Experimental investigation on the macro- and micromechanical properties of water-cooled granite at different high temperatures

Abstract

To investigate the damage mechanisms in granite’s physical and mechanical properties after high-temperature water quenching, this study employed MTS815.04 for uniaxial compression tests on thermally treated specimens, with concurrent acoustic emission monitoring, and utilized nanoindentation for micromechanical analysis. The results show that with increasing temperature, granite's peak strength and elastic modulus decrease, with a sharp decline after 400–500 °C, corresponding to a significant increase in the internal damage, which can be detected by acoustic emission monitoring. Below 500 °C, macroscopic mechanical degradation is due to mineral thermophysical property differences, while above 500 °C, microcrack development is the main deterioration factor. The failure mode shifts from tensile to tensile-shear complex to shear failure, with transition points at 400 °C and 800 °C. The results of this study are of certain reference value for improving the efficiency of extracting thermal energy from dry-hot rocks and providing security guidance for the tunnel restoration process following fire damage.

Introduction

Rapid water quenching of high-temperature rocks induces thermal “compression-tension” stress, leading to increased damage and macroscopic cracks, which diminish the rock's mechanical properties¹. The deterioration of rock properties directly impacts the stability of post-disaster underground structures and exploitation of geothermal energy. Therefore, it is of great significance to study the influence of high temperature water cooling on rock properties and fracture morphology in underground engineering and geothermal resource engineering.

Thermal treatment alters rocks’ macroscopic mechanical properties, differing from those at ambient temperatures. This discrepancy is associated with high-temperature-induced changes in mineral composition and structural damage. Increased temperature results in physical alterations of rock-forming minerals, including thermal expansion, dehydration, fusion, and phase transitions². Rocks containing carbonates or reducing minerals may undergo chemical changes such as decomposition or oxidation³. Extensive experimental research and theoretical analysis indicate that: Under high-temperature conditions, the strength properties of rocks deteriorate. The variation in strength characteristics is influenced not only by temperature but also by the methods of heating, cooling, as well as the rates of temperature increase and decrease^{4,5,6,7,8,9,10,11,12}. The weakened strength of thermally treated rocks is chiefly attributed to the enhanced internal crack density, which is referred to as thermal damage. For most rocks, parameters like density, P-wave velocity, thermal diffusivity, uniaxial compressive strength, tensile strength, fracture toughness, and Young's modulus decrease with temperature. Conversely, porosity, permeability, and peak strain increase with temperature elevation^{7,8,13,14,15,16,17}. High temperatures also alter the macroscopic fracture patterns of rocks. Numerous studies have found that as temperature increases, rocks transition from brittle to ductile failure^18,19,20,21. The research findings generally exhibit similar trends. However, due to variations in rock-forming minerals and diagenetic environments, there are discrepancies in the range of numerical variations. Additionally, the temperature thresholds corresponding to abrupt changes in evolutionary trends still exhibit subtle differences. Existing research conclusions are typically applicable to only certain specific cases. To effectively resolve related engineering challenges, a more profound investigation into the underlying mechanisms is necessary.

As an aggregate of mineral particles, the distribution and micro-mechanical properties of mineral components collectively influence the macroscopic mechanical behavior of rocks. Maruvanchery et al²². analyzed nanoindentation and microscopic mineral imaging data. They found that the meso-elastic modulus and hardness of minerals and grain boundaries were in accordance with the decrease in macroscopic properties of sandstone. Sha et al^23,24. used thin section analysis and SEM to study the damage evolution and mechanism of high-temperature granite post-cooling shock. Both methods confirmed that microcracks were the primary cause of the rock’s mechanical degradation. Peng et al²⁵. studied the thermal cycling impacts on marble cracks using optical microscopy and established a correlation between mechanical properties and crack density. Mo et al²⁶. discovered that thermally induced microcracks alter granite’s macro-mechanical properties due to mineral micro-mechanical changes during heating and cooling. It can be seen that studying rock microstructures and mineral micro-mechanical properties is crucial for understanding rock macro-mechanical degradation.

Summarily, significant findings exist on the macro-mechanical effects of high-temperature rapid water cooling on rocks, yet few studies merge macro and micro viewpoints on this topic. The typical temperature range of geothermal resources (100-300 °C) and the potential for unexpected events like tunnel fires (up to 900 °C)^27,28. The relationship between granite’s physical properties and mechanical performance at 900 °C is not fully researched. This study uses MTS815.04 for macro-mechanical tests and acoustic emission to trace crack damage in high-temperature quenched granite (25–900 °C). Nanoindentation testing is employed to investigate the mesoscopic mechanisms of macro-mechanical property damage in granite after high-temperature quenching.

Experimental procedures

Sample preparation

The granite samples utilized in this study were sourced from Yueyang, Hunan Province, China. Characterized by a predominantly off-white appearance, the samples are composed mainly of plagioclase, quartz, and biotite. To minimize variability in the test results, all samples were extracted from a single intact rock specimen using a core drilling method. According to ISRM (International Society for Rock Mechanics)²⁹, granite was prepared into cylindrical standard specimens with a diameter of 50 mm and a height of 100 mm for macro-mechanical testing, as shown in Fig. 1a.

