高温后灰岩单轴压缩的分形特征研究

Fractal Characteristics of Limestone after High Temperature under Uniaxial Compression

摘要:

温度是影响岩石物理力学性质的重要因素,对不同温度作用后灰岩单轴压缩的碎块进行统计分析,结果表明灰岩的块度分布是个分形,分形维数D是反映高温后灰岩破碎程度恰当的特征统计量,同时D表现出随温度的增大而减小的性质。在此基础上,通过扫描电镜分析,获得温度对岩石力学性质的影响主要与组成岩石矿物性质和内部微观结构有关,而不同温度的作用会影响岩石矿物组成成分和岩石的晶格结构,在灰岩的扫描电镜结果对比中已发现微观形貌特征的差异,这可从内在机制方面解释不同温度下灰岩分维值变化特征。

Abstract:

Temperature is an important factor that influences the physical and mechanical properties of rocks. Many scholars have studied various types of rock crushing, but few have examined the characteristics of crushed limestone previously exposed to different temperatures. This study involved a heating limestone specimen, 50 mm in diameter and 100 mm in height, in a high temperature furnace heating cylinder before performing a uniaxial crushing experiment. Various phenomena were observed including axial splitting and bursting strength at low temperature. However, the strength of fracturing and splitting at high temperature decreased significantly as cleavage was simultaneously reduced. Sample pieces were completely reassembled and similarly sized fragments grouped together. Fractal mathematical theory applied to the analysis of rock fragments indicated that limestone block distribution is a fractal. Therefore, the fractal dimension D was considered an appropriate statistic to represent the characteristics of limestone crushed after exposure to high temperatures. The fractal dimension D has a negative correlation with temperature where it decreases with increasing temperature. The experimental data was represented in the rectangular coordinate system by plotting temperature on the X axis and fractal dimension on the Y axis in a scatter plot. Linear regression used for curve fitting the data resulted in y=0.001 9+2.506 2x and a very large correlation coefficient. The observed influence of temperature on limestone mechanical properties mainly involved mineral physical properties and changes in microstructure. Mineral composition and crystal lattice structure may change when a rock is exposed to different temperatures. Fragment comparison with scanning electron microscope (SEM) revealed that limestone subjected to high temperature had different microstructural characteristics than untreated limestone. Room temperature limestone had a smooth surface. Limestone subjected to high temperatures had an uneven surface and partially broken cement.The rock surface appeared to contain long, wide fissures across grains, rough mineral grain surfaces, and increased detrital material. These characteristics reduced the mechanical properties and caused the change in fractal dimension. This internal mechanism in the limestone explains why fractal dimension decreases with increased temperature. In addition, limestone samples were gray-black at room temperature and light gray when heated to high temperatures. High-temperature exposure caused calcium carbonate formation and magnesium carbonate oxidation that led to the observed color change.