Salt Weathering Mechanism Of Ancient Earthen Material In Arid Area Of Northwest China

The earthen heritage sites?EHSs?are one of historical remains of human cultural activities and carry plenty of historical information and cultural connotations,which are also the crystallization of history development and the precipitation of wisdom during the human history.The EHSs not only have profound scientific and cultural background,but also play important roles in the construction of socialist political,material and spiritual civilizations,in the promotion of social economy and culture development.Due to the arid climate conditions in the northwest of China,a large number of EHSs have been preserved to date,which provides us an opportunity to understand the ancient earthen archaeological sites in detail.Meanwhile,after the successful application for World Cultural Heritage of the Silk Road and the national initiative of “the Belt and Road”,the protection of those EHSs in the northwest of China has received extensive attention.Most of EHSs are immovable,and many types of deteriorations have been developed by natural forces during last thousand years.Based on the comprehensive investigations into the evolution processes of soil degradation and site structure instability,it is wildly considered that salt weathering is the key factor of deterioration development and is also the urgent problem for EHS protection.It is also recognized that salt weathering processes for EHSs involve multidisciplinary knowledge including analytical chemistry,material science,geotechnical engineering and so on.Meanwhile,many kinds of salts can exist within the EHSs and form complex analysis system.These limitations make the research topics relatively scattered and independent,and the systematic studies for EHSs from physical chemistry standpoint are still sorely absent.Since the serious lack of fundamental studies for EHSs greatly restricts the protection of these precious cultural relics.Therefore,it is necessary to systematically study the mechanisms of EHSs weathering,especially due to salt weathering and the related factors,and then it is expected that more scientific protection for EHSs can be achieved through taking more effective mitigation and prevention countermeasures.Considering the research topics of salt weathering and the salt contents of EHSs in the arid region of northwestern China,in the present study,the mixture salts of Na₂SO₄-NaCl were used to further understand the salt weathering mechanisms based on the salt characteristics and their response to the external environment.The equilibrium conditions of Na₂SO₄-NaCl-H₂ O system were calculated by utilizing thermodynamic theory.The crystallization-dissolution characteristics and crystallization failure ability of Na₂SO₄-NaCl solutions were gained by means of chemical analysis,which provides the theoretical basis for interpreting the soil-salt weathering processes.The significant effects of crystal locations on the deterioration modes were discussed by analyzing the water-salt mitigation characteristics of the mixture solutions.Based on the macroscopic and microscopic observation results,the influence of temperature on the salt weathering of Na₂SO₄-NaCl mixtures for EHSs was discussed as well.Considering the environmental changes of these EHSs in the arid regions of northwestern China,a series of tests under wet-dry cycles were carried out to simulate the moisture absorption and desorption processes by using different water/vapor sources?gaseous and liquid water?.At last,the weathering differences for salty EHSs under the water-heat-salt interactions were compared and analyzed.According to the present study,the research contents and findings are mainly summarized as follows.?1?Accurate prediction of the solubility of Na₂SO₄-NaCl-H₂ O ternary system was achieved based on extended Pitzer model.Prediction result shows that Na₂SO₄·10H₂O only exists in the ternary system at temperature lower than 32.4?.Meanwhile,transition temperature of Na₂SO₄·10H₂O-Na₂SO₄?V?decreases and Na₂SO₄·10H₂O becomes unstable with the increase of NaCl content.Stable phase of Na₂SO₄-NaCl mixed solutions at any temperature gradually changes from Na₂SO₄·10H₂O to Na₂SO₄?V?with increasing of NaCl content in the temperature range 17.6<T<32.4?.?2?Results of crystallization experiment under varying concentration ratios and solubility experiment in mixed solutions show that NaCl inclusion has no influence on the crystallization pathway of Na₂SO₄ during evaporation process and Na₂SO₄?V?is the only stable Na₂SO₄ crystal in the whole crystallization process.In the dissolution process,suspected Na₂SO₄·10H₂O crystal was formed in some mixed-salt solutions,but didn't exhibit large volume changes.In the condition of Na₂SO₄ is regarded as the first precipitated crystal in the mixed solutions,it exhibits low supersaturation and NaCl inclusion has no significant influence on supersaturation of solution when crystallization of Na₂SO₄?III?from the capillary evaporation experiment.On the contrary,in the condition of NaCl is regarded as the first precipitated crystal in the mixed solutions,it shows high supersaturation and Na₂SO₄ content