摘要
为研究可溶盐侵蚀导致的潮湿环境下露天石窟寺的劣化机理。以重庆石佛寺遗址摩崖造像为研究对象,采用扫描电子显微镜及能谱仪、X射线衍射分析仪、X射线荧光分析仪、离子色谱分析仪、红外热成像仪等多重分析手段对石佛寺遗址摩崖造像表面病害及劣化产物进行科学分析与表征,厘清了可溶盐主要的种类与来源。在分析数据的基础上,结合石质文物本体构成与环境变化综合分析,探讨了潮湿环境下石质文物中可溶盐的作用模式,为石质文物保护提供了科学依据。
摩崖造像一般指在崖壁雕凿浅龛造像(或直接造像),一般进深较浅,没有明显的三壁之分,仅正面雕造一尊或一组造
石佛寺遗址摩崖造像千年来由于长期处于露天条件下,受环境中不利因素侵蚀,岩体普遍出现泛盐、粉化、剥落等表面劣化问题。内层岩石继续在环境中劣化,泛盐、粉化等病害进一步发展,在自身重力因素的影响下会逐层剥落(见图

图 1 泛盐
Fig. 1 Salt efflorescence deterioration

图 2 泛盐
Fig. 2 Salt efflorescence deterioration

图 3 泛盐
Fig. 3 Salt efflorescence deterioration

图 4 粉化、剥落
Fig. 4 Chalking and crumbing deterioration

图 5 粉化、剥落
Fig. 5 Chalking and crumbling deterioration

图 6 片状剥落
Fig. 6 Scaling deterioration
扫描电子显微镜及能谱仪:美国FEI公司QUANTA-650型扫描电子显微镜(ESEM)及EDAX公司的APOLLO-X型能谱仪(EDS);实验条件:高真空模式,样品烘干粘在导电胶上;工作电压为15、25、30 kV;二次电子像及背散射像,能谱扫描时间30 s;能谱仪采用点扫与面扫工作模式。
X射线荧光光谱仪:德国BRUKER公司S2 RANGER光谱色散型X射线荧光光谱仪;实验条件:EQUA Oxides模式,样品烘干粉碎后,过100目筛,采用光谱纯硼酸压片。
离子色谱仪:德国Thermo Fisher公司DionexICS1100型离子色谱仪;砂岩与去离子水质量比为1∶100;首先称取经0.2 mm(80目)筛孔筛选的烘干待测砂岩样品0.500 g(精确到0.001 g),置于250 mL的烧杯中,加入50 mL的去离子水,超声提取15 min,每隔5 min震荡一次,保证固相分离完全;然后将溶液置于50 mL的离心管中,静置10 min,在离心管中以4 000 r/min转速离心10 min,离心后的溶液经0.45 μm微孔过滤膜过滤后,再用C18固相萃取小柱进行再次过滤,得到浸出液,最后进行离子色谱检测分析。
X射线衍射分析仪:德国布鲁克公司D8 ADVANCEX射线衍射分析仪对试样进行分析;实验条件:辐射源为Cu/Kα,波长为15.4 nm,管压40 kV,管流40 mA,扫描速度5 (°)/min,扫描范围10°~80°,样品烘干后过100目筛。
从剥落劣化岩样的扫描电镜微观形貌图(见图

