首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 35 毫秒
1.
The crystallisation of soluble salts within the pores of the stone is widely recognised as a major mechanism causing the deterioration of the stone-built architectural heritage. Temperature, in turn, is one of the main controls on this process, including salt precipitation, the pressure of crystallisation and the thermal expansion of salts. Most laboratory experiments on decay generated by salts are just carried out with convective heating regimes, while in natural environments building stones can undergo radiative and convective heating regimes. The thermal response of stone to these different heating regimes is noticeably different and might influence the crystallisation patterns of a salt within a stone. The aim of this work is to raise awareness on the different patterns of crystallisation of NaCl within a porous stone tested with different heating regimes (convection and radiation) and the implications that this could have on the design of experimental modelling of natural weathering conditions in laboratory simulations. Results show that heating regime affects the sodium chloride distribution within a stone with high percentage of microporosity. In this case, radiation heating facilitates the generation of subefflorescences, while convection heating promotes efflorescences. This has a clear implication both on the stone decay in natural environments and on the methodologies for testing salt decay, as subefflorescences are more destructive than efflorescences. In this sense, the use of convective heating in laboratory experimentation might underestimate the potential damage that sodium chloride may generate. This counsels the use of radiation heating test methods in addition to convection for the laboratory study of salt crystallisation.  相似文献   

2.
Environmental controls on stone decay processes are rapidly changing as a result of changing climate. UKCP09 projections for the 2020s (2010–2039) indicate that over much of the UK seasonality of precipitation will increase. Summer dryness and winter wetness are both set to increase, the latter linked to projected precipitation increases in autumn and spring months. If so, this could increase the time that stone structures remain wet and possibly the depth of moisture penetration, and it appears that building stone in Northern Ireland has already responded through an increased incidence of algal ‘greening’. This paper highlights the need for understanding the effects of climate change through a series of studies of largely sandstone structures. Current and projected climatic trends are therefore considered to have aesthetic, physical and chemical implications that are not currently built into our models of sandstone decay, especially with respect to the role played by deep-seated wetness on sandstone deterioration and decay progression and the feedbacks associated with, for example surface algal growth. In particular, it is proposed that algal biofilms will aid moisture retention and further facilitate moisture and dissolved salt penetration to depth. Thus, whilst the outer surface of stone may continue to experience frequent wetting and drying associated with individual precipitation events, the latter is less likely to be complete, and the interiors of building blocks may only experience wetting/drying in response to seasonal cycling. A possible consequence of deeper salt penetration could be a delay in the onset of surface deterioration, but more rapid and effective retreat once it commences as decay mechanisms ‘tap into a reservoir of deep salt’.  相似文献   

3.
The Orval Abbey, a major monument of southern Wallonia, Belgium, was partly destroyed and rebuilt several times between the Middle Ages and the present time. The oldest parts are made of natural stones of local origin (Bajocian and Sinemurian limestones) and the most recent parts are mostly made of reconstituted stone. The process of reconstituted stone making is not known. Although confronting the same environmental conditions, the reconstituted stone is much more susceptible to weathering than the natural limestones, especially to salt crystallisation. The present study compared the mineralogical and petrophysical properties of these building materials to gather information on the making of the reconstituted stone and to understand the difference in salt susceptibility between natural and reconstituted stones. Microscopic observations and petrophysical measurements showed that the reconstituted stone is composed of debris of Sinemurian and Bajocian limestone and cement, and the salt efflorescences were thenardite. Within the cement, amorphous grains were found that may correspond to grains of clinker, which have not reacted during stone making. Although its porosity and water transfer properties were close to that of the Bajocian limestone, its pore access distribution was centred around 0.1 μm. Furthermore, the details of the pore size distribution allowed calculating salt susceptibility indices that were very high in the case of the reconstituted stone. Thus, the composition of the cement and the pore size distribution are likely the two factors explaining a high susceptibility of the reconstituted stone to salt weathering.  相似文献   