Figure 1

Samples of uniaxial compression and nanoindentation: (a) uniaxial compression samples and (b) nanoindentation samples.

Heating and cooling treatment

The specimen was placed in a KSL-1200X box-type high-temperature sintering furnace for heating treatment, with a heating rate set at 5 °C/min. The selected temperature range was 25–900 °C. After reaching the designated temperature, the specimens were held at a constant temperature for 2 h to ensure uniform heating. Subsequently, the specimens were swiftly quenched in cold water upon removal from the furnace, as illustrated in Fig. 2. Processed rock samples were subjected to uniaxial compression tests. Representative specimens were selected based on their deformation and failure characteristics for nanoindentation testing. Figure 1b illustrates the relevant sample was created using an abrasive cutter and formed into a cylinder with a diameter of 25 mm and a height of 10 mm. The mineral composition and content of water-cooled granite under different high temperatures were analyzed through X-ray diffraction (XRD) tests, as shown in Table 1.

Figure 2

Heating equipment and schematic diagram of granite heating and cooling path: (a) KSL-1200X box-type high-temperature sintering furnace and (b) schematic diagram of the granite heating and cooling path.

Table 1 Mineral composition of water-cooled granite under different high temperatures.

Uniaxial compression test

Uniaxial compression tests were performed using the MTS815.04 rock mechanics testing system, as depicted in Fig. 3. Axial displacement control was applied with a loading rate of 0.001 mm/s. Simultaneously, the PCI-2 multi-channel acoustic emission (AE) system was employed to capture AE signals during the loading process. To minimize environmental noise interference, the gain of the preamplifier was set to 40 dB³⁰, with a threshold of 45 dB, and a sampling frequency of 1 MHz.

Figure 3

Test loading system and sensor layout: (a) test loading system and (b) sensor layout.

Nanoindentation test

Experimental device and test procedure

The nanomechanical testing was conducted using the Hysitron Ti Premier nanoindenter, as illustrated in Fig. 4. The principle uses high-resolution actuators and sensors to measure the indentation and withdrawal on the sample surface, yielding high-resolution load–displacement curves. By analysis and computation, the nanoscale mechanical properties such as hardness and elastic modulus of the sample can be obtained³¹.

Figure 4

Bruker nanoindentation instrument.

Cross-sectional imagery via 3D profilometry is shown in Fig. 5a, with quartz (white to gray), plagioclase (gray), and biotite (dark, lustrous) demarcated. Figure 5b uses color-coding for mineral distinction in the rock. The nanoindentation microscope was used to observe and identify mineral components, with tests performed in smooth areas in a 3 × 3 grid pattern. Mica, softer with finer grains, was tested with a maximum load of 2000 μN and an indentation spacing of 10 μm; quartz and feldspar at 8000 μN with 50 μm spacing³². The test controlled maximum load, loading to the set maximum in 5 s, holding for 2 s to reduce creep-induced strain hysteresis³³, then unloading in another 5 s.

Figure 5

Cross-sectional image of granite and mineral color mapping: (a) cross-sectional image and (b) mineral color mapping.

Fundamentals of nanoindentation

The nanoindenter records the load–displacement curve via a sharp tip indentation. This curve is then used to calculate the material’s elastic modulus, hardness, fracture toughness, and other properties ³⁴. Figure 6a, b respectively show the typical load‒displacement curve for nanoindentation and the schematic diagram of the cross section.

Figure 6

Schematic diagram of the nanoindentation principle: (a) typical nanoindentation load‒displacement curve and (b) schematic diagram of the nanoindentation cross section.

S represents the elastic contact stiffness of the sample, which is typically calculated by fitting the upper section of the unloading curve; P_max is the maximum indentation load; h_max is the maximum indentation depth; h_f is the final depth after full unloading; and h_c is the indentation contact depth. The equations for the elastic modulus (E) and hardness (H) of the relevant materials are shown as follows:

$$ S = \left( {\frac{dP}{{dh}}} \right)_{{h = h_{\max } }} $$

(1)

$$ h_{c} = h_{\max } - \varepsilon \frac{{P_{\max } }}{\varepsilon } $$

(2)

$$ E_{r} = \frac{\sqrt \pi }{{2\beta }}\frac{S}{{\sqrt {A_{c} } }} $$

(3)

$$ \frac{1}{{E_{r} }} = \frac{{1 - v^{2} }}{E} + \frac{{1 - v_{i}^{2} }}{{E_{i} }} $$

(4)

$$ H = \frac{{P_{\max } }}{{A_{c} }} $$

(5)

where E_r is the reduced modulus and E_i and v_i represent the elastic modulus and Poisson’s ratio of the indenter, respectively. For the diamond indenter, E_i = 1140 GPa, and v_i = 0.07. β is the indenter correction coefficient, and ε is the coefficient related to the indenter shape. In this test, a modified Berkovich indenter with a regular triangular pyramid is used, β = 1.034 and ε = 0.75³⁵. Ac is the projected contact area, which can be calculated as follows:

$$ A_{c} = \frac{3\sqrt 3 \tan \theta }{{\cos \theta }}h_{c}^{2} $$

(6)

where θ is the angle between the centre axis of the indenter and the side, which is taken as 65.27°, that is, A_c = 24.5h_c².