图7 劣化砂岩微观图片
Fig. 7 SEM morphology of deterioration sandstone

图8 劣化砂岩微观图片
Fig. 8 SEM morphology of deterioration sandstone

图9 可溶盐结晶破坏图片
Fig. 9 SEM morphology of soluble salts erosion

图10 劣化产物(42-1号样品)元素分布
Fig. 10 Elements distributions of deterioration components
使用XRF对多处劣化样品表面进行原位无损分析以得到其元素组分,分析样品及结果如
编号 | 成分含量 | ||||||
---|---|---|---|---|---|---|---|
w(Na2O) | w(MgO) | w(Al2O3) | w(SiO2) | w(P2O5) | w(SO3) | w(Cl) | |
41-1 | 1.38 | 3.120 | 15.500 | 72.100 | 0.349 | 0.580 | 0.127 |
42-1 | — | 2.400 | 15.600 | 73.900 | 0.366 | 0.836 | — |
44-1 | — | — | 4.350 | 16.500 | 0.206 | 51.300 | — |
45-1 | — | 2.460 | 10.800 | 53.100 | — | 17.000 | — |
45-3 | — | 1.950 | 12.200 | 67.400 | — | 7.520 | 0.142 |
45-4 | — | — | 5.510 | 32.900 | — | 41.000 | — |
46-1 | — | 4.050 | 15.300 | 71.500 | — | 0.770 | 0.197 |
47-1 | — | 1.830 | 9.710 | 47.100 | — | 21.500 | — |
48-1 | — | 3.320 | 12.500 | 65.500 | — | 8.510 | — |
49-1 | — | 3.180 | 10.200 | 66.900 | — | 9.440 | 0.140 |
50-1 | 1.340 | 3.410 | 12.000 | 74.900 | — | 0.639 | 0.158 |
编号 | 成分含量 | ||||||
w(K2O) | w(CaO) | w(TiO2) | w(MnO) | w(Fe2O3) | 备注 | ||
41-1 | 1.970 | 0.625 | 0.349 | 0.359 | 2.850 | 轻度劣化 | |
42-1 | 1.530 | 0.709 | 0.495 | 0.593 | 3.180 | 轻度劣化 | |
44-1 | 0.396 | 26.000 | 0.143 | — | 0.949 | 重度劣化 | |
45-1 | 0.940 | 13.300 | 0.402 | — | 1.460 | 中度劣化 | |
45-3 | 1.160 | 7.690 | 0.363 | — | 1.370 | 中度劣化 | |
45-4 | 0.474 | 18.600 | 0.187 | 0.139 | 0.955 | 重度劣化 | |
46-1 | 1.420 | 4.240 | 0.446 | 0.121 | 1.680 | 轻度劣化 | |
47-1 | 0.886 | 16.700 | 0.337 | 0.133 | 1.610 | 中度劣化 | |
48-1 | 1.400 | 6.200 | 0.454 | 0.118 | 1.700 | 中度劣化 | |
49-1 | 1.090 | 6.830 | 0.372 | — | 1.500 | 中度劣化 | |
50-1 | 1.170 | 4.210 | 0.333 | — | 1.380 | 轻度劣化 |
通过离子色谱进一步对造像劣化区域的可溶盐阴离子进行定量分析,结果如
样品编号 | 样品重量/g | 阴离子种类 | 样品浸出液中阴离子的含量/(mg· | 样品中阴离子的含量/(mg· | 备注 |
---|---|---|---|---|---|
41-1 | 0.501 0 |
C | 49.656 4 | 4.965 64 | 中度劣化 |
SO | — | — | |||
NO | 4.436 1 | 0.443 61 | |||
42-1 | 0.502 3 |
C | 18.961 8 | 1.896 18 | 轻度劣化 |
SO | — | — | |||
NO | 0.036 7 | 0.003 67 | |||
44-1 | 0.501 9 |
C | 4.235 5 | 0.423 55 | 重度劣化 |
SO | 762.535 7 | 76.253 57 | |||
NO | 60.014 8 | 6.001 48 | |||
45-1 | 0.501 2 |
C | 1.478 2 | 0.147 82 | 中度劣化 |
SO | 264.548 5 | 26.454 85 | |||
NO | 16.225 0 | 1.622 50 | |||
45-4 | 0.501 2 |
C | 1.085 0 | 0.108 50 | 重度劣化 |
SO | 830.438 1 | 83.043 81 | |||
NO | 22.744 2 | 2.274 42 | |||
52-1 | 0.5007 |
C | 0.897 3 | 0.089 73 | 中度劣化 |
SO | 160.371 6 | 16.037 16 | |||
NO | 13.254 8 | 1.325 48 |



图11 劣化产物离子色谱图
Fig. 11 Ion chromatogram of deterioration components
XRD对劣化区域样品的结晶物质进行表征(见图

图12 41-1X射线衍射谱图
Fig. 12 XRD spectrum of 41-1

图13 42-1X射线衍射谱图
Fig. 13 XRD spectrum of 42-1

图14 44-1X射线衍射谱图
Fig. 14 XRD spectrum of 44-1

图15 45-1X射线衍射谱图
Fig. 15 XRD spectrum of 45-1

图16 45-3X射线衍射谱图
Fig. 16 XRD spectrum of 45-3

图17 45-4X射线衍射谱图
Fig. 17 XRD spectrum of 45-4
石佛寺遗址摩崖造像可溶盐主要为二水硫酸钙以及很少量的氯盐与硝酸盐,因此,二水硫酸钙是其中最具破坏作用的可溶盐。目前的研究认为,硫酸钙的来源主要有地下水中盐迁移、环境污染物沉降、硫氧化性细菌作用等几种途

