Experimental Study on the Structural Features and Corrosion Characteristics of Dolomite and Limestone in the Karst Stone Mountainous Areas in Northern Guangxi

: Taking dolomite and limestone in Guilin and Liuzhou regions in the north of Guangxi Province as research objects, this paper analyzed their mineral composition and chemical composition, and then carried out the chemical corrosion test, the corrosion test under the chemical-temperature actions, and the corrosion test under the action of vibration load, respectively. The results showed that: (i) the dolomite in northern Guangxi mainly has fine crystalline texture and massive structure with low content of acid insoluble matters, while limestone mainly has powder crystalline texture and massive structure with high content of acid insoluble matters, and the purity of both dolomite and limestone are very high; (ii) the difference of corrosion between dolomite and limestone mainly depends on the ratio of CaO/MgO in their chemical composition, and the content of silica and acid insoluble matters; (iii) the corrosion rates of the pure dolomite and the pure limestone are basically the same under the same external conditions; (iv) temperature and vibration load have relatively large influence on the corrosion rates of dolomite and limestone, and the corrosion rates of dolomite and limestone increase with the increase of temperature, but the influence of vibration load on the corrosion rate is more significant than temperature. This research can provide theoretical basis and technical support for large-scale engineering construction and prevention of karst geologic disasters in karst stone mountainous areas in the northern


Introduction
In carbonate rock areas, due to the differences in mineral composition and chemical composition of rock mass, the differences in internal structure and pore characteristics, and the influence of external conditions such as groundwater occurrence, temperature and vibration load, the dolomite and limestone distributed in these areas have differential corrosions. The differential corrosions often destroy the integrity of the rock mass, affect the mechanical properties and stability of the rock mass, and have a great impact on the safety of buildings and constructs attached to it. Therefore, it is of great engineering practical significance to profoundly study the structural characteristics and differential corrosions of the limestone and dolomite.
The research on karst geologic disasters is mainly divided into two categories: theoretical research on corrosion mechanism and research on karst detection methods.
The corrosion test of carbonate rocks is an important content of the theoretical research. Scholars in the past have done a lot of experiments on the corrosion characteristics of carbonate rocks, and have achieved many results. Liu et al. (2018) used the two-scale continuous medium model to simulate the dissolution reaction process and predict the optimal injection rate during acidification treatment of carbonate rocks, and put forward a general numerical method to simulate structured and unstructured reaction flow problems. Zhao et al.(2018Zhao et al.( , 2017 proposed a new alternative method for theoretically dealing with the evolution process of chemical dissolution surface in water-saturated porous rocks when propagating in a chemical system. In this method, porosity, pore fluid velocity and acid concentration were taken as independent variables, so that the problem of water-rock interaction could be solved by numerical methods and algorithms. Liu et al. (2013)  (2012) revealed that the rock microstructure plays a controlling role in the corrosion rate, and the corrosion rates of dolomites with different structures are as follows: mesocrystalline to fine crystalline dolomites > fine crystalline to powder crystalline dolomites > powder crystalline to micritic dolomites; the specific surface area is related to the total corrosion amount, but independent of the corrosion rate per unit surface area. The corrosion starts from intergranular and intercrystalline pores, structural microfractures and cleavage, and gradually expanded to make pores and fissures connected with each other. The above researches on the corrosion investigated the corrosion characteristics of limestones and dolomites from the aspect of water-rock interaction mechanism with the consideration of the influence of lithology, the characteristics of acidic solution and microorganisms, etc., which provides a theoretical basis and practical reference for the research of this project. On the basis of the above research results, taking the action factors of temperature and vibration load into account can further confirm and improve the above research conclusions and would be more in line with the actual situation, so it has certain feasibility and important significance.
Temperature is one of the important factors affecting the mechanical properties of rocks. Rock is composed of solid mineral particles and tiny gaps between the particles. The solid mineral particles and the channel networks formed by gaps between the mineral particles are often filled with fluid media. When changes in the environment of such porous media of rocks occur, the water presenting in the pores and fissures inside the rocks will undergo phase change, leading to changes in the physical and mechanical properties of the rocks. These changes are not only related to their physical structure, but also affected by the existing water, temperature and stress  Scientific problems such as differential corrosion characteristics of rocks and identification of influencing factors in dolomite and limestone development areas have not been completely solved. Therefore, it is of great theoretical and practical significance to systematically carry out tests to determine the structural characteristics and differential corrosions of limestone and dolomite and explore the influencing mechanism of differences between dolomite and limestone on karst geologic disasters.

Collection and analysis of the test samples
Several limestone and dolomite samples were collected from large-scale construction sites such as highways, main roads and urban railway transits in Guilin and Liuzhou regions in the north of Guangxi. After returning to the laboratory, the representative samples were selected and analyzed for the minerals and structural characteristics of the rocks with an optical microscope, so as to roughly obtain the mineral composition, structural characteristics and name of the samples. The specific test results are shown in Table 1.

Description of characteristics of the test samples
In the process of mineral and rock identification, it is difficult to accurately judge the mineral and rock only by naked eyes and simple tools for some fine-grained, micro-grained, aphanitic or vitreous rocks; so it is necessary to triturate the 0.03 mm thick rock or mineral slices and identify with the aid of an optical microscope. The transmission polarizing microscope is used for identification of transparent minerals, while the reflection polarizing microscope is used for identification of opaque minerals. The test samples shown in Table 1 are photographed with an optical microscope and further described in details below. color, and is evenly distributed in the rock. The dolomite is irregularly granular, with a particle size of 0.05-0.10 mm.

