Zhang et al. Herit Sci (2020) 8:108 https://doi.org/10.1186/s40494-020-00452-6 RESEARCH ARTICLE Open Access What factors contribute to high mechanical strength of tabia? Case study of the tabia from coastal forts sites at Zhejiang, China Cuisong Zhang1, Bingjian Zhang1,2* , Biao Cui3 and Guocong Lin4 Abstract Tabia is one of the most widely used construction materials in ancient China. In this work, the tabia samples from three coastal defense fort sites at Zhejiang Province, China, we analyzed to determine their component composition and physiochemical properties. The results show that these tabia samples exhibited high compressive strength, which could be as high as 4–9 MPa. Furthermore, the systematic analysis demonstrated that the high compressive strength was related to the following factors: (1) a suitable mass ratio of sand, lime, and clay; (2) an appropriate sand particle size ratio; and (3) the formation of hydrated calcium silicate (C-S-H). Moreover, sticky rice was also detected in the tabia samples. These fndings are fundamentally important, which could be benefcial to further study of the crafts- manship of the coastal defense forts and could further provide essential guidance for the protection and restoration of the tabia relics. Keywords: Tabia, Coastal defense fort, Compressive strength, C-S-H, Sticky rice Introduction tabia used in building the forts was 5:3:2. After being well Tabia, a mixture of clay, lime, and sand, was widely used mixed with sticky rice soup, it was pounded from eight in the urban wall construction [1–3], tombs [4–6], and inches to two inches thick. Te fnished material was said water conservancy facilities [7] in ancient China for its to be as hard as iron [11]. Due to long-term exposure to super mechanical properties. Zhang and co-workers the natural environment, these tabia coastal defense forts have discovered starch in the mortar sample of a tomb in were greatly subjected to water erosion, wind erosion, Eastern Han Dynasty (25–220 AD) in Xuzhou, Jiangsu, freezing and thawing erosion and temperature changes, China [8]. About 1500 years ago, organic additives such which have caused some damages such as spalling and as sticky rice, tung oil, egg white, sugar, blood, and plant collapse. Terefore, it is in urgent to protect these tabia vine juice have been widely applied to improve mechani- coastal defense forts [12–16]. When the tabia coastal cal properties of the tabia [9]. After the Second Opium defense fort sites are to be subjected to the necessary in- War (1856–1860 AD), the construction of the tabia situ replacement and repair work, the “original materials coastal defense forts in China’s coastal area began, and and original craftsmanship” needs to be done at the early the sticky rice tabia coastal defense forts were also con- stage. Terefore, understanding the relationship between structed [10]. Xu Jiagan’s “Outline of Coastal Defense” the strength, raw material ratio, and construction meth- recorded the craftsmanship of sticky rice tabia coastal ods is fundamentally important not only for studying his- defense forts. Te proportion of lime:clay:sand of the torical of building materials, but also providing insights into the construction of tabia, and further severing for all *Correspondence: [email protected] purpose of restoring tabia coastal defense fort sites. 1 Department of Cultural Heritage and Museology, Zhejiang University, In recent years, there have been many studies on the Hangzhou 310028, People’s Republic of China mechanical properties and material composition of tabia Full list of author information is available at the end of the article coastal defense forts in China. Li et al. [17] investigated © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat​iveco​ mmons​.org/licen​ses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creat​iveco​mmons​.org/publi​cdoma​in/ zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Zhang et al. Herit Sci (2020) 8:108 Page 2 of 15 the tabia coastal defense forts in Dagu, Tianjin city using Materials and methods XRD, XRF, and SEM–EDS methods. Tey found that Materials the formula of clay, lime, and sand for “Hai” fort is 1:2:1, Te chemicals used in this work were as follows: calcium while the formula of clay and lime for “Wei” fort is 3:1. carbonate, hydrochloric acid, acetate, sodium carbonate Ruan et al. [18] compared mechanical properties between anhydrous, sodium hydroxide, copper sulfate anhydrous, the tabia coastal defense