4.
Salt crystallisation is a major problem of deterioration in historic stone buildings, monuments and sculptures. The capillary rise of soil water is one of the primary sources of salts in stone structures, which evaporates leaving the salts behind. It has been noted that the spatial distribution profile of different species of salts crystallised in historic stone buildings is not homogeneous, i.e. different salts crystallise at different locations. The capillary transport and inhomogeneous spatial distribution of different salts in the porous building materials has been considered to be a result of solubility-dependent crystallisation; however, the factors responsible for this phenomenon are not clearly known. This paper aims to investigate the factors influencing the differential distribution of salts during capillary rise of soil water. In this study, the capillary transport of salts was simulated on two different sandstones—Locharbriggs, a Permo–Triassic, red sandstone and Stoke Hall, a Carboniferous, buff sandstone. The experiments were carried out under controlled environmental conditions to eliminate the possibility of evaporation-driven crystallisation of salts depending on their solubilities. The results indicate that fractionation or differential distribution of salts takes place even in the absence of evaporation and crystallisation. The sandstones exhibit properties like an ion exchange column, and ionic species present in the salt solution show differential distribution within the porous network of sandstone.  相似文献   

5.
Crystallisation of salts in the pores of stone is a major concern in the preservation of heritage buildings, monuments and sculptures, but the mechanism of transport and distribution of salts is still not properly understood. The fractionation and distribution of salts in the porous matrix has, in building material research, conventionally been attributed to the solubility and concentration of salts present in the groundwater. We propose another mechanism contributing to the control of the salt distribution based on the interaction of ions in the salt solution with the charged mineral phases within the stone. The transport of mixed salt solutions was studied in laboratory simulated flow-through experiments on two fluvial sandstones – a Permo-Triassic red bed sandstone and a Carboniferous sandstone, both from the UK. The experiments were carried out under non-evaporative conditions, eliminating the possibility of solubility-dependent crystallisation. The results indicate that the process of ion exchange significantly controls the transport of ions in the pores and leads to fractionation of solutes in the course of transport even in the absence of evaporation and crystallisation. The sandstones behave like a chromatographic column and retention of various ionic species is significantly controlled by ion exchange processes. A quantitative estimation of cation exchange capacity (CEC) indicates that sandstones with higher CEC have greater influence on retention and fractionation of salts in the course of capillary transport than those with lower CEC. Simple scoping calculations using a geochemical modelling code and the ion exchange properties based on those determined in the laboratory experiments, demonstrate that ion exchange can have a significant effect on mineral precipitation during evaporation.  相似文献   

6.
In this study, an experimental salt weathering simulation and porous stone durability classification are proposed. There are many laboratory tests that quantify durability against salt crystallisation weathering action. These are usually based on the total immersion of samples into a saline solution, which is not representative of the salt weathering mechanism. An experimental test based on partial immersion is suggested. This is a comparable study of weight loss and degradation of visual appearance due to salt crystallisation using, on the one hand, a standard durability test (UNE), and, on the other, the proposed durability test. The weight loss and visual appearance in our test is comparable to the degradation of building stone. The differences between weight loss data in both tests depend on the petrophysical properties: porous media and degree of coherence.

From this testing, a new durability classification as a function of dry weight loss in the partial immersion test is proposed. Four divisions of different types of materials can be made in this classification, which quantifies salt weathering action mainly in environments and mild climatic conditions.  相似文献   


