Abstract

The thermal effect of rocks not only depends on the temperature level but also may be influenced by the factors including heating environment, heating rate, and cooling method. In this study, approximate vacuum (V) and air circulation (A) heating condition are, respectively, applied on the limestone specimens in the whole heating process. Then, physical, mechanical, and nuclear magnetic resonance (NMR) tests were carried out to investigate the effect of heating conditions on the rock properties. The results show that heating conditions have significant effects on mechanical properties of limestone specimens (including peak strength, elasticity modulus, secant modulus, and crack initiation stress), which are due to the interference effect on the oxidation and thermal decomposition. It is worth noting that the significant temperature range of the heating condition is 450 ∼ 750°C, during which the mechanical performances of heat-treated specimens under V condition obviously outperform those under A condition. Combining the NMR results and the microstructure images from scanning electron microscope (SEM) technology, the evolution of pore distribution was revealed. As temperature increases from room temperature to 900°C, porosity increases gradually. However, pore distribution changes from small and medium pores dominating to large pore dominating and then to medium pore dominating. For limestone specimens after high-temperature treatment above 450°C, mineral crystals may melt and reconsolidate, filling in some of the previously large pores generated by thermal decomposition.

Highlights

  • In many countries, such as England, Switzerland, and China, energy application has entered the stage of cleaning, efficiency, and diversification

  • Limestone specimens were naturally cooled to room temperature in the furnace. e whole heating treatment process was accomplished under the approximate vacuum or air circulation condition. e heating methods and limestone specimens after heat treatment are shown in Figures 3(b) and 3(c)

  • Pore distribution transforms from small and medium pores dominating to large pore dominating

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Summary

Introduction

In many countries, such as England, Switzerland, and China, energy application has entered the stage of cleaning, efficiency, and diversification. As a recognized green resource, deep geothermal energy can provide a great amount of heat, and the impact of rock thermal effect on engineering is a problem that cannot be ignored [1, 2]. Ere is a growing demand for knowledge on mechanical properties of thermally treated rock to provide a basis for the performance and safety analysis of those corresponding projects [12,13,14,15]. Kim et al [16] conducted laboratory tests on three types of rock specimens to explore the effect of rapid thermal cooling on the physical and mechanical properties, and they found that rocks with stronger heterogeneity and coarser grain are more likely to exhibit crack growth. Peng et al [18] investigated the evolution characteristics of fracture by carrying out semicircular three-point bending tests on granite specimens treated with different high temperature and concluded that the temperaturesensitive ranges of granite are different under different

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