forts in Jiaoshan, Jiangsu prov- trisodium citrate dihydrate, iodine, potassium iodide, ince with simulated tabia samples made by traditional zine powder, periodic acid, acetyl acetone, phenolphtha- craftmanship. And the comparative analysis results lein, and Coomassie Brilliant Blue G-250 dye. All chemi- showed the formula of clay, lime, and sand is about 4:3:3 cals were procured from Sinopharm Chemical Reagent for the Jiaoshan ancient fort. Te result was performed Co., Ltd., Shanghai. on the repair of this fort with good result. Zhang and All the tabia samples were collected from the ground Wan [19] investigated the tabia coastal defense fort at that have fallen of from the coastal defense forts and Xiaoshi harbor, Huangshan city, Jiangsu province. Tey will not cause damage to the sites. When the samples found that the formula of the tabia clay and lime is 3:1, were cut to reveal a new interface, it was found that the with about 0.5% sticky rice soup. Tey also improved the components of the sample JY, PY, ZY, NW, and LW were formula for the fort’s restoration. Wu et al. [20] investi- evenly mixed, while the lime of sample HY and TFG were gated the formula and construction techniques of three uneven distributed. tabia coastal defense forts at Minjiang river estuary, Fujian province. Zhou et al. [21] investigated the physical, Methods mechanical and water performance of four tabia coastal Surface Leeb hardness, compressive strength, density defense forts in Guangzhou. All these researches show and porosity test that the formula of raw materials and craftsmanship of Te surface Leeb hardness of the tabia samples was tabia coastal defense forts difers from place to place. Tis tested using an Equotip 550 Portable Rockwell Leeb D is of great help for us to understand the characteristics of Impact Device (Proceq SA). Five tests were determined tabia coastal defense forts, and can provide us with sup- and then averaged for each tabia sample. Te compres- port for the necessary restoration project. However, there sive strength was assessed using a HANDPI electronic are few reports on the tabia coastal defense forts in the tensile testing machine (Yueqing Aidebao Instrument coastal areas of Zhejiang Province, China. In this study, Co., Ltd.). Tree tests were performed for each tabia sam- 3 seven tabia samples were collected from Pinghu coastal ple and the size of the tabia sample is 3 × 3 × 3 cm . Te defense forts sites, Ningbo coastal defense forts sites, and density and porosity were measured by a QL120C Den- Wenzhou coastal defense forts sites in Zhejiang Province, sity Tester (MatsuHaku, Inc.) based on the Archimedes’ China. Surface hardness, compressive strength, density principle. Tree tests were determined and averaged for and porosity, component composition, distribution of each tabia sample. sand and clay, organic additives, morphology, and aging resistant properties, and capillary water absorption of the Determination content of calcite, sand, and clay minerals tabia samples were studied to have a better understand- in tabia samples ing of the properties of the tabia coastal defense forts. Te CaCO3 content of tabia samples was calculated Jingyuan coastal defense fort, Pingyuan coastal defense based on an acid dissolution method. First, 1.8 g well fort, Zhenyuan coastal defense fort, and Hongyuan dried stone-free tabia was transferred to a conical fask, coastal defense fort are in Zhenhaikou, Ningbo City, in which 20 mL 1 M HCl solution was added. Secondly, Zhejiang Province, China. As a part of the Zhenhai- the fask was shaken for 15 min, to accelerate the chemi- kou Coastal Defense Site, the above four coastal defense cal reaction: CaCO3 + 2HCl = CaCl2 + CO2↑ + H2O. forts have been listed as the national key cultural relics Finally, the volume of CO 2 was recorded by a home-made in China since 1996. Tianfeigong coastal defense fort CO2 gas volume measuring instrument and the content and Nanwan coastal defense fort in Zhapu Town, Pin- Q1 of CaCO3 in tabia samples was calculated according to ghu City, Zhejiang Province, China, have been listed as the aforementioned-chemical equation. Tis process was the national key cultural relics in China since 2013. Long- repeated three times to obtain the average for each tabia wan coastal defense fort in Longwan District, Wenzhou sample. City, Zhejiang Province, China, has been listed as a key Te sand and clay contents were also determined by cultural relic of Wenzhou City since 2002.