An experimental workflow has been designed to simulate the synergic impact of salt weathering and frost damage under reality-based climatic conditions in the laboratory (Fig. 1). The experiment included the following steps: preparation of experimental materials, simulation ageing test, appearance observation and measurement of physic-mechanical properties. Soluble salts contained in the historic stone building, Chuyangtai, north of the West Lake Cultural Landscape of Hangzhou (WLCL, a World Heritage Site in Hangzhou, China) were identified through ion chromatography and ECOS-RUNSALT thermodynamic model [12, 13] for salt selection. Stones were sampled from the mountains included in the WLCL and the quarry near Hangzhou. They were cut and grouped at the beginning of the experiment and partially immersed in saline solution until they reached stable weight for salt impregnation. Then, the stone specimens were wrapped with polyethene foil with one vertical face open to mimic the exposure of the vertical façade of rock-hewn heritages, followed by 25 cycles of the salt ageing and freeze-thaw test, appearance observation and physic-mechanical measurements. The mineral composition of the stone samples was subjected to Bruker SMART APEX II X-ray diffractometer (operating at 40 kV and 40 mA with a Cu source) for mineralogical identification in the State Key Laboratory of Silicon Materials, Zhejiang University.
Selection of soluble salt
To identify the salt types existing in the stone heritages in the West Lake, a salt assessment trial was conducted on the SE façade of the Chuyangtai. Three loose rock debris were collected from the façade (Fig. 2d). Soluble salts contained in the three debris were extracted and analysed through the following steps:
a. grind rock sample to fine powder,
b. weigh 1.0g rock sample and place into 50ml plastic sample pot,
c. add 50ml deionised water into the pot,
d. sonicate for 1 hour at room temperature, and agitate for 1 hour on a flask-shaker at 250 strokes/min,
e. filter the sample into 100ml volumetric flasks and make to volume with deionised water,
f. subject to ion chromatography for ion concentration determination,
g. input ions content into the ECOS-RUNSALT model for salt type prediction.
A mixture of sodium sulfate and magnesium sulfate (1.4mol/1.5mol/kg) was chosen for this study because bloedite (Na2SO4·MgSO4·4H2O), thenarite (Na2SO4), starkeyite (MgSO4·4H2O), kieserite (MgSO4·H2O) are the most common salt types found in our salt assessment trial on the SE façade of the Chuyangtai through ion chromatography and ECOS-RUNSALT prediction (Figure 2a-2c). And more importantly, it is convincingly believed that these salt types would impose destructive effects on stone-built heritages [14], especially the bloedite, which has been proven to have high damage potential to rock because of its characteristic of incongruent dissolution [15,16].
Rock samples
This study utilised five types of rock (Table 1). The first one is dolomite (YM) sampled from the Ziyun cave on the top of the Yuhuang mountain, which is south of the West Lake and hosts several ancient rock-hewn heritages, such as the Ciyunling rock-hewn statues which were initially hewn in the Wuyue Kingdom (907–978 CE). YM is composed of 97.6% dolomite and 2.4% calcite. The second one (LA) is flint collected from the Lin'an district which is 35 km away from the West Lake and contains abundant mineral resources, such as limestone, kaolinite, tuff and fluorite [17]. LA is composed of 44.6% quartz, 54.4% muscovite and 2% clay minerals. The third one (FL) is dolomitic limestone sampled from the Feilai peak, situated northwest of the West Lake and is well-known for the ancient Buddha statues (North Song dynasty, 1282 CE) hewn from its outcrop. FL is made up of 74.6% calcite and 25.4% magnesian calcite. The forth one (LQ) is freshly quarried dolomite from Lin’an district. LQ is a significant carbonate rock for the architecture industry and is composed of 45.5% Fe-dolomite and 54.5% dolomite. And the fifth one (GM) is sedimentary tuff sampled from the Gu Mountain, an island in the West Lake with a similar diagenetic process and rock-forming minerals to the Baoshi Mountain [18].
Because of the limited quantity, shape and size of raw materials that can be obtained from the field, YM was cut into four cuboids with 2.5cm length, 2cm width and 5 cm height, LA was cut into three cuboids with 2.5cm length, 2cm width and 3cm height, and FL was cut into two cuboids with 4cm length, 3cm width and 7.3 cm height. LQ and GM were cut into two shapes. One is the cuboid with 3cm length, 1.2 cm width and 3cm height, and another is the cuboid with 3cm length, 3cm width and 12cm height, with three samples produced from each shape.
Experimental conditions
The ageing conditions selected for the experiment (Figure 3) were based on the extreme heat wave event in 2022 summer in Hangzhou, China [19] and the environmental parameters in the standard freeze-thaw test methods ASTM C666/C666M-15[20]. The ageing cycle includes 3 phases. Rock samples were partially immersed in saline solution for 3 hours at room temperature (20℃) in the first phase, namely the cooling and salt impregnation phase, followed by the freeze-thaw phase, which consisted of 15 hours at -10℃, followed by the high-temperature phase (evaporation phase) which consisted of 6 hours at 50℃, ready for the next phase 1 to start. The ageing test was conducted in the oven (JINGHONG DHG-9000) and lab fridge (AUCMA BC/BD-143DNE) and run for 25 cycles in total.
Dimensional change evaluation
The dimensional change coefficient (\(\epsilon\)) of rock samples was calculated as the ratio between the length/width/height change of the sample after the ageing test (\(\varDelta x\)) and the original sample length/width/height \({x}_{0}\), as shown in the following equations:
$$\epsilon =\varDelta x/{x}_{o}\bullet 100$$
$$\varDelta x={x}_{i}-{x}_{o}$$
Where \(\epsilon\) is the dimensional change coefficient, \({x}_{0}\) is the original sample length/width/height, and \({x}_{i}\) is the sample length/width/height after the ageing test.
Physic-mechanical properties measurement
To evaluate the synergic impacts of salt weathering and frozen damage on the rock samples, the following measurements were taken before and after the ageing test: surface hardness, water absorption coefficient by capillary (WAC), and pore structure analysis. Because the measurement of splitting tensile strength needs to break off the rock samples, it was only carried out after the ageing test.
Surface hardness measurements were taken with a Leed hardness tester (BH200C, BoTai Co., Ltd). The magnitude of surface hardness has been widely used as a proxy for the strength and weathering extent of rock-built heritages [21]. Based on the data sampling methods introduced by Aoki & Matsukura [22], ten rebound readings were obtained by randomly hitting different points across the front vertical surface and calculating the mean of the ten rebound readings to represent the surface hardness of the rock samples.
Pore structures of YM, LA, FL and GM were determined by mercury intrusion porosimeter AutoPore IV 9510 (MIP) in the State Key Laboratory of Chemical Engineering, Zhejiang University. And pore size distribution of LQ was analysed by Quantachrome AUTOSORB-1-C gas sorption analyser (GS) through density functional theory (DFT) and Barrett-Joyner-Halenda method (BJH) [23] in the State Key Laboratory of Chemical Engineering, Zhejiang University because the porosity of LQ samples is far below the detection limit of the mercury intrusion porosimeter. Furthermore, the aged rock sample were all subjected to the gas sorption analyser for the surface area calculation using the Brunauer-Emmett-Teller (BET) theory.
The measurement of the water absorption coefficient followed the standardising test protocol BS EN 1925:1999 [24]. Moreover, the determination of splitting tensile strength followed the ASTM standard c1006/c1006m-20a [25].