Study On Thermal Damage Characteristics Of Limestone Under High Temperature

Based on the underground engineering(geothermal mining,underground coal gasification,etc.)which involves the thermal damage of high temperature rock,a series of physical and mechanical properties tests are carried out with limestone as the research object.The effects of different temperatures and thermal shock cycles on the physical and mechanical properties of limestone were investigated.The physical and mechanical parameters of limestone tested include: surface chroma L~*a~*b~* value,fracture development,P-wave velocity,thermal conductivity,porosity,tensile strength,etc.Through the analysis of the above physical and mechanical parameters,the damage mechanism of limestone caused by temperature and thermal shock times is further revealed,which provides theoretical support for energy exploitation,development and utilization of underground space and tunnel fire.The main conclusions are as follows:(1)Surface characteristics of limestone after high temperature treatmentThrough testing and observing the surface chroma of rock samples,it is found that the chroma scalar L~*a~*b~* value and fracture development are greatly affected by temperature and thermal shock times.Especially after 300?,a~* and L~* fluctuate obviously,and cracks increase with the increase of temperature and thermal shock times.The b~* value increases slowly with the increase of temperature,and the surface color becomes yellowish brown gradually.(2)Variation characteristics of longitudinal wave velocity,thermal conductivity and tensile strength of limestone after high temperature treatmentAfter high temperature treatment,the wave velocity,thermal conductivity and tensile strength of limestone have significant changes,but there are differences.The thermal conductivityand longitudinal wave velocity decrease with the increase of temperature and number of thermal shocks,especially after 300?.The thermal conductivity and wave velocity decrease rapidly when the thermal shock occurs once.The tensile strength fluctuates with the increase of temperature and number of thermal shocks,but decreases as a whole.(3)Pore and fracture evolution characteristics of limestone after high temperature treatmentThe porosity and pore development of limestone after heat treatment were tested by mercury injection test.It is found that the pore size of limestone tends to be complicated with the increase of temperature by using the curves of phase mercuryintake and pore size,cumulative mercury intake and pore pressure,cumulative mercury intake and pore pressure,porosity and temperature,fractal dimension and temperature.When the limestone is below 200?,there are mainly macropore and super-macropore,while when the limestone is above 200?,the small and medium pore increase greatly,and the macropore and super-macropore expand further.In addition,there is a strong negative correlation between wave velocity and porosity,and the wave velocity decreases significantly with the increase of porosity.(4)Variation characteristics of limestone permeability under high temperatureBased on the porosity and maximum and minimum pore size data of limestone at different temperatures,the fractal permeability model was established by combining fractal theory.The permeability rate was calculated by the permeability model,and it was found that the rate of change of permeability has a strong positive correlation with temperature,pore fractal dimension and temperature.The change of fractal dimension can be estimated to reflect the change of reflection,pore and permeability.The abrupt temperature of porosity and permeability is around 300-400?.