Zhang, Y., et al.

INFLUENCE OF TEMPERATURE ON PHYSICAL

AND MECHANICAL PROPERTIES OF A SEDIMENTARY ROCK

Coal Measure Mudstone

by

Yuan ZHANG

a,b*

, Zhijun WAN

a,b

, John MCLENNAN

c

,

Bin GU

b

, and Xupeng TA

b

a Key Laboratory of Deep Coal Resource Mining, China University of Mining and Technology,

Ministry of Education, Xuzhou, Jiangsu, China b School of Mines, China University of Mining and Technology, Xuzhou, Jiangsu, China

c Energy and Geoscience Institute, University of Utah, Salt Lake City, Ut., USA

Original scientific paper https://doi.org/10.2298/TSCI190101297Z

Determining the physical and mechanical behavior of sedimentary rocks is one of the most common challenges in deep rock mass engineering. Experiments were con-ducted to study the physical and mechanical properties of coal measure mudstone with SEM, XRD, and uniaxial compression testing. The results show that tempera-ture has a significant effect on the physical and mechanical properties of coal meas-ure mudstone. The presence of clay minerals in the evaluated mudstone contributes to the unique characteristics seen at high temperature. The mudstone experiences obvious color changes on the surface as temperature rises. This is mostly attributed to the iron-bearing clay minerals. Internal color change is caused by thermal de-composition of kerogen associated with the clay minerals. As the major clay mineral in mudstone, kaolinite undergoes significant phase changes at high temperatures, which leads to changes in mechanical properties. From 25 °C to 200 °C, due to the evaporation of absorbed water from the clay minerals, the strength of the mudstone increases significantly. As the temperature continues to rise beyond this, water evaporation continues and the rock strength increases gradually from 200 °C to 400 °C. When the temperature reaches 400 °C, this mudstone was strengthened as a result of decomposition of the kaolinite and thermal expansion of crystalline min-erals. Above 600 °C, dehydration of the clay minerals ends while thermal cracking initiates gradually, which results in decreasing strength.

Key words: rock mechanics, mechanical property, coal measure mudstone, high temperature

Introduction

Currently, humans have been able to exploit solid mineral resources at depths to

around 4 km, and drilling has reached strata of more than 10 km [1, 2]. Generally, deep rocks

are in a high temperature environment, which may affect their physical and mechanical prop-

erties. Deep rock mass engineering, such as underground coal gasification, geothermal energy

exploitation, underground nuclear waste storage, and rock mass support of tunnels after fire, all

involve high temperature rocks. The temperature ranges from normal temperatures (10-50 °C)

to extreme temperatures (1000-1500 °C) in these rocks [3]. Consequently, research related to

–––––––––––––– * Corresponding author, e-mail: zhangyuan@cumt.edu.cn

Zhang, Y., et al.

the physical and mechanical properties of rocks in high temperature environments has broad

engineering significance and need to be studied in detail.

Mudstone is a typical sedimentary rock that is widespread in the crust. During the last

few years, some investigations have been conducted in this field. Due to its weak lithological

characteristics and tendency to soften when water is encountered, mudstone is always a focus

of attention in deep rock engineering. It is widely believed that the thermal expansion and ther-

mal cracking are dominant factors for the effect of temperature on rocks [4, 5]. However, the

physical and mechanical properties of coal measure mudstone at different temperatures are very

complicated, involving the effects of water, organic matter and clay minerals. Liu et al. [6]

conducted an experimental study of mudstone, from a coal mine in China, at temperatures rang-

ing from 25 °C to 800 °C. They found that the strength and elastic modulus of mudstone have

an obviously increasing trend from 25 °C to 400 °C, and then decrease rapidly when the tem-

perature reaches 600 °C, and have a little change or decrease from 600 °C to 800 °C. Zhang

et al. [7] also studied the subject, and the results are similar to those in paper [6]. These studies

demonstrate that evaporation, thermally-induced softening and thermal cracking are the main

factors relevant to mechanical property variation at high temperature and brittleness is the main

failure pattern below 600 °C. Different from that at high temperature, the strength and elastic

modulus of a kind of mudstone were found to decrease with increasing temperature from 25 °C

to 55 °C [8]. In regard to the failure mechanism of mudstone at high temperature, Zhang et al.