7.
Historic sandstone structures carry an inheritance, or a ‘memory’, of past stresses that the stone has undergone since its placement in a façade. This inheritance, which conditions present day performance, may be made up of long-term exposure to a combination of low magnitude background environmental factors (for example, salt weathering, temperature and moisture cycling) and, superimposed upon these, less frequent but potentially high magnitude events or ‘exceptional’ factors (for example, lime rendering, severe frost events, fire). The impact of complex histories on the decay pathways of historic sandstone is not clearly understood, but this paper seeks to improve that understanding through the use of a laboratory ‘process combination’ study. Blocks of quartz sandstone (Peakmoor, from NW England) were divided into subsets that experienced different histories (lime rendering and removal, fire and freeze–thaw cycles in isolation and combination) that reflected the event timeline of a real medieval sandstone monument in NE Ireland, Bonamargy Friary (McCabe et al. 2006b). These subsets were then subject to salt weathering cycles using a 10% salt solution of NaCl and MgSO4 that represents the ‘every-day’ stress environment of, for example, sandstone structures in coastal, or polluted urban, location. Block response to salt weathering was monitored by collecting, drying and weighing the debris that was released as blocks were immersed in the salt solution at the beginning of each cycle. The results illustrate the complexity of the stone decay system, showing that seemingly small variations in stress history can produce divergent response to salt weathering cycles. Applied to real-world historic sandstone structures, this concept may help to explain the spatial and temporal variability of sandstone response to background environmental factors on a single façade, and encourage conservators to include the role of stress inheritance when selecting and implementing conservation strategies.  相似文献   

8.
Salt and ice crystallisation in porous sandstones   总被引：1，自引：0，他引：1  
Salt and ice crystallisation in the pore spaces causes major physical damage to natural building stones. The damaging effect of these processes can be traced back to physically induced stress inside of the rock while crystallizing. The increasing scientific research done during the past century has shown that there are numerous parameters that have an influence on the weathering resulting from these processes. However, the working mechanisms of the stress development within the rock and its material dependency are still subject to discussion. This article gives an overview of salt and ice weathering. Additionally, laboratory results of various sandstones examined are presented. Salt crystallisation tests and freeze/thaw tests were done to obtain information about how crystallisation weathering depends on material characteristics such as pore space, water transportation, and mechanical features. Simultaneous measuring of the length alternating during the salt and ice crystallisation has revealed detailed information on the development of crystal in the pore spaces as well as the development of stress. These findings can help to understand the damaging mechanisms.  相似文献   

9.
El Paular Monastery (eleventh century) is one of the most important Carthusian monasteries in Spain and is highly affected by crystallisation of Mg-sulphates, together with chlorides and nitrates. Urgent remediation of the decay process is needed to guarantee the stability of the building materials from the cloister and to make their hallways suitable for the exhibition of an important collection of seventeenth century paintings. This paper aims to characterise the building materials, salts and their interaction to suggest preservation strategies to minimise the impact of salts both in the short and the long term. These strategies include architectural solutions (such as a ventilation system to avoid increasing dampness and hence the dissolution, mobilisation and crystallisation of salts), petrophysical-based solutions (i.e. exploiting the porosity differences between building materials and poultices to maximise salt reduction) and strategies based on the physicochemical behaviour of salts and relative humidity transfer through the stone (to determine the most suitable environmental conditions to prevent crystallisation of the most harmful salt species). This research represents both a practical and experimental exercise that is useful for conservation scientists and restorers involved in the field of preservation of monuments, and for environmental control to avoid salt crystallisation.  相似文献   