It is clear from Eq. (4) that there is very little variation in the elastic modulus obtained from the potential Poisson's ratio values of the samples. Additionally, since E_i >  > E, the elastic modulus derived with E_i = 1140GPa and E_i = ∞ only differs by approximately 1.6%³⁶. Therefore, the effect of the diamond indenter stiffness on the calculation results can be disregarded in the absence of very high precision requirements, and the streamlined Eq. (7) can be employed instead:

$$ E = E_{r} (1 - v^{2} ) $$

(7)

Experimental results and analysis

Macromechanical properties

Stress‒strain curve

The stress–strain curves of granite after water cooling at different high temperatures are shown in Fig. 7. For different loading stages, it is evident that: (1) During the compaction stage, the gradual closure of initial internal cracks in the granite under axial stress occurs. This results in all stress–strain curves showing notable nonlinear characteristics. As treatment temperature rises, the strain during the compaction phase increases, indicating more nonlinear behavior. This suggests that water cooling at higher temperatures may generate more internal microcracks. (2) The elastic segment of rock samples treated with water cooling at lower temperatures is longer. The proportion of the linear stage decreases as processing temperatures increase. (3) In the fissure damage stage, increasing stress causes internal microcracks to recur, grow, and propagate, showing nonlinear traits near the peak stress point. (4) Granite samples treated at different temperatures show varied post-peak behaviour. Below 400 °C, water-cooled granite experiences a sudden stress drop after reaching the peak, displaying distinct brittle failure characteristics. Above 400 °C, granite’s deformation gradually increases, demonstrating typical plastic failure characteristics. Higher temperatures accentuate the post-peak plastic behaviour. Therefore, it can be inferred that the critical temperature for the brittle-ductile transition of granite lies between 400 and 500 °C. Additionally, the stress level of the granite treated with water cooling at 900 °C remains consistently low. This indicates that the internal damage of the granite after such temperature treatment is excessive, resulting in a loss of its load-bearing capacity.

Figure 7

Stress‒strain curve of water-cooled granite at different high temperatures.

Macroscopic deformation strength characteristics

Table 2 presents the macroscopic mechanical parameters of high temperature granite after water cooling. Figure 8 illustrates the temperature-dependent evolution of peak strength, peak strain, and elastic modulus (the stress-to-strain ratio in the elastic phase) for the sample. The graph shows a gradual decline in peak strength and elastic modulus below 500 °C, with a steep drop above this temperature. Peak strain slowly rises with temperature up to 400 °C, then rapidly increases beyond this point. Thus, the macroscopic mechanical parameters trends of samples can be split into three stages: (1) Stage I (25–400 °C): With increasing temperature, the degradation of the macroscopic mechanical properties of granite occurs relatively slowly; (2) Stage II (400–500 °C): The peak strain increases significantly, while the decrease in peak strength and elastic modulus is not pronounced. This stage marks the critical temperature range for granite’s macroscopic mechanical performance deterioration; (3) Stage III (500–900 °C): Peak strength and elastic modulus decrease rapidly with increasing temperature, while peak strain increases with temperature. It is evident that the macroscopic mechanical properties of granite deteriorate rapidly with increasing temperature during this stage. When the temperature reaches 900 °C, the granite structure is severely damaged, completely losing its load-bearing capacity.

Table 2 Macroscopic mechanical parameters of high temperature granite after water cooling.

Figure 8

The peak strength, peak strain and elastic modulus of high-temperature granite after water cooling as a function of temperature: (a) peak strength and peak strain versus temperature and (b) Elastic modulus versus temperature.

AE characteristics

The damage evolution of water-cooled granite at high temperatures under loading can be investigated by analysing the AE signals generated from the rock samples during uniaxial compression tests.

AE ringing counts

The distribution of AE ringing counts during the loading process for water-cooled granite at different high temperatures is shown in Fig. 9. The inset in the figure illustrates lower AE ring counts, which could not be depicted in the main figure due to the excessively high AE ring counts at the stress peaks. Granite treated at different temperatures undergoes three stages during the loading process: active period I, quiet period, and active period II³⁷. The three stages are demarcated by AE ring counts, with two critical points: the first where AE counts exceed the background level and remain unchanged in hit rate, and the second where the AE hit rate rapidly increases. Active period I, early in loading, features minimal yet continuous AE signals from the initial closure of internal rock fractures³⁸. This stage reflects the initial damage degree within the rock sample. A higher proportion indicating more thermal cracks due to high temperature and water cooling. During the quiet period, initial cracks are further compressed, placing the rock sample in an elastic phase with no discernible damage. AE events are relatively fewer and intermittent compared to active period I. A smaller proportion of this stage corresponds to lower rock brittleness and, correspondingly, enhanced ductility (plasticity)³⁹. In active phase II, as the external force rises and microfractures regrow and spread, AE events surge dramatically.