图18 细菌在门水平的丰度分布图
Fig. 18 Relative abundance of bacteriophyta

图19 可溶盐来源示意图
Fig. 19 Schematic of sources of soluble salts
酸性物质对于砂岩的作用是不可逆的,长期处在酸性环境中,砂岩的钙质胶结物会流失,内部结构被破坏,岩体表面变得酥松,强度降低,最终剥离石窟本
摩崖造像本体在酸雨作用下可能存在以下2种反应方式:
1)直接反应生成CaSO4。在酸性环境中,砂岩中的化学反应会持续发生,直到反应物质消耗完为止。首先,是碳酸钙反应,当碳酸钙被酸性物质耗尽,砂岩中钙长石继续溶
, |
, |
。 |
2)直接反应生成CaSO4∙2H2O。环境中渗入的SO2在水的参与下会与CaCO3反应生成CaSO3∙1/2H2O,之后CaSO3∙1/2H2O在氧气与水的缓慢作用下最终形成CaSO4∙2H2O,或当反应条件适宜时,CaSO3在初始反应后很快达到平衡,然后,随着反应的进行石膏的量继续增加,反应可能一步发
, |
, |
。 |
或是在大气环境中,SO2首先完成氧化过程,在H2O的作用下生成H2SO4,CaCO3与H2SO4直接反应生成CaSO4∙2H2
, |
, |
。 |
通过对石佛寺劣化砂岩样品的微观分析,发现劣化的砂岩整体结构疏松、胶结物流失、孔隙度较大、颗粒与胶结物质结合变松散以及部分颗粒破碎。这些劣化现象是由水、热、酸、盐等多方面因素协同作用导致的,其中,可溶盐是主要的影响因素。学术界普遍认为,砂岩内部盐的作用会导致岩石颗粒间胶结作用破坏,增大岩石孔隙率,岩石内部结构劣化严重影响岩石的刚度与强度等力学参
潮湿露天环境下(见

图20 摩崖造像赋存环境变化
Fig.20 Impact of environmental changes on the cliff caves
盐在岩石的劣化过程中,通过增加其热膨胀系数,从而增强与温度波动有关的劣化机

图21 摩崖造像本体热量分布
Fig. 21 Heat distribution of the cliff caves
湿度变化是导致可溶盐状态变化另一重要影响因素,湿度与温度共同影响了水蒸气压力(见
相对湿度/% | 温度/℃ | ||||
---|---|---|---|---|---|
21.1 | 23 | 25.6 | 27 | 37 | |
100 | (2.50) | 2.81 | (3.28) | 3.56 | 6.28 |
95 | 5.97 | ||||
93 | 2.61 | ||||
91 | 5.71 | ||||
87 | 2.44 | ||||
84 | 2.36 | 5.24 | |||
81 | 2.28 | ||||
79 | 2.22 | ||||
75 | 2.67 | ||||
73 | 4.58 | ||||
72 | 2.02 | ||||
70 | 1.75 | [2.30] | 3.45 | ||
55 | |||||
50 | 1.41 | 1.64 | |||
42 | 2.64 | ||||
25 | 1.57 | ||||
5 | 0.31 |

图22 温湿度变化监测数据
Fig. 22 Temperature and humidity change monitoring data
从矿物组成来看,石佛寺遗址摩崖造像由石英、方解石、钠长石、钙长石、可溶盐组成。可溶盐的侵蚀作用方式主要有结晶压、水合压、热膨胀
温湿度变化会导致可溶盐发生物理及化学变化,物理变化包括溶解-结晶-再结晶过程和热膨胀过程,化学变化包括水合过程,常温下CaSO4向CaSO4∙1/2H2O或CaSO4∙2H2O转化较为容易。且由于潮湿环境下湿度波动较大,溶解-结晶-再结晶过程是循环多次发生的。虽然CaSO4∙2H2O的溶解度不高,但其结晶压和水合压较
砂岩表面温度与环境温度呈正相关,砂岩表面以内0~5 cm范围内的温度梯度最大,是热膨胀应力最集中的区
温湿度变化时,蒸发速率越大,盐分越容易被携带到表面,发生表面盐害现象;相反,盐分越容易滞留在砂岩内部,发生内部盐害现
潮湿露天环境下,不可移动石质文物通常会受到多种影响因素的协同作用,可溶盐对于石质文物的侵蚀是导致其材料表面劣化的重要原因。而由于盐类的本身特性和文物保护的相关要求制约,又无法彻底将岩石中的盐分彻底脱出,这也就造成了文物表层材料劣化难以阻止。在长期的盐劣化循环之后会使砂岩孔隙度增大、内部结构变疏松,更加容易为可溶盐的渗透与迁移创造条件。以石佛寺遗址摩崖造像为代表的潮湿露天环境下石质文物中,CaSO4∙2H2O是造成表面劣化的主要可溶盐种类。因此,抑制石质文物表面可溶盐导致的劣化,首先应在遵循文物保护原则的基础上采用修建窟檐、保护设施等方式避免酸雨的直接侵蚀;此外,除采用脱盐方法外,还应针对影响不同种类可溶盐结晶、水合、热膨胀的外部赋存环境,尽量削弱其不利影响。
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