control sample (no.1) and archaeological tabia mortar sample (no.2). c Coomassie Brilliant Blue test for protein detection: from left to the right, the control sample is the whey protein powder with 73% protein content (no.1) and archaeological tabia mortar sample (no.2). d Ban’s reagent test for sucrose detection, from left to the right, the samples are lab-made tabia without sugar additives (no.1), and archaeological tabia mortar sample (no.2)

Full size image

Microstructural features of the tabia mortar samples

Polarized thin sections of the tabia samples were micrographically photographed (Fig. 5a) and processed using image analysis with a polarizing microscope to determine statistical grain size distribution.

Thin-sectioned sample and grain size distribution (GSD) measurements reveal the textural arrangement of the inorganic component in this material. The particle size distribution of the tabia mixtures ranges from fine sands to coarse sands, exhibiting a nearly symmetrical distribution (skewness range: -0.5 to 0.5), with sizes ranging from 0.012 to 1.2 mm. The sandy fraction consists of rounded to elongated (circularity values: 0.3 to 1) and sub-angular clasts of polygenic origin, including loose crystals of quartz, quartz-rich plutonic rocks, volcanic rocks with fluidal textures, and cherts. No hydraulic products or reaction rims were observed in the current micrographs.

The petrography of sand grains is consistent with the intermediate to acidic volcanic rocks dating from the Late Jurassic to Early Cretaceous periods, as well as intrusive counterparts of the Coastal Volcanic Belt that extend along the coastal regions of the Zhejiang and Fujian provinces [39].

Fig. 5
figure 5

a Thin section polarizing micrographs of archaeological tabia sample; b, c GSD curve of the tabia mortar: axis φ = minimum Feret phi size; axis C = circularity

Full size image

The microscopic morphology of the tabia sample was further analyzed using Scanning Electron Microscopy (SEM). SEM images depicted a dense structure indicating a high degree of cohesion among the components within the tabia mortar (Fig. 6).

Fig. 6
figure 6

a SE image of the surface of archaeological tabia (b–g) EDX analysis of a showing elemental distributions

Full size image

Physical properties of archaeological tabia sample

To investigate the physical properties of the archaeological tabia samples from Lin Yimu’s tomb, we characterized hardness, compressive strength and pore structure using a Leeb hardness tester, an electronic universal testing machine and Mercury Intrusion Porosimetry (MIP), respectively. The Leeb hardness of the sample from the Lin Yimu’s tomb measured 293 ± 15 HL, and its compressive strength was 5.5 ± 0.2 MPa. This is slightly higher compared to tabia mortar samples from other regions of China, such as the those from Ming Great Wall in Jiayuguan, Gansu Province (4.56 MPa) [40], the Great Wall at Banchangyu, Hebei Province (5.23 MPa) [40], the Jiaoshan ancient battery in Jiangsu Province (4.74 MPa) [35].

Regarding pore structure, the archaeological tabia samples exhibited a porosity of about 27.5%. This is lower than some other tabia and lime mortar samples, such as the minimum porosity of 33.6% found in mortars from the Dazhuangke section of the Ming Great Wall [41] and a minimum of 44.86% for the roof mortar in the ancient building complex of the Southern Three Shops of the Forbidden City [42]. The pore size distribution of the tabia sample is dominated by mesopores (0.1–2500 μm) and micropores (< 1 μm), with a notable concentration around 95 nm (Fig. 7). This distribution indicates a dense structure with small internal pore sizes overall [43].

Fig. 7
figure 7

Pore distribution in the archaeological tabia

Full size image

These findings suggest that the tabia mortar sample achieves high mechanical strength despite the absence of hydraulic products, attributed to its dense structure, low porosity, and fine particle size distribution.