Figure 9

Variation curve of the AE ringing count of water-cooled granite at different high temperatures.

In the 25–200 °C temperature range, the AE ringing counts during loading exhibit a similar trend, as shown in Fig. 9a–c. Active phase I accounts for about 45%, and the quiet phase for 35%. At 300–400 °C, the active phase I rises to 55–60%, while the quiet phase falls to approximately 25%. At 500 °C, the quiet period ends, and the loading process is marked by continuous active periods I and II with ongoing AE events. This shows that the rock’s brittle characteristics vanish and are primarily replaced by ductile (plastic) deformation characteristics. The phenomenon shows that above 500 °C, rapid water cooling causes significant internal thermal damage and cracks in granite the damage increases significantly with temperature, transforming the rock’s deformation properties from brittle to ductile.

AE accumulative energy

The cumulative energy during compression damage of water-cooled granite at different high temperatures is plotted as a function of temperature, as shown in Fig. 10. It can be divided into two stages: below 400 °C, the accumulated energy decreases gradually with rising temperature. Above 400 °C, the energy released prior to granite failure diminishes rapidly. Similar to the results of Shao et al¹²., the temperature at this stage has a significant effect on the accumulated energy. Only less energy is required to break the rock sample under external loading due to internal damage from high temperature and water cooling.

Figure 10

Curve of the cumulative energy of water-cooled granite failure versus temperature.

Based on trends in macroscopic deformation and AE characteristic parameters, the study reveals a critical transition at 400–500 °C for the macroscopic physical and mechanical properties of granite.

Micromechanical properties

The differential expansion and contraction of various minerals within granite under high-temperature water cooling affect the rock’s macro-mechanical properties^8,23. Investigating mineral micro-mechanical properties contributes to a deeper understanding of granite damage mechanisms under high-temperature and water-cooling treatments. Macroscopic tests reveal 300 °C and 500 °C as critical points for granite’s properties. Therefore, a detailed micro-mechanical study is conducted on rock samples subjected to water quenching at four temperatures: 25 °C, 200 °C, 400 °C and 600 °C.

Mineral stress–strain curve

Figure 11 shows the typical micro-load–displacement curves of the three main minerals in granite at room temperature. The load–displacement curves of quartz and plagioclase appear relatively smooth. In comparison to quartz, plagioclase demonstrates lower hardness, leading to a prolonged compacting stage and deeper indentation. As for biotite, its load curve exhibits noticeable concave-convex inflection points. Under the same load (2000 μN), biotite experiences the greatest total deformation, approximately 1.83 times that of quartz and 1.73 times that of plagioclase. Additionally, the creep deformation during the constant load stage is also the most significant, primarily due to biotite’s weakest mechanical properties and distinct pore structure²⁶.

Figure 11

Load–displacement curves of granite mineral nanoindentation at room temperature.

Figure 12 illustrates the micro load–displacement curves of the three primary minerals in granite after undergoing various high-temperature and water-cooling treatments (three typical characteristic curves were chosen for each mineral for analysis). The figure suggests that below 400 °C, quartz’s load–displacement behavior closely resembles that at room temperature, with minimal curve dispersion and curves converging closely. At 600 °C, there is a significant increase in curve dispersion and plastic deformation, as well as creep deformation under constant load. A noteworthy observation is that at room temperature, the peak deformation of quartz exceeds that at 200 °C and 400 °C. This can be attributed to the slight expansion of mineral particles due to lower high-temperature effects, leading to the occupation of pore space. Consequently, the available space to accommodate deformation during loading is reduced, resulting in a decrease in peak deformation. Increasing temperature causes the divergence of load–displacement curves for plagioclase minerals to gradually increase, leading to more concave and convex folds. Additionally, the indentation depth and creep deformation during the constant load phase also increase. Below 400 °C, biotite minerals exhibit smoother and more discrete load–displacement curves compared to room temperature. After heating to 600 °C, the curves show concave-convex folds, multiple horizontal segments, and significantly increased indentation depths. The analysis above indicates that at temperatures below 400 °C, quartz shows good microscopic homogeneity. However, above 600 °C, a transition from homogeneity to anisotropy occurs in quartz minerals, accompanied by increased plastic deformation, corresponding to a phase transition at around 573 °C. Additionally, the mechanical properties of plagioclase gradually decrease with rising temperature, accompanied by an increase in plastic deformation. Biotite minerals consistently exhibit pronounced anisotropic characteristics and significant total deformation across different temperature treatments. According to the load–displacement curves, the mechanical properties of biotite at 400 °C is slightly better than that at ambient temperature. As the temperature rises, plastic deformation progressively increases, aligning with findings from the research of Mo et al^26,32. The nanoscale mechanical characteristics started to degrade at approximately 600 °C.