has great influence on supersaturation of solution?3?The decay of specimens with different dry density and initial water content were analyzed by evaporation and crystallization of the specimens which were saturated highly by solutions with different concentrations at room temperature.The test results showed that a great deal of Na₂SO₄·10H₂O crystal precipitate on the surface of specimens prepared by pure Na₂SO₄ solution,which was a representative phenomenon of efflorescence.However,the decay changed to the combined patterns of efflorescence and subflorescence if NaCl was added into the Na₂SO₄ solution.When the mole fraction of NaCl was more than 0.8,only the crystalline shell could be found on the surface of specimens,and no weathering peeling occurred.The corresponding decay belonged to subflorescence.Additionally,for the soil with mass loss,there was a general trend that the mass loss ratio decreased gradually with the increase of dry density and initial water content,and increased with the increase of NaCl solution concentration.?4?In order to study the mechanism of salt weathering under different conditions of temperature,some saline soil samples with varieties of drying temperatures were tested by drilling resistance test?DRMS?.And also the method of inner salt transportation,micro-structure and the distribution of pores were studied as well.?1?The results showed that the variation of the mechanical strength was mainly distributed within 1mm from the surface of samples.The values of drilling resistance of saline soil samples were influenced by the combination function of salt contents and drying temperature.Specifically,the samples with low salt content had a little improvement in surface strength,expect for the samples with Na₂SO₄·10H₂O appeared on the out layer,which had a decrease in surface strength.However,the surface strength of samples with high salt contents were increased.The samples treated by 105? drying temperature had a lower strength,compared with samples treated by other drying temperatures.And also,the law of strength had a little difference when the drying temperature changed.?2?The ions had a trend to enrich within the 1mm of out layer,and the total amount of ions decreased with the increase of drying temperature to a certain extent.For the first 1mm out layer of samples,the content of NaCl was lower than the initial value,and then became larger than initial value when the total NaCl content increased.?3?The crystal layer was cemented and covered on the surface of the soil.The higher the drying temperature was,the less concentration of the surface crystal was.Among them,the maximum concentration of salt in the sample surface was found under the drying temperature of 40?.On the other hand,the drying temperature had a little influence on the inner part of samples.The porosity of samples was reduced with the aggregate clusters increasing due to the existence of salts.?4?The variation of the pores were mainly focused on large pores?about 5?m in diameter?and middle pores?about 3?m in diameter?.When the drying temperatures were relatively high,such as 60? and 105?,the salts always deposited in large pores.However,when the drying temperatures were relatively low,the larger part of salts were deposited in middle pores,and also had a small part existed in large pores.The number and diameter of the pores were increased when the drying temperature increased.And also,the smaller the salt content was,the more obvious the effect of drying temperature on the pore size was.?5?Take saline earthen site soil as the object in the salt weathering experiment of site soil under water-heat-salt interaction.The salt weathering process of earthen site which have surface salt enrichment in water was simulated by repeatedly add water-dry at different temperature conditions.At the same time,the dry and wet cycle?water vapor?experiment at room temperature was designed for comparative analysis.The results showed that:?1?Using liquid water for different temperature control conditions of dry and wet cycle,at 5 ? and room temperature,Na₂SO₄?V?crystals dissolve in water to form supersaturated solution,then the formation of hydrate is induced to produce the crystallization pressure.So that the surface of the specimen in which hydrate?Na₂SO₄·10H₂O?Na₂SO₄·7H₂O?can be produced in the process of dissolution developed cracks.And the degree of cracking increases with the number of cycles.At 5?,the degree of supersaturation of the solution is larger,the damage degree of the whole specimen is more serious than that at room temperature.For different dry density saline soil the smaller the dry density,the more serious the degree of damage.And under the same temperature control conditions,smaller dry density samples require fewer cycles to be experienced.There was no obvious damage to all the samples in the dry wet cycle at 40?,and the color of the surface of sample which subjected multiple wet and dry cycles is irregular deepening because of the uneven deposite of salt.?2?The samples of dry and wet cycling were carried out at room temperature.In addition to the obvious migration and enrichment of salt,there is no obvious surface damage.