10.
中国不同气候带盐风化作用的地貌特征   总被引：2，自引：0，他引：2  
盐风化作用是地球表面普遍存在的一种物理风化作用,由于盐类的周期性结晶作用而造成地表岩石和建筑材料的破坏,形成诸如风化穴或蜂窝石构造等地貌景观。盐风化作用也是差异风化的主要表现形式之一。然而,到目前为止盐风化作用在中国地学界仍然被严重忽视,以至于盐风化作用造成的地貌景观常常被地学研究者和科普人士误读为海浪冲蚀、流水侵蚀、风蚀作用等。经过近十年的野外观察与探讨,笔者等对盐风化的形成机理和表现形式有了深入的理解。本文以中国境内东部海岸带、华北半干旱区、西北干旱区和东南湿热气候带基岩露头为例,系统地分析了盐风化作用的机理及其在不同气候带的表现形式。盐风化的必要条件是:适当的可溶性盐类(如Na_2SO_4、NaCl等)供应、周期性的干湿交替和温度变化。盐风化作用主要在发育可渗性孔隙的砂砾岩类和富含微裂隙的花岗岩类之露头表面表现明显,可以形成特征显著的盐风化穴。盐风化作用形成的地貌景观在东部海岸带和西北干旱区表现尤为明显,常常形成蜂窝石构造和大型风化穴,与风蚀作用的痕迹明显有别;而在华北半干旱区和南方湿热气候带虽然受到降雨等其他因素的影响而常常遭受改造、叠加甚或清除,但在某些露头区仍然保留有重要的识别标志,形成大型风化穴以及小型蜂窝石构造。笔者等强调:地表各种地貌景观形成过程中都有盐风化作用的贡献,而建筑物和景观保护也必须考虑到盐风化作用的影响。建议地学同仁重视盐风化作用的普遍性和重要性,在相关教材中补充更新盐风化的概念,并以科普的方式通过多种媒体纠正过去的错误认识。  相似文献   

11.
The facing masonry of the Gothic Charles Bridge in Prague (Czech Republic) has been largely altered during nineteenth and twentieth century repairs, due to extensive replacements of weathered sandstone ashlar blocks. Natural stone varieties used during these replacements show different responses to a variety of weathering processes which lead to their rapid decay. The decay of the newer stone has been accelerated by use of Portland cement paste as the binding and fill material, instead of the original hydraulic lime-based and cocciopesto-type mortars. The hardened Portland cement paste makes an almost impermeable barrier, with permeability three to four orders of magnitude lower, compared to the original mortars and natural stone. The low permeability of this new alien material results in the accumulation of water-soluble salts (specifically nitrates, but also sulphates and chlorides) in the facing masonry ashlars, with their crystallisation coming in the form of both sub- and efflorescence, as well as the development of various decay forms (blistering, granular disintegration and/or scaling and flaking). The source of the water-soluble salts can be traced back to previous restoration attempts. These included chemical cleaning (black crust removal) by nitric acid, ammonium hydroxide (‘ammonium water’), or ammonium hydrogen carbonate, all which can be linked to high nitrate content in the efflorescence.  相似文献   

12.
The Early Christian Munazio Ireneo cubicle in Cagliari (Sardinia) is carved into the rock and is one of the rare monuments of Sardinia belonging to Early Christian Age. It is 166 cm under the planking level and is a semi-confined chamber in which a gate allows exchange with the outside. In 1888, when it was discovered, it was completely painted, but over the years, it suffered a serious damage and all the paintings are nearly disappeared. Now the site shows a high moisture and a differential damage characterised by delamination, powdering, salt crystallisation and biological colonisation. The research offers a multidisciplinary approach to study the salt crystallisation damage, which is a phenomenon still not completely understood today. Studies and analyses highlight that different kinds of damage can be recognised. The cement mortar and the air pollution cause crystallisation of the sulphates. Constant capillary rising and the presence of water infiltration due to the rains, which travelled into the site from the vault, were detected during qualitative inspection. Although the microclimate did not change much in the site and the humidity was almost constant during the monitoring period, the variations in temperature allowed the phase transitions of sodium sulphate, especially in summer. The greatest damage of the porous stone is associated with the phase transitions and crystallisation inside the stone of sodium sulphate, one of the most harmful salts for porous materials because of its high crystallisation pressure.  相似文献   