Figure 12

Load‒displacement curves of nanoindentation of water-cooled minerals at different high temperatures.

Nanoindentation load–displacement curves indicate exhibit escalating plastic deformation in mineral crystals above 400 °C, aligning with macroscopic strain amplification in granite after water cooling at this temperature. It is inferred that the heightened plastic deformation of minerals beyond 400 °C is one of the factors contributing to the macroscopic plastic deformation in granite.

Elastic modulus and hardness of minerals

The elastic modulus and hardness of the minerals can be calculated by Eqs. (5) and (7) by using the parameters obtained from the nanoindentation test. According to the available research data, Poisson’s ratio is 0.15 for quartz, 0.2 for plagioclase and 0.3 for biotite³².

Figure 13a, b show the microscale elastic moduli and hardness of the three primary minerals. Both parameters exhibit similar trends of variation with increasing temperature. This study utilizes the elastic modulus of minerals as a case to analyse their evolutionary pattern with temperature variation. Combined with Fig. 12, the load–displacement curve of quartz within 400 °C shows a similar trend. This indicates good homogeneity and mechanical properties. Furthermore, Fig. 15 shows no significant morphological changes in quartz within this temperature range. Moreover, in Fig. 13a, the elastic modulus of quartz is increased at 200 °C and 400 °C compared to 25 °C. The phenomenon might result from initial microcracks closing under low-temperature thermal stress, with negligible quartz microstructure damage. There is a positive correlation between the elastic modulus and thermal stress. Based on this, it can be inferred that quartz primarily exhibits elastic expansion within 25–400 °C, followed by elastic recoil after rapid water quenching. Hence, its micro-elastic modulus is less affected by this temperature range. At 600 °C, the phase transition of quartz induces structural changes, leading to a rapid decrease in its micro-elastic modulus, consistent with conclusions drawn by Kimizuka et al^40,41. Similar to the pattern of plagioclase performance with temperature investigated by Liu et al^42,43., the elastic modulus of plagioclase gradually declines with rising temperature. Biotite exhibits an initial increase followed by a decrease in its elastic modulus with rising temperature. The analysis of this trend is as follows: at high temperatures, the internal bound water and structural water in biotite gradually evaporate. Thermal damage in biotite hinders its return to the initial state after water cooling, decreasing plasticity and increasing elasticity. This is similar to the conclusion attained by Ma et al^6,44. that high temperature at 500 °C can improve the toughness of biotite. When exposed to 600 °C, the high temperature and rapid water cooling create numerous internal pores and cracks, sharply reducing the elastic modulus of the biotite.

Figure 13

Curve of the micromechanical properties of water-cooled granite minerals changing with temperature: (a) elastic modulus and (b) hardness.

Additionally, nanoindentation tests reveal that the mechanical properties of the three minerals exhibit distinct trends with increasing temperature below 400 °C. At 200 °C and 400 °C, quartz and biotite exhibited higher elastic moduli compared to ambient temperature, with quartz increasing by 19% and 16%, and biotite by 20% and 3% at these temperatures. While plagioclase's elastic modulus decreased with temperature, being 14% lower at 200 °C and 19% lower at 400 °C than ambient. At 600 °C, the elastic moduli of quartz, plagioclase, and biotite all decreased significantly, with reductions of 25%, 32%, and 44% compared to 25 °C. This deterioration corresponds to the macroscopic peak strength degradation of the granite after water quenching. Changes in the elastic modulus of the three main mineral at 200 °C and 400 °C are relatively minor compared to those at 600 °C. This consistency with the macroscopic strength, showing moderate changes below 500 °C and rapid deterioration above. Notably, below 500 °C, the microscopic mechanical strength of granite minerals do not fully correspond with its macroscopic strength. Quartz and biotite demonstrate increased elastic modulus and hardness, contrasted with diminishing macroscopic mechanical strength.

Failure modes

Macroscopic failure modes

Figure 14 shows the granite’s macroscopic fracture morphology under uniaxial compression tests. High-temperature treated granite samples with water cooling display surface cracks and varying degrees of spalling. The water cooling-induced temperature gradient creates maximum secondary stress, which is tensile stress⁵, on the high-temperature rock surface. With an increase in processing temperature, the resulting larger temperature gradient leads to higher tensile stress. Consequently, the specimen surface is severely damaged under secondary thermal stress, making it more prone to failure during uniaxial compression. Below 400 °C, granite specimens exhibit vertical cracks and surface spalling during uniaxial compression. The internal horizontal stress reaches the tensile strength of the specimen, leading to failure, with the fracture plane parallel to the axial stress. At this stage, the predominant failure mode of the rock sample is tensile failure. When the temperature ranges from 400 to 700 °C, the elevated heat exacerbates internal damage within the rock. In uniaxial compression, the specimen surface initially spalls, followed by the propagation of internal cracks until the stress reaches the shear strength, resulting in the formation of a shear plane. During compression, the shear surface slip causes the specimen to spall into blocks. With increasing temperature, the shear failure surface becomes more pronounced. At this temperature range, the failure pattern of granite exhibits a combination of tension and shear failure. At 800 °C, shear cracks extend and converge, producing numerous fine particles from shear plane movement, ultimately resulting in shear failure under load.