Chemical composition analysis of the test samples
As shown in MgO, CO 2 , SiO 2 , acid insoluble matter, and loss on ignition by using high-frequency infrared sulfur-carbon analyzer and X-ray fluorescence spectrometer, respectively.
The specific test results are shown in Table 2. Then the ratio of CaO/MgO was calculated to judge the purity of dolomite and limestone (Zeng,1986;Zhu, 2008).
Among them, dolomite and limestone with high purity were selected, and then the subsequent corrosion tests were carried out.

Corrosion Tests
The corrosion (corrodibility) of water is an internal factor of water-rock interaction, and the mobilization property and chemical characteristics of water will (iv) calculating the corrosion amount of the rock samples. In order to eliminate the influence of surface area differences of the rock samples, the results of this experiment are expressed by the corrosion amount per unit surface area. A shown in Fig.1, the corrosion rates of dolomite and limestone are basically the same under static conditions, and the corrosion amount per unit surface area for dolomite is 0.341 mg (average value), which is slightly larger than that of limestone (0.328 mg). And there is little difference between them for a long time.

Comparison of dolomite test results under different conditions.
Temperature  Table 4), and the corrosion tests were carried out at different temperatures (0°C and 40°C ) and vibration conditions. The corrosion results of the selected samples under different conditions are shown in Table 4 and Fig.2. As shown in Fig.2, the corrosion rate of dolomite is the lowest at 0°C, and is increased at 40°C, but the difference is not significant. However, the corrosion rate and amount of dolomite are increased significantly under the vibration condition, and the corrosion rate under the vibration condition is about 1.75 times of the corrosion rate at 0°C. The results indicate that vibration load has a more significant effect on the corrosion rate of dolomite than temperature.

Analysis and discussion of the test results
The corrosion of erosive aqueous solution is an internal factor of water-rock interaction, and the mobilization property and chemical characteristics of water affect the characteristics of the action of erosive aqueous solution on dolomite and limestone.
With the progress of water-rock interaction, the change of internal structure and pore characteristics of rock would inevitably lead to the unceasing change of groundwater seepage field. At the same time, the hydrochemical property in the rock mass also changes correspondingly, which is mainly reflected in the coupling process of chemical field, temperature field and stress field of water-rock interaction.
The dolomite (from Liuzhou area) selected in this study has a CaO/MgO ratio of less than 2.2, an acid insoluble content between 0.31%-0.68%, and a SiO 2 content between 0.24%-1.32%, so it belongs to a pure dolomite. The selected limestone (from Guilin area) has a CaO/MgO ratio of greater than 10, and the contents of acid insoluble matter and SiO 2 are generally higher, and the purity is slightly lower. In the pure carbonate rocks, the corrosion rate of calcite is generally higher than that of  The corrosion rate of both dolomite and limestone increases with the increase of the experimental temperature, and the increasing amplitude for limestone is larger than that for dolomite. This is possibly because the dolomite has more pores on the surface, and mostly has stratification structures, and is easily to form corrosion films and pores on the surface, while the limestone has fine mineral particles (microcrystalline to powder crystalline), has compact structures, and is more conducive to the surface corrosion with the increase of temperature.
Corrosion includes chemical dissolution and mechanical damage, and vibration load is one of causes for mechanical damage. With the rapid development of economy and society and the adjustment of national strategy, more and more high-speed railways and highways need to pass through the karst areas in northern Guangxi. In the process of construction and operation, the influence of vibration loads such as vehicles (especially high-speed trains and urban railway transits) is more conducive to increasing the corrosion rate of dolomite and limestone, especially obvious in the districts with better hydrodynamic conditions. Vibration loads will aggravate the degree of chemical corrosion of water on the rocks, and the coupling of mechanical destruction and chemical corrosion will lead to the generation of secondary minerals and secondary pores in the rocks, which will accelerate the rate of corrosion reaction.
The corrosion rate of dolomite in northern Guangxi is increased significantly under the vibration load conditions, which is about 1.75 times of that at 0°C. The corrosion rate of limestone is also increased significantly, which is about 1.88 times of that at 0°C. Under the action of vibration load, physical disintegration is dominant in the corrosion process of dolomite, and the rock fragments provided by physical disintegration are more conducive to chemical corrosion. In addition, the uniform intercrystalline pores in dolomite are beneficial to the overall corrosion. However, the distribution of fissuring gaps is extremely uneven when the limestone is stressed, and thus the limestone is easy to form rock cracks and cave systems, which is characterized by obvious differential corrosion.

Conclusions
(1) The dolomite in northern Guangxi mainly has a fine crystalline texture and massive structure, and the ratio of CaO/MgO is generally less than 2.2. The dolomite has high purity. The limestone mainly has a powder crystalline texture and massive structure. The ratio of CaO/MgO is generally greater than 10, and the highest ratio can reach about 140. The limestone also has high purity. The content of acid insoluble matter is low in dolomite, but is high in limestone due to the high content of carbonaceous matter. at 0 °C and are increased with the increase of temperature, but the influence of vibration load on corrosion rate is more significant than temperature.