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Lanzhou University (2017)

Salt Weathering Mechanism of Ancient Earthen Material in Arid Area of Northwest China


Titre : Salt Weathering Mechanism of Ancient Earthen Material in Arid Area of Northwest China

Auteur : 沈云霞

Grade : Doctoral Dissertation 2017

Université : Lanzhou University

Résumé partiel
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

Mots clés : Earthen heritage sites; Salt weathering; Na2SO4-NaCl-H2O system; Phase diagram; Crystallization pressure; Temperature and Relative humidity

Présentation (CNKI)

Page publiée le 20 septembre 2017