Figure 14

Failure mode of high-temperature granite after water cooling.

Micromorphology of minerals

Figure 15 shows the microscopic morphology of water-cooled granite with different high temperatures via a nanoindenter’s electron optical microscope. The red dashed lines represent mineral boundaries. Figure 15a shows the untreated mineral surfaces are very smooth, with a dense structure, good integrity and no apparent microcracks. At 200 °C, Fig. 15b indicates a few intragranular cracks in plagioclase and biotite. The interconnection of quartz minerals and other particles shows little change from 25 °C. This is because the thermal stress has not reached the particle bonding strength, and minor volume expansion of particles tightens their connections. Under uniaxial compression, granite fails by tension similarly to normal temperature. At 400 °C, rapid water cooling induces significant thermal shock stress, creating numerous microcracks in quartz, plagioclase, and biotite, primarily intracrystalline fractures. Intergranular fractures appear at of large-grain quartz boundaries, with some intragranular cracks linking to them. This leads to the formation of a small number of transgranular fractures within the quartz and plagioclase minerals, as depicted in Fig. 15c. The significant increase in intragranular and transgranular fractures of miners at 400 °C leads to more internal cracks in granite. Under external loading, microcracks propagate and coalesce, increasing peak strain and plasticity, leading to a distinct shear failure surface. The rise in internal fractures align with a notable increase in the granite’s AE ring counts at this temperature. At 600 °C, Fig. 15d shows roughened mineral particle surfaces in granite, with a marked rise in intragranular microcracks. A few transgranular and intergranular cracks in quartz and plagioclase reduce the mineral bonding strength. This further enhances the plasticity of the granite sample, in line with the experimental findings of Guo et al⁴⁵.

Figure 15

The mineral composition morphology of granite water-cooled at different high temperatures. Q quartz, P plagioclase, B biotite. I intragranular crack. II transgranular crack. III intergranular cracks. Only the more prominent fissures are marked in the illustration for observational purposes.

Temperature rise leads to more microcracks and larger crack sizes in minerals. On one hand, differential thermal expansion coefficients of the various minerals lead to uneven expansion and deformation at higher temperature. This results in additional microcracks due to mutual constraint. On the other hand, during water cooling, the increased rock’s temperature differential induces greater thermal shock stress, creating numerous microcracks. These internal microcracks within the rock propagate and coalesce, ultimately forming larger cracks. In summary, below 400 °C, granite’s internal microcracks mainly form intracrystalline cracks, with their with their number increasing with temperature. At 600 °C, there is a notable increase in internal cracks, with intracrystalline, transgranular, and intergranular cracks developing concurrently.

Analysis of granite’s macroscopic damage and mineral microscopic morphology, below 200 °C, intracrystalline microcracks are minimal. At this point, the rock’s internal structure and mineral cementation are less affected by high temperature and water cooling. The specimens exhibit brittle tensile failure under uniaxial compression. As temperature reaches 400 °C, internal intracrystalline cracks develop, with a small number of transgranular cracks. This initiates a microcrack network, degrading the granite structure^14,24. External loading leads to an increase in microcracks, eventually penetrate and produce a macroscopic shear-damaged surface. Approaching 600 °C, transgranular and intergranular fissures start forming in granite, weakening inter-mineral cementation. At this point, the granite’s macroscopic shear damage pattern becomes more apparent. It is clear that high-temperature and water-cooling treatments transform granite’s macroscopic failure mode from tensile to tensile‒shear composite and then to shear as temperature rises.

Discussion

Thermal damage analysis of water-cooled granite at different high temperatures

This research indicates that high temperature and water cooling induced thermal damage significantly impacts the physical and mechanical properties of granite. As the temperature increases, the failure mode of the rock under uniaxial compression (see Fig. 14) transitions from tensile to tensile-shear composite and ultimately to shear failure. The critical temperature is within the range of 400–500 °C. At temperatures between 25 and 400 °C, rocks sustain less damage. The non-uniformity of the specimens results in variable strengths across areas, leading to local cracking under stress surpassing local strength. Increasing axial stress induces stress concentration at the microcrack tip, causing the crack to propagate towards the weaker tip area⁴⁶. Meanwhile, secondary cracks may emerge alongside the primary crack, linking local microcracks into macroscopic tensile cracks. At 500 °C, the rock experiences severe internal damage, resulting in increased pores, weak spots, and localized cracking during loading. Microcrack growth ceases at large pores. As the stress increases, primary and secondary cracks merge into short en-echelon fractures⁴⁷. When the stress reaches its peak, adjacent cracks link to form a macroscopic shear surface.

