Evaluating the Impact of Elevated Temperatures on Engineering Properties of Sedimentary Rocks: Insights and Current Trends

Abstract

This paper presents a systematic review of research investigating the effects of elevated temperatures on sedimentary rocks. The literature was selected using keyword-based searches of titles, abstracts, and keywords in the Scopus and Web of Science databases. In total, 107 relevant articles published between 2010 and 2024 were critically examined to address research questions on temperature-treated sedimentary rocks. Furthermore, both bibliometric analysis and systematic synthesis of experimental data were performed. The review identifies sandstone as the most-studied rock type, followed by limestone. It reveals that standard experimental methods include unconfined compressive strength (UCS), Brazilian tensile strength (BTS), and P-wave velocity tests. The study’s findings indicate that a temperature threshold of 400–600 °C governs deterioration in engineering properties, driven by the quartz α–β transition in sandstones and calcite decomposition in limestones. Normalized data show that UCS, BTS, and elastic modulus decline significantly beyond this threshold, while porosity increases. The study highlights the influence of fabric anisotropy, mineralogy, and heating conditions on rock behaviour, and identifies research gaps related to confined testing, real-fire scenarios, and anisotropic rocks. Based on a comprehensive analysis of the literature, the principal factors and processes occurring at different temperature ranges were identified and discussed.

1. Introduction

Several underground engineering projects have been conducted in sedimentary rock formations [1,2], which are exposed to elevated temperatures and pressure variations due to varying geological environments. Such projects have been related to the exploitation of geothermal energy systems [3,4], deep mining [5,6,7], nuclear waste disposal [8,9], oil and gas extraction [10,11], underwater oil and gas extraction [12], shale gas extraction [11,13,14,15], and the construction of large tunnels at great depths. These projects are associated with elevated temperatures and potential geohazards. For example, underground coal fires can cause heat stresses of up to 1300 °C [4,16,17,18,19], thermal energy storage materials are exposed to temperatures of about 500–700 °C [4,20,21], and nuclear waste can release significant heat over long periods [8]. As elevated temperatures can affect the properties and stability of the rock mass, understanding their behaviour is crucial to ensuring project safety.

Recently, several studies have investigated the effects of temperature on rock [22,23,24,25], including heating time [26], pressure [27,28,29,30], and heating cycle [31,32,33]. Researchers have examined the mineralogical transformations and microstructural changes in rocks after heating [19,30,34,35], indicating that these temperature-related changes could be significant factors affecting rock engineering properties. Research has found that elevated temperatures could result in the evaporation of crystalline water, changes in crystal structure, and decomposition and transformation of minerals [31,36,37,38,39,40,41,42,43,44,45]. As a result, the compressive strength, Young’s modulus, and tensile strength of rocks, as well as permeability, density, and P-wave velocity, could also change [11,40,46,47,48,49,50]. However, such changes appear rather complex and can vary widely within the same rock type, depending on geological conditions such as temperature, pressure, and mineral composition [51].

Despite significant interest in the elevated-temperature behaviour of sedimentary rocks, a systematic, quantitative literature review that synthesizes and consolidates experimental trends, and critically, translates them into actionable insights for geotechnical engineering is lacking. The existing reviews usually focus on qualitative discussions of physical and mechanical properties, specific rock types, and countries [44]. This study intends to bridge this gap by providing a comprehensive systematic review and meta-analysis of the research published from 2010 to 2024. The main objective is not only to list the effects of elevated temperatures on sedimentary rocks, but also to synthesize the findings into a coherent framework that identifies critical thresholds, generalizes trends, and highlights implications for practical applications. Therefore, this paper addresses the following research questions:

• What are the current methods/approaches and testing procedures used to investigate the behaviour of heated rocks, and what are their advantages and limitations?
• How can elevated temperatures affect the engineering properties of rocks?
• What is the mechanism of changes in the properties of heated rocks?

The study is organized into the following sections: (1) search criteria and literature collection using systematic quantitative literature review (SQLR) methods and the Scopus and Web of Science (WoS) databases; (2) summary of search data through meta-analysis; (3) a critical review of key research articles on the effects of elevated temperature on the engineering properties of sedimentary rocks, considering rock types and temperature ranges; (4) discussion of findings, including reflections on results, identification of research gaps, and recommendations for future research directions; and (5) conclusions, providing a concise summary of the review. In this review and critical analysis, we aim to advance the current knowledge and offer valuable insights to guide future research and practical applications in this field.

2. Materials and Methods

2.1. Overview of SQL Procedure

The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) protocol [52] was followed to develop this SQL. PRISMA guidelines are a universally adopted methodology that provides a strict checklist for this work. This methodology significantly enhances the quality assurance of revision procedures and the consistency of screening processes, while ensuring their reliability. A protocol was developed for this review, outlining the article search strategy and the selection, data extraction, and analysis procedures, which are shown in Figure 1. In this study, bibliometric analysis refers to the quantitative mapping of publication trends and research using Bibliometrix version 4.1. The systematic review refers to the comparative review and normalization of experimental data from selected studies to detect trends and thresholds.