Discussion

Tabia mortars, renowned in ancient China for their exceptional mechanical properties, have garnered substantial scholarly attention aimed at elucidating the factors contributing to their unique characteristics. Many researchers initially proposed that tabia mortars are hydraulic materials, requiring a humid environment for proper curing [16, 45, 46]. Zhang et al. proposed that the high mechanical strength arises from factors such as high density, an optimal sand- lime-clay mass ratio, suitable particle size distribution of sand, and the formation of hydraulic material C-S-H [44].

However, recent studies indicate that not all tabia mortars contain hydraulic components. For example, research on the tabia mortar from Ming Dynasty Xu Jie’s tomb revealed constituents such as calcite, quartz, montmorillonite and feldspar [5]. Similarly, rammed earth unearthed in Zhengzhou city consists solely of calcite and quartz, yet exhibits a dense structure and high hardness [45]; In other examples, tabia from fortresses in Tianjin, Dagu and at the mouth of the Minjiang River, as well as ancient coastal defense fortresses, consist solely of calcium carbonate, earth and sand [31]. Additionally, tabia mortar from tombs at Lanjiazhai Site in Wushan, Chongqing, which include sticky rice in their composition, are composed of calcium carbonate, earth, and sand, yet exhibit high strength without hydraulic components [6].

Fusade et al. reported that the compressive strength of lime mortar prepared with NHL2 (St Astier) and quartz sand (Chardstock) or sand-sized crushed limestone (Portland) with a 1:2 and 1:3 binder-to-aggregate ratio by volume remains below 1.8 MPa after 90 days of curing [47]. As compared to those materials, the tabia mortar from Lin Yimu’s tomb in the Qing Dynasty showcases high strength without the presence of hydraulic products, enriching archaeological data on non-hydraulic tabia mortars and suggesting new research avenues for understanding tabia strength. This underscores that the formation of hydraulic products may not always be necessary for developing tabia material strength. Further comprehensive studies and mock-up experiments are currently underway to investigate the factors contributing to the high mechanical strength of tabia mortars compared to other lime-based mortars.

Furthermore, while we estimated the lime-earth-sand mass ratio, precise determination of raw ingredient ratios in archaeological tabia mortar poses significant challenges, primarily due to quartz’s predominant presence in both river sand and earth. This potential overlap complicates clear categorization between ‘sand’ and ‘earth’, leading to arbitrary distinctions. Ongoing research, including mock-up experiments, aims to address these complexities and establish more accurate methods for understanding ingredient ratios in archaeological tabia mortars.

Conclusions

This paper presents a comprehensive characterization and analysis of tabia samples from Lin Yimu’s tomb, yielded the following conclusions regarding their composition, physical phases, proportions, microstructure, and macroscopic properties:

  1. (1)

    The primary minerals in the archaeological tabia samples include calcite, quartz, feldspar and phyllosilicates (illite/muscovite). Calcite originates from the carbonation of the lime. The aggregate consists of a selectively sourced sand deposit likely obtained locally, composed of quartz, feldspar, and rock fragments from mixed igneous and sedimentary sources. Phyllosilicates are residual phases attributed to the earth fraction. Archaeological tabia mortar also contains a certain amount of oil additives.

  2. (2)

    Quantitative methods such as thermogravimetric analysis and acid sieving revealed that lime constitutes approximately 20% of the physical composition of the archaeological tabia. The ratio of earth to sand is approximately 1:2, resulting in a roughly estimated lime-earth-sand ratio of 3:4:8.

  3. (3)

    Tabia samples from Lin Yimu’s tomb exhibit a dense structure, with a porosity of 27.5% and a predominant pore size of 95 nm. These samples demonstrate high mechanical properties, including a Richter hardness of 293 ± 15 HL and a compressive strength of 5.5 ± 0.2 MPa, making them outperform some common lime-based conservation materials, such as aerial lime mortars, and certain natural hydraulic lime mortars such as NHL2.

While the findings of this study may not apply universally to all burial tabia mortars, insights into raw materials, composition, mechanical and physical properties, and microstructures offer valuable references for studying tabia mortars. They also provide practical suggestions for the restoration and conservation of tabia mortar materials.