[9] found that the overall structure undergoes a phase change around 600 °C, leading to a sud-

den change in the mechanical properties of this mudstone. At 600 °C, the crystalline state de-

grades and kaolinite disappears. However, some illites are found, indicating chemical reaction

of the structure and correlating with the sudden drop of bearing capacity of the mudstone. Liu

et al. [10] investigated the mechanical behavior of an Australian mudstone subjected to tem-

peratures up to 900 °C. They showed that the uniaxial compressive strength (UCS) changes

non-monotonically with temperature. This can be explained by two dominant mechanisms: dif-

ferential thermal expansion of the constituent minerals and thermal reactions of muscovite and

kaolinite over the temperature range of interest. However, as the geological environment of the

mudstone formation is complicated, there are great differences in the mineralogy and grain

structures in different mudstones, especially the clay minerals. Temperature has a large influ-

ence on clay minerals [11, 12]. Therefore, the effect of temperature on the physical and me-

chanical properties of coal measure mudstone could also differ substantially.

Mudstone specimens were sampled from coal measure strata to study the temperature

effect on physical and mechanical behavior. Uniaxial compression system was selected to test

the mechanical properties of these samples at different temperatures (25, 200, 400, 500, 600,

and 800 °C). This loading was carried out in a custom resistance furnace. After UCS tests, SEM

was used to observe the internal structure of the failed samples. The XRD analysis was also

performed to measure the mineralogical composition after heating. It was expected that this

research program would provide useful data relevant to the physical and mechanical behavior

of coal measure sedimentary rock.

Materials and methodologies

Sample preparation

The mudstone samples were all from the Baoli coal mine. This is an open pit mine

located in Inner Mongolia, China. The mineralogy of the mudstone included quartz and clay min-

erals, with some organic matter. Twenty cylindrical samples, 50 mm in diameter and 100 mm

Zhang, Y., et al.

long, were obtained. The ends of the cylinders were ground to ensure flatness and perpendicular-

ity to the axis. These samples were all prepared following ISRM suggested methods. Before test-

ing, the samples were placed in a drying oven at 105 °C for at least 24 hours to remove all mois-

ture content. Then, they were cooled slowly to room temperature in the oven. In general, three

samples were put grouped together and labeled for evaluation at the experimental temperatures.

Experimental procedures

First, sample heating was performed in a furnace with a temperature controller. The

maximum design temperature of the furnace is 1000 °C, with a control precision of ±1 °C. Ac-

cording to the experimental program, the target temperatures were 200, 400, 500, 600, and

800 °C. The heating rate was set to 2 °C per minute to minimize thermal shock [13, 14]. Once

the sample reached the target temperature, the furnace maintained the target temperature for

2 hours at which time the uniaxial compressive test was conducted to measure the mechanical

properties of the sample at the target temperature. Second, uniaxial compression tests were per-

formed with a hydraulic universal testing machine (MTS C64) in the State Key Laboratory of

Coal Resources and Safety Mining. Stress control mode was adopted, with a load rate of

0.5 MPa/s. More mudstone samples would be added when the variation of test data was high.

Third, subsamples were taken at the corners of the mudstone after the uniaxial compression

tests. A hammer and chisel were used to fracture the blocks to minimize post-experiment dis-

ruption to the rock. Then, SEM observations were carried out to view the microstructures using

the SIGMA SEM in the Wuxi Graphene Industry Development Demonstration Area Detection

Centre. At last, XRD was also used to detect mineralogical changes after heating at different

temperatures. These measurements were carried out in the Advanced Analysis & Computation

Center.

Results and discussion

Appearance change

Some of the samples after uniaxial compression are shown in figs. 1 and 2. These

photographs demonstrate the sample failure patterns and color changes after heat treatment and

uniaxial compression.

As shown in fig. 1, samples are more likely

to generate shear failure below 200 °C (25 °C

and 200 °C) and tensile failure above 400 °C

(400-800 °C) under uniaxial compression. It can

be seen in fig. 2 that the failed samples are more

fragmented at 500 °C and 800 °C, compared

with other temperatures. Meanwhile, these mud-

stones show significant color changes on the sur-

face as temperature rises. This may be due to

chemical reaction of minerals, especially the

iron-bearing clay minerals [15-17]. We can see from fig. 1 that the color alteration is not obvi-

ous before 200 °C, but the samples become reddish when the temperature reaches 400 °C. As

the temperature rises, the red coloration becomes deeper. Apparently, there is new mineral

forming at high temperature levels, related to the oxidation of iron. In general, when the tem-

perature reached 400 °C, dehydration of iron-bearing clay minerals occurs and the structure is

