Experimental Study On Structural Damage Mechanism Of Undisturbed Loess Leached By Weak Acid

Under the guidance of the national strategy of high-quality development of the Yellow River Basin,industrialization and urbanization in the loess region have been steadily promoted,and the infiltration of weakly acidic water,such as industrial wastewater,coal mine wastewater and urban sewage,into the loess has been gradually increasing.Due to the characteristics of multi-porosity,weak cementation,strong water sensitivity and weak alkaline sedimentary environment,the loess is prone to geological disasters caused by weakly acidic water infiltration.It is of great theoretical significance for the improvement of disaster prevention and reduction in the loess area to study the degradation mechanism of engineering properties of the weakly acid dissolved loess.Taking undisturbed loess as the research object,this paper carried out weak acid leaching experiments.We focused on the cation migration of soluble salts and the changes of mineral composition,and deeply analyzed the micro-meso-structural damage evolution process and mechanism of undisturbed loess under the action of weak acid dissolution.The main research results include:(1)After leaching,the changes of calcite,dolomite and albite are obvious.However,under different leaching conditions,the relative changes of the above minerals are different.Under higher pH leaching conditions,the relationship is as follows:albite≈calcite<dolomite,and the dissolution reaction of dolomite is easier to generate Mg ions;When the pH is low,albite<chlorite<dolomite<calcite,but the dissolution reaction of chlorite releasing Si ions is faster and more intense than the release of Fe,Al,K ions.By means of SEM images and EDS function,we can observe that layer-by-layer denudation of layered calcite and albite leads to gradually clear outline of particles in each layer,and even small particles gather in the surface layer.Layer-by-layer erosion of quartz and muscovite results in gradient descent between layers and contour grinding.(2)The contact of the particles before and after leaching were analyzed by SEM.It was found that the direct contact between particles were mainly surface-surface contact and line-surface contact.The changes of cementation bonding are as follows:①fine particle aggregates disappear;②The flocculated cementation structures appear alternately with the increase of leaching time.The roundness and fractal dimension of the particles before and after leaching were obtained by image analysis software Image Pro Plus(IPP).The fractal dimension did not change much,while the roundness increased with the lengthen of leaching time or the decrease of leaching solution pH,and this phenomenon was more significant when the leaching solution pH decreased.(3)According to the changes of quantitative parameters of pore structure and frequency ratio in the interval of pore gray ratio before and after leaching at each layer of the sample,there are two evolution trends in the development of different depths pores:①lateral expansion or even coalescing or initiation;②lateral closure,longitudinal plugging;With the prolongation of leaching time,the above two evolution trends appear alternately when leaching with distilled water and weak acid at pH=5.When leaching with weak acid at pH=3,the deeper pores in each layer of the sample mostly develop in the first evolution trend from beginning to end,while the shallower pores still show the above two evolution trends alternately.(4)With the help of industrial CT and image processing software(AVIZO),the different pore size ratios of Φ100×150mm soil body were calculated,and the boundaries of macropore,mesopore,fine pore,and micropore were determined as>330μm,200～330μm,120～200μm and<120μm in the cumulative ratio curve based on the self-similarity hypothesis.The distribution of smaller micropores(<10μm)was also obtained by using NMR and liquid nitrogen adsorption tests.To analyze its growth over time,we found that there is a reciprocating and unidirectional development phenomenon between different pores.In the leaching process of distilled water and weak acid with pH=5,the larger pores develop to smaller pores first,while in the leaching process of weak acid with pH=3,the larger pores develop to smaller pores first,and in the micropore interval,the relatively larger micropores first moved towards smaller pores and then continue to develop in a reciprocally manner.(5)The damage characteristics of the pore structure of the undisturbed loess leached by weak acid are as follows:brittle damage caused by the expansion of initial pores and holes and toughness damage caused by the initiation of shallow pores.During the leaching of distilled water,the pore expansion is caused by the loss of aggregate cementation and the dislocation between grains.In addition to the above internal mechanism,the expansion space for pores after lattice sliding and rupture of calcite and albite,and the shedding of cementing coating between skeleton particles should also be considered in the leaching process of weak acid.