The formation and spread of cracks are mainly influenced by stress and microstructure. Granite’s diverse mineral components have varying thermal expansion coefficients, differing even within the same mineral along crystallographic axes¹⁶. Heating causes thermal expansion mismatch due to differing expansion rates and mineral anisotropy, resulting in interparticle constraint and thermal stress in the rock. When the thermal stress exceeds the local limit strength of the rock, localized thermal microcracks will form. At 200 °C, biotite dehydration-induced shrinkage during heating leads to high stress concentration around the biotite particles. Furthermore, due to its weak mechanical properties and porous structure, microcracks initially form in biotite and at grain boundaries (see Fig. 15b). At 400 °C, intergranular cracks emerge at quartz grain boundaries, as their formation energy is lower than intragranular crack⁴⁸. Cracks tend to propagate along preferential paths, primarily along mineral boundaries and large-grain minerals. As intergranular cracks extend, microcracks tend to occur in fractured weak minerals with larger or pre-existing cracks, forming intragranular cracks⁴⁹. Furthermore, at this temperature, organic matter in cement and minerals degrade, leading to crack formation at grain boundaries. Intracrystalline and intercrystallite cracks connect, developing transgranular cracks in the quartz and plagioclase. Quartz has a high, anisotropic thermal expansion coefficient⁵⁰. This results in frequent intracrystalline and transgranular cracks within its grains and near boundaries (see Fig. 15c).

At higher heating temperatures, granite’s macroscopic physical and mechanical properties deteriorate more, with thermal cracks expanding, signifying increased thermal damage. The increase in heating temperature leads to an enhancement of thermal stress within the rock. This triggers more microcracks and promotes crack propagation. Consequently, higher temperatures causes more prominent degradation in physical and mechanical properties, and greater microstructure damage. Particularly at 500 °C, higher temperature supply more energy for thermal crack initiation and quartz phase transformation-induced volume expansion. The rock’s peak strain increases with temperature, while peak strength correspondingly deteriorates rapidly (see Fig. 7).

After being heated, the sample reaches a thermal equilibrium state. Water cooling induces thermal shock from the extremely high temperature gradient, disrupting thermal equilibrium and generating thermal shock stress. The transient thermal stress typically manifests as tensile stress on the rock surface and compressive stress at its center⁵¹. The rock has minimal tensile resistance. Intense tensile stress forms tensile microcracks on its surface. Rapid cooling in water after heating allows water to infiltrate surface microcracks, inducing tensile stress that enlarges and deepens the cracks. As the heating temperature rises, the enhanced temperature gradient induces greater thermal shock stress, generating numerous microcracks. Cracks widen, penetrate the rock interior, and merge to form a network. This significantly reduces the rock’s mechanical properties and shifts its deformation from brittle to ductile. Quenching at 900 °C, the granite undergoes severe thermal damage. It nearly loses its load-bearing capacity and can be easily fractured with minimal energy.

Microcracks arise due to thermal stress caused by uneven expansion within and between minerals during heating. These microcracks then propagate due to tensile stress from thermal shock. The combined effects of high temperature and thermal shock lead to the deterioration of granite’s physical and mechanical properties following high temperature water quenching.

Macro and micro mechanical properties of water-cooled granite at different high temperatures

Studies show that high-temperature and water-cooling treatments significantly affect granite’s mechanical properties at both macro and micro scales. To link macroscopic and microscopic mechanical rock properties, a homogenization analysis was conducted using the Mori–Tanaka method⁵² based on the inclusion theory. The equivalent elastic modulus of granite was calculated and compared to macroscopic experimental data, as shown in Fig. 16. The figure illustrates a general decreasing trend in the homogenized elastic modulus of granite with rising temperature. The comparison with macroscopic elastic modulus measurements show a similar trend with temperature: little change before 400 °C, then a sharp decline by 600 °C. There are differences between homogenization method calculations and measured values within a certain range. However, these results effectively reflect the trend of granite’s mechanical parameters with temperature. Therefore, they retain significant practical engineering value for assessing material mechanical properties.

Figure 16

The homogenized elastic modulus and macroscopic measured value change with temperature of high temperature granites after water cooling.

At 200 °C, the homogenized elastic modulus is higher than at room temperature. According to previous studies, below 400 °C, granite’s elastic modulus and strength may increase, not decrease, under different heating and cooling conditions^12,16,32. This study observed a slow decrease in granite’s macroscopic mechanical properties within the temperate range. This is associated with variations in the thermophysical properties and compositional content of minerals. AE monitoring and mineral microscopic morphology (Figs. 9 and 15) indicate that microcrack quantity in granite rises with temperature, but no significant increase is seen below 500 °C. According to the macroscopic strength characteristics (see Fig. 8), the critical temperature of peak strain is 400 °C, and for peak stress, it’s 500 °C. Below 500 °C, the mechanical strength of granite changes slowly with increasing temperature. In other words, compared to strength parameters, deformation parameters are more sensitive to the increase in internal microcracks⁴⁷. Therefore, below 500 °C, thermal damage from high temperatures and thermal shock is minimal. Microcracks in granite are compressed and closed during initial loading, which had a minor impact on rock strength.