13.
Cavernous weathering is commonly found on sandstone slopes in different environments. Either a single dominant process or polygenetic agents require to be invoked in order to interpret the development. The Yongningshan hill of the central Loess Plateau is representative of cliff dwellings in Northwest China, which is characterized by well-developed cavernous weathering features and provides a good opportunity for the better understanding of sandstone weathering in the Loess Plateau. Multiple methods, including field survey, in-situ rock strength measurement, along with experiments on samples for microscopic observation, element composition and salt chemistry, were employed to investigate the controlling factors of cavernous weathering. The results show that cavern development is different on the four slopes with the western slope hosting massive honeycombs, tafoni and hardened surfaces. The porous and permeable aeolian sandstones are fundamental, because they provide space and pathways for the transportation of water and salt, honeycombs dominantly aligning within the lamination of cross-beds. The environmental factors such as the seasonal wetting and drying cycle, aeolian salt, moisture and water vapor are key factors for the development of cavernous weathering forms. The northern and northwestern wind-blown dust storms have brought abundant salts, the lengthier dry periods of the wetting and drying cycle being beneficial for salt accumulation within caverns, favoring salt weathering.  相似文献   

14.
A record of mortar and salt analyses from the last 20 years on various monuments in Saxony demonstrates the close relation between the occurrence of magnesium sulphate salts and the use of dolomitic lime mortars in a regional frame. The regional distribution scheme of dolomitic lime in historic mortars depends on the geological setting of the territory, i.e. the natural occurrence of dolomite deposits. Sulphation of these mortars with the formation of magnesium sulphate and gypsum is mainly due to environmental pollution, as can be shown by sulphur stable isotope analyses. Exposure of the sulphated mortar surfaces to rain leads to the preferential dissolution and advective transport of the novel formed magnesium sulphate salts, which effloresce or subfloresce on the mortar surface or can be accumulated in the structure of adjoining porous building materials like natural stones. These processes may cause severe damages in mortars by the weakening of the binder and in porous stones by salt crystallisation of phases like epsomite and hexahydrite.  相似文献   

15.
Carbonate rocks have been extensively used as building stones because of their availability, workability, and attractiveness. Unfortunately, however, some stones suffer from chemical attack or freeze-thaw activity. The environmental affects, especially those due to pollution of the atmosphere, are discussed in terms of weathering processes, salt crystallization, and acid deposition. A review of some recent research in relation to deterioration of stone in urban environments is given and carbonate rocks are classified in relation to position within a building, geographical location, and degree of pollution. Some comments on stone preservation are provided, especially in terms of the properties a preservative should possess.  相似文献   

16.
Al-hambra is an immense and valuable archaeological site in Spain built on Sabika hill with red brick and natural sandy limestone. It exhibits weathering features indicating salt weathering process. The main aim of this study is to examine weathering processes and intensity acting on Al-hambra. Rock petrography and mineralogical composition have been examined using thin sections, scanning electron microscope, X-ray diffraction and X-ray fluorescence; limits of rock’s physical parameters using ultrasonic waves and mercury porosimeter; rock salt content through hydrochemical analysis. Salts attacking this structure are mainly from wet deposition of air pollutants on the long term chemical alteration of rock’s carbonate content to its equivalent salts. The salts’ concentration limit within the examined rock samples is considerably low but it is effective on the long run through hydration of sulphate salts and/or crystallization of chloride salts. Rock texture type and its silica as well as clay content reduces its resistance to internal stresses by salts as well as wetting and drying cycles at such humid area. The recession in limits of physical parameters examined for deep seated and weathered limestone samples quantitatively reflects weathering intensity on Al-hambra.  相似文献   