2.2. Article Record Identification

A systematic literature search strategy [53] was adopted to target peer-reviewed research articles on the physical and mechanical properties of heated sedimentary rocks. This study only included peer-reviewed articles published in English between 2010 and 2024. In addition to the explanation of the article screening and selection process utilizing the PRISMA method presented in the previous section,

Table 1. Keywords, limiters, and initial search records from each online database.

Database	Search Terms	Number of Search Results
Scopus	TITLE-ABS-KEY ((“Sedimentary rock” OR sandstone OR siltstone OR mudstone OR limestone OR “Coal deposit *” OR “Carbonate rock” OR shale OR conglomerate OR arenite OR breccia) AND (“thermal behaviour” OR “thermal behaviour” OR “thermal Stress” OR fire OR “Elevated temperature environment”) AND (mechanical OR structural OR porosity OR “Engineering Property” OR characteristics OR “Mineral composition” OR behavioural OR behaviour OR strength OR ucs OR uct OR “Brazilian tensile strength” OR bts OR “Shear Strength” OR “Tensile Strength” OR “Point load Index” OR pli))	210
WoS	TS = ((“Sedimentary rock” OR sandstone OR siltstone OR mudstone OR limestone OR “Coal deposit *” OR “Carbonate rock” OR shale OR conglomerate OR arenite OR breccia) AND (“thermal behaviour” OR “thermal behaviour” OR “thermal Stress” OR fire OR “Elevated temperature environment”) AND (mechanical OR structural OR porosity OR “Engineering Property” OR characteristics OR “Mineral composition” OR behavioural OR behaviours OR strength OR ucs OR uct OR “Brazilian tensile strength” OR bts OR “Shear Strength” OR “Tensile Strength” OR “Point load Index” OR pli))	160

Note: (*) represents a truncation symbol used to retrieve different word forms sharing the same root.

illustrates the search strategy. Table 1 lists the keywords used to search for relevant literature in the Scopus and Web of Science (WoS) databases. The search strings were written in the format recommended by the databases and used Boolean operators (e.g., “OR” and “AND”). In the end, 370 English-language articles were retrieved for further screening. These keywords helped to identify relevant literature, which was screened using PRISMA protocols.

2.3. Screening, Eligibility, Extraction, and Inclusion

The duplicates (n = 46) of the recorded articles from the database search were removed using EndNote 21, a reference manager. The remaining 324 articles were screened based on title and abstract to identify relevant articles for full-text screening. The title and abstract were evaluated based on whether the research article discusses the effects of elevated temperature on sedimentary rock’s physical and mechanical properties. The title and abstract screening excluded 250 articles, and 120 articles were judged relevant for full-text screening. During the full-text screening, 16 articles were excluded for the following reasons: 8 failed to report quantifiable engineering properties, 5 focused on igneous rocks, and 3 were non-English publications. Finally, 104 articles, plus 3 additional articles identified from cross-references, totalling 107, were included in the bibliometric analysis, and 46 articles were selected for systematic synthesis based on data completeness and innovation. Therefore, 46 articles featuring innovative methods, breakthroughs, and rigorous experimental methodologies were selected for meta-analysis.

3. Results

The study of the engineering properties of rocks at elevated temperatures has attracted growing attention in recent years (Figure 2). Although data collection began in 2010, no eligible articles were identified between 2010 and 2012, resulting in zero publications during that period. Since 2013, related research has expanded rapidly, with an average annual growth rate of 23% that peaked in 2021. This surge has been driven by the increasing demand for natural resources, sustainable resource development, and the construction of complex underground engineering projects.

3.1. Meta-Analysis Results

As shown in

Table 2. Summary of some of the experimental studies on the behaviour of rocks at elevated temperatures.

Reference	Rock Type	Temperature (°C)	Heat Duration (h)	Experiments Conducted
Ozguven and Ozcelik [42]	Limestone	Rm, 200, 400, 600, 800, 1000	1	Colour analysis
Ugur et al. [54]	Limestone	Rm, 100, 200, 300, 400, 500	3	P-wave, porosity
Ozguven and Ozcelik [55]	Limestone	22, 200, 400, 600, 800, 1000	1	UCS, BTS, abrasion resistance, Mohs hardness, porosity, density
González-Gómez et al. [56]	Limestone	25, 100, 200, 300, 400, 500, 600	1	UCS, TGA, reflectance spectra, colour analysis
Liu et al. [40]	Sandstone, mudstone	100, 300, 450, 600, 750, 900, 1200	24	TGA, XRD, UCS, MIP