Based on macroscopic strength and mineral composition, quartz and biotite show improved mechanical properties below 400 °C, while the sample’s peak strength decreases as temperature rises. At the microscale, the mechanical properties of quartz and biotite exhibit an different trend with temperature that from their macroscopic behaviour (see Fig. 13). At 200 °C and 400 °C, the evaporation of internal bound water and structural water in biotite leads to particle shrinkage. This results in reduced plasticity and increased elasticity, enhancing its mechanical properties. Quartz exhibits higher elastic modulus and hardness at 200 °C and 400 °C compared to room temperature. This is due to lower thermal expansion-induced thermal stress, which is insufficient to disrupt the grain structure. The slight particle volume expansion closes inherent pores and tightens inter-particle bonding, thereby enhancing quartz’s mechanical properties. This demonstrates that the thermal expansion mechanism within 400 °C is beneficial for certain minerals. However, the impact of multiple mineral components on stress distribution and microcracks outweighs the thermal expansion of individual minerals. Non-matching thermal expansion is a more critical factor than bulk thermal expansion⁵⁰. The differences in thermal physical properties lead to varied micro-mechanical characteristics of minerals as temperature changes. Non-matching thermal expansion causes thermal damage from mineral interactions, reducing macroscopic strength. Zhang et al³². discovered that the macroscopic mechanical properties of granite improved with the enhancement of its main mineral components’ properties after heating to 300 °C. Vázquez et al⁵⁰. also noted that higher or lower quartz content leads to less stress within the rock, thereby reducing microcracks. This underscores the significance of mineral composition in affecting granite’s overall mechanical properties. Hence, the high plagioclase content (65%-70%) in granite samples (Table 1) means its mechanical decline partly contributes to the samples’ overall mechanical weakening.

Based on the above analysis, the alignment of homogenization and the measured results with temperature suggests that the microscale mineral mechanics dominate granite’s macroscopic temperature response. Considering the influence of mineral properties on crack development: at 200 °C and 400 °C, different minerals exhibit varying changes in mechanical strength. Discrepancies in mineral thermal expansion and the degradation of high-content minerals govern granite’s macroscopic strength changes. Simultaneously, the mineral microstructure primarily develops intragranular cracks. This leads to a gradual reduction in the peak strength of granite after water quenching as the temperature rises. At 600 °C, the micro-mechanical properties of the primary mineral components in granite all decrease significantly. This results in a notable increase in internal microcracks within the granite, along with the growth of intragranular, transgranular, and intergranular cracks. Consequently, the macroscopic peak strength of the granite deteriorates rapidly following water cooling. In general, the slow decline in granite’s macroscopic mechanical properties below 500 °C is due to variations in thermophysical characteristics and mineral composition. The rapid decrease of mineral micromechanical properties and the generation and propagation of microcracks are probable primary drivers of the degraded macroscopic mechanical properties of water-cooled granite above 500 °C^22,23,24.

Conclusion

In this research, uniaxial compression tests were conducted on granite subjected to high-temperature water quenching, with simultaneous acoustic emission monitoring. Subsequently, nanoindentation was employed for representative samples. The conclusions regarding the macro- and micro- physical and mechanical properties of granite are presented as follows:

• (1) Following high-temperature water quenching, the failure modes of granite under uniaxial compression are as follows: tensile failure below 400 °C, tensile-shear combined failure at 400–700 °C, and shear failure above 800 °C. Peak strength and modulus decrease with increasing temperature, while peak strain increases. The critical temperature range for macroscopic mechanical property degradation is 400–500 °C.

• (2) The decrease and eventual absence of quiet periods in AE ring counts, accompanied by a rapid drop in cumulative released energy, indicate that the ductility of granite increases after 400 °C, with substantial internal damage occurring after 500 °C. Below 400 °C, the energy released before failure decreases gradually, but above this temperature, it decreases sharply, indicating that high temperature and rapid cooling-induced cracks intensify internal damage and reduce the energy needed for failure.

• (3) Macroscopic and microscopic testing indicates that below 500 °C, the primary minerals of water-cooled granite exhibit diverse responses to temperature changes: quartz shows minimal micro-mechanical property changes, feldspar's properties decrease with temperature, and mica's properties are slightly enhanced compared to room temperature, with intracrystalline cracks predominating the internal fractures.

• (4) Variations in thermophysical characteristics and mineral composition of the rock samples result in a gradual change in the macroscopic mechanical properties of water-cooled granite below 500 °C. At 600 °C, the mechanical properties of the minerals sharply decline, accompanied by a marked increase in internal cracks. The rapid decrease in micro-mechanical properties and the development of microcracks are the main causes of the deterioration of the macroscopic mechanical properties.

This study contributes to a deeper understanding of the physico-mechanical behavior of granite under high-temperature quenching. Our exploration of the deterioration mechanisms in high-temperature quenched granite can provide a reference for enhancing the efficiency of dry hot rock extraction. When assessing the stability of tunnel surrounding rock after a fire, it is necessary to consider the influence of temperature thresholds to enhance the safety of post-disaster engineering repairs.