17.
Viscosity contrasts displayed in flow structures of a mountain namakier (Kuh-e-Namak - Dashti), between ‘weak’ second phase bearing rock salt and ‘strong’ pure rock salt types are studied for deformation mechanisms using detailed quantitative microstructural study. While the solid inclusions rich (“dirty”) rock salts contain disaggregated siltstone and dolomite interlayers, “clean” salts reveal microscopic hematite and remnants of abundant fluid inclusions in non-recrystallized cores of porphyroclasts. Although the flow in both, the recrystallized “dirty” and “clean” salt types is accommodated by combined mechanisms of pressure-solution creep (PS), grain boundary sliding (GBS), transgranular microcracking and dislocation creep accommodated grain boundary migration (GBM), their viscosity contrasts observed in the field outcrops are explained by: 1) enhanced ductility of “dirty” salts due to increased diffusion rates along the solid inclusion-halite contacts than along halite–halite contacts, and 2) slow rates of intergranular diffusion due to dissolved iron and inhibited dislocation creep due to hematite inclusions for “clean” salt types Rheological contrasts inferred by microstructural analysis between both salt rock classes apply in general for the “dirty” salt forming Lower Hormuz and the “clean” salt forming the Upper Hormuz of the Hormuz Formation and imply strain rate gradients or decoupling along horizons of mobilized salt types of different composition and microstructure.  相似文献   

18.
The crystallisation of soluble sulphate salts is one of the most important factors of stone monument degradation. The origin of these salts is variable: marine, air pollution, building or restoration material. The lack of certainty about these sources represents a problem for restoration campaigns. The use of sulphur and oxygen isotopic tracers allows to discriminate the origins of materials and some stone deterioration patterns like black crusts (e.g. [Šrámek J., 1988. Sulfur Isotopes in the revealing corrosion mechanism of stones. 6th International Congress on Deterioration and Conservation of Stone,. Proceedings, ed. J. Ciabach. Nicholas Copernicus University, Torun, Poland, 341–345.]). First results obtained on the Bourges cathedral (France) show that the sulphur and oxygen isotopic composition of sulphates from external (atmospheric pollution) and internal (mortars, plasters and sulphates coming from stone sulphide oxidation) origins constitute well differentiated poles. The isotopic composition of sulphates implied in different stone deterioration patterns is well explained by a combination of these poles. The present study will be extended to other French monuments located in different lithological and hydroclimatic settings where contributions of sea salts and ancient chemical treatments are suspected.  相似文献   

19.
We report on the crystallization behavior and the salt weathering potential in natural rock and porous stone of single salts (NaNO3, Na2SO4) and salt mixtures in the ternary NaNO3–Na2SO4–H2O system. Geochemical modeling of the phase diagram of the ternary NaNO3–Na2SO4–H2O system was used to determine the equilibrium pathways during wetting (or deliquescence) of incongruently soluble minerals and evaporation of mixed electrolyte solutions. Experiments were carried out in order to study the phase changes during dissolution either induced by deliquescence or by the addition of liquid water. In situ Raman spectroscopy was used to study the phase transformations during wetting of pure Na2SO4 (thenardite) and of Na3NO3SO4·H2O (darapskite). In both experiments crystallization of Na2SO4·10H2O (mirabilite) from highly supersaturated solutions is demonstrated confirming the high salt weathering potential of thenardite and darapskite wetting. In order to study the damage potential of darapskite experimentally, wetting–drying experiments with porous sandstone with the two single salts (Na2SO4, NaNO3) and two NaNO3–Na2SO4 salt mixtures were carried out. Different destructive and non-destructive techniques were tested for damage monitoring. NaNO3 was found to be the least damaging salt and Na2SO4 is the most damaging one. The classification of the two salt mixtures was less obvious.  相似文献   

20.
Moisture in stone material is the key factor for all stone deterioration processes and also in weathering of cultural heritage. With additional presence of salts in the material the situation gets even more critical. While the properties of pure salts with moisture are well known, knowledge about the interaction of salt mixtures with moisture is still poor. In different approaches the reactions of salt-contaminated stone material on changing moisture were tested in the laboratory. Experiments with different solutions in the Na-Mg-SO4-NO3-H2O system revealed interesting new results on the moisture behaviour of salt-contaminated samples. Theoretical considerations and computer simulations are helpful to interpret the data obtained, but are not yet sufficient to explain the real processes acting on site at the monuments. More encouraging to this fact are complementary studies on visible efflorescences in the same salt system. It is shown how by experimental approaches the understanding on salt-induced stone deterioration is strongly complemented.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号