Figure 1. Exterior photographs of failed mudstone samples

Zhang, Y., et al.

disrupted. Fe2+ and Fe3+ in the clay minerals are transformed into Fe2O3, which makes the sam-

ples reddish [18]. Many investigations have confirmed this observation. Figure 2 demonstrates the changes in the

interior appearance of the samples. The interior

of the mudstone is grayish-white with black tex-

ture associated with organic matter at tempera-

tures below 200 °C. As the temperature rises,

the black texture disappears gradually. When the

temperature rises to 400 °C, the interior be-

comes slightly black and the intensity of the

black becomes progressively deeper with tem-

perature increasing. It reaches its greatest inten-

sity, black, at around 600 °C. The black texture

is more likely higher in plant debris. Hence,

these are likely kerogenous clays, with mainly

type III kerogen. At lower temperatures (about

50-120 °C), thermal decomposition of the kero-

gen begins, primarily dehydration. When the

temperature reaches about 350 °C, the organic functional group of the kerogen decomposes.

The decomposition is exhausted at about 520 °C to 620 °C [19, 20]. Before decomposition of

the kerogen ends, some new organic byproducts are formed, such as asphalt and liquid hydro-

carbon. This organic matter itself, or new condensation polymerization from them, may affect

the samples’ appearance after cooling to room temperature, as shown in fig. 2.

Microstructure and composition

Microstructural characteristics

Figure 3 illustrates the morphological characteristics of mudstone samples after heat

treatment of different temperatures. It can be seen that there are many clay minerals in the

mudstone at each experimental temperature. Many of the clay mineralogical crystals are flaky,

irregular, hexagonal-like, foliated, and scale-like. Most of the flakes are straight and flat with

some frizzy edges, as shown in fig. 3(b). These flakes are stacked together to form aggregations,

lamination-like, blossom cluster-like or worm-like configurations. Therefore, kaolinite is one

of the most possible minerals in these mudstones.

Many microscopic pores are found in the SEM images in figs. 3(c)-3(f), which indi-

cates that the organic matter in the mudstone may produce gas at high temperature above 400 °C.

It is also observed that clay minerals are like cushion and filling materials in the mudstones. At

lower temperatures, e. g. 25 °C, the surface of mudstone samples is relatively smooth. However,

when the temperature reaches to 200 °C, the mudstone starts peeling, as shown in figs. 3(b) and

3(c). At 500 °C, the mudstone peels off and becomes clastic. What’s more, the samples begin to

crack at 600 °C, as shown in fig. 3(e). Above 600 °C, huge fractures appear in the mudstones.

The whole process is a bit similar to clay soil cracking because of drought.

Meanwhile, Energy Dispersive X-Ray Spectroscopy (EDX) was also performed to

quantify the elements in the samples after testing at 25, 200, and 600 °C. The results are shown

in fig. 4 and tab. 1. As shown in tab. 1, the percentage of oxygen declines continuously from

25-600 °C. This illustrates that dehydration occurs at elevated temperatures. Depending on the

atomic percentage of Si and Al, there must exist clay minerals in the mudstone.

Figure 2. Interior photographs of failed mudstone samples

Zhang, Y., et al.

Figure 3. The SEM images of mudstone samples at different temperatures; (a) 25 °C, (b) 200 °C, (c) 400 °C, (d) 500 °C, (e) 600 °C, and (f) 800 °C

Composition changes

Figure 5 illustrates the XRD patterns for mudstone samples at different temperatures

as determined from the diffraction intensity of X-rays at different detection angles. The miner-

alogical compositions of the mudstone samples are determined by XRD at 25 °C. The major

compositions are quartz (large), kaolinite (medium) and elpidite (small). The spectral lines at

different temperatures are similar, indicating that the mineralogical composition of the mud-

stone at these temperatures changes little.

Zhang, Y., et al.

Figure 4. Main elements in mudstone samples, measured by EDX; (a) 25 °C, (b) 200 °C, and (c) 600 °C

(for color image see journal web site)

Table 1. Atomic percent of elements in mudstone samples [%]

T [°C] O Si C Al Fe Mg Na K Zr Total

25 62.82 12.16 12.82 6.69 3.40 1.05 - - 1.03 99.97

200 61.79 13.16 13.27 6.42 2.53 0.85 1.52 0.46 - 100

600 58.49 13.36 15.21 7.17 3.58 1.49 - 0.69 - 99.99

Zhang, Y., et al.

It can be found that the height of the char-

acteristic diffraction peaks (2θ = 26.619°) of

SiO2 at lower temperatures (25, 200, and 400 °C)

is greater than that at high temperatures (500,

600, and 800 °C). This shows that the quartz in

the mudstone has better crystallinity at lower

temperatures. A similar phenomenon is found in

the characteristic peaks of kaolinite when

2θ = 12.359°. This suggests that kaolinite may

undergo significant phase changes at high tem-

peratures. In general, the moisture content has a

strong influence on the behavior of the clay min-

erals. When temperature reaches 400~500 °C,

dehydration of the kaolinite begins and the regu-

lar crystal structure of the kaolinite is broken.

Then, the kaolinite (Al2O32SiO22H2O) is trans-

formed to be metakaolinite (Al2O32SiO2), with an amorphous structure, which leads to the

degradation of the kaolinite XRD patterns [21, 22].

Mechanical properties

Stress-strain characteristics

Figure 6 indicates the stress-strain characteristics of these mudstones during uniaxial

compression. Eighteen curves are presented in figs. 6(a)-6(f). There are consolidation stages

and elastic stages during uniaxial compression at different temperatures, but the consolidation

stage is not obvious at 25 °C. With temperature increasing, consolidation becomes more and

more apparent. With the stress control loading mode, residual deformation after the peak cannot

be clearly observed in the stress-strain curves.

Peak stress and strain

The variation in peak stress and strain of these mudstone samples as a function of

temperature are shown in fig. 7.

It can be seen from fig. 7(a) that the peak stress shows an increasing trend with an

increase in ambient temperature, but does not change consistently. There is a substantial and

huge increase in strength from 25 °C to 600 °C. However, there is a slight decline from 600 °C

to 800 °C. At 25 °C, the peak stress is 14.54 MPa and then increases sharply to 18.92 MPa at

200 °C, rising by 30%. From 200 °C to 500 °C, the peak strength increases continuously as the

temperature rises. The strength at 500 °C reaches 20.87 MPa, an increase of 10.33% compared

to the peak stress at 200 °C and by 43.52% relative to the measurement at 25 °C. There is a

sharp rise from 500 °C to 600 °C. The peak stress reaches 29.49 MPa, rising by 41.28% relative

to the strength at 500 °C and 102.76% from 25 °C. When the temperature is 800 °C, the peak

stress decreases slightly (by 13.6%), compared with that at 600 °C, but still is 75.16% greater

than the strength at 25 °C.

Figure 7(b) demonstrates the trends peak strain of mudstone at different temperatures.

From 25 °C to 200 °C, the peak strain declines sharply, decreasing by 33%. However, it in-

creases slowly from 200 °C to 800 °C, and the rate of increase from 400 to 600 °C is higher

Figure 5. The XRD patterns of mudstone

samples at different temperatures; Q – quartz, K – kaolinite, E - elpidite

Zhang, Y., et al.

Figure 6. Stress vs. strain for mudstone samples subjected to uniaxial compression at different

temperatures; (a) 25 °C, (b) 200 °C, (c) 400 °C, (d) 500, (e) 600 °C, and (f) 800 °C

than at the other temperatures. The peak strain at 800 °C increases by 51.5% compared with

that at 200 °C.

Figure 7. Diagrams of peak stress and strain of the mudstone samples at different temperatures; (a) peak stress, (b) peak strain

Elastic modulus and deformation modulus

The variation of elastic and deformation modulus with temperature is shown in fig. 8.

The deformation modulus is a secant modulus in the curve of stress-strain, equal to peak stress

divided by peak strain.

Zhang, Y., et al.

From 25 °C to 200 °C, both the elastic

modulus and the deformation modulus increase

sharply, by 122.6% and 97.8%, respectively.

This means that the deformation resistance in-

creases rapidly at 200 °C compared with 25 °C.

However, from 200 °C to 400 °C, the increase in

rate of elastic modulus (with temperature) slows

and the deformation modulus decreases slightly.

This indicates that the capacity for deformation

resistance reduces as the temperature increases.

The elastic modulus and the deformation modu-

lus decline by 8.5% and 7% at 500 °C, respec-

tively, compared with the values at 200 °C. Both

the elastic and deformation moduli increase rap-

idly from 500 °C to 600 °C, by 24.5% and

27.1%, respectively. Above 600 °C, both moduli

begin to decrease.

Mechanical mechanisms

The main cementing agents in the mud-

stone are crystalline grains and clay minerals, es-

pecially SiO2 forming the skeleton. The degree

to which filling eliminates holes is likely to be

incomplete. Figure 9 schematically illustrates

mineralogical contact forms in a generic mud-

stone.

When the temperature reaches 200 °C, release of water absorbed in sample pores re-

duces the lubrication among mineralogical grains [23]. The strength of the mudstone conse-

quently increases as the absorbed water evaporates. As the temperature continues to rise, evap-

oration continues and the rock strength gradually increases. However, when the temperature

reaches 400 °C, the clay minerals begin to decompose, and the regular crystal structure of those

minerals is broken. This has been confirmed by many previous research programs and by the

XRD analysis in this paper. Decomposition products gradually fill the holes in the mudstone.

This increases the friction among large grains and strengthens the mudstone. At the same time,

because of the large concentration of clay minerals in the mudstone, the quartz grains are not

close enough to be in intimate contact, and the large crystalline grains are more homogeneous.

In addition, the clay minerals serve a buffering function. Therefore, thermal cracking is not dra-

matic at high temperatures. In contrast, as the temperature rises, the crystalline grains become

thicker and larger because of thermal expansion, leading to densification of the larger grains and

strength increase; this is the reason for the mudstone to have such a large strength at 600 °C.

However, when the temperature is above 600 °C, dehydration of the clay minerals

ends and thermal cracking is gradually enhanced. The skeleton, consisting of the crystalline

grains, softens at high temperatures. The capacity of deformation resistance declines gradually,

which results in the decreasing of strength.

Overall, the mudstone contains significant amounts of clay minerals and water, which

undergo phase changes at elevated temperature. During this period, the composition and struc-

ture of a mudstone may change causing variation in the mechanical properties.

Figure 8. Plot of elastic and deformation

modulus of mudstone at high temperature

Figure 9. Grain model of the mudstone

Zhang, Y., et al.

Conclusions

Temperature has a significant influence on the physical and mechanical properties of

mudstone. The characteristics of mudstone at high temperature are unique because of the clay

minerals. The mudstone samples tested showed significant surficial color changes as tempera-

ture was increased. This is largely attributed to the iron-bearing clay minerals. Due to thermal

decomposition of kerogen intermixed with the clay minerals, the interior of the samples was

blackened. The major constituents of the mudstone tested were quartz, kaolinite and elpidite.

Kaolinite undergoes significant phase changes at high temperatures. When the temperature

reaches 400-500 °C, dehydration of the kaolinite begins and the regular crystal structure of ka-

olinite is broken.

Overall, the strength of these mudstones increased with an increase in ambient tem-

perature. A dramatic increase in strength occurs between 25 °C and ~ 600 °C and a slight de-

cline occurs from 600 °C to 800 °C. From 25 °C to 400 °C, because of evaporation of absorbed

water from the clays, the strength of the mudstone increases gradually. From 400 °C to 600 °C,

due to the decomposition of kaolinite and the thermal expansion of the crystalline minerals,

holes in the mudstone are filled and densification of large grains increases, resulting in an in-

crease in strength. Above 600 °C, dehydration of the clay minerals ends and thermal cracking

gradually increases, resulting in decreasing strength.

Acknowledgment

This work was financially supported by the Fundamental Research Funds for the Cen-

tral Universities (No. 2017DXQH01). The authors would express thanks to Qun Wang, Xiaolin

Wang, Lijun Chen and Xunze Zhang for their help in the experiments. Dr. Yuan Zhang would

also give thanks to China Scholarship Program.

Nomenclature

E – elastic modulus, [GPa] T – temperature, [°C]

Greek symbols

2θ – detection angles, [°] ε – strain εp

– peak strain σ – stress, [MPa] σp – uniaxial compressive strength, [MPa]

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Paper submitted: April 1, 2019 © 2021 Society of Thermal Engineers of Serbia. Paper revised: June 15, 2019 Published by the Vinča Institute of Nuclear Sciences, Belgrade, Serbia. Paper accepted: June 22, 2019 This is an open access article distributed under the CC BY-NC-ND 4.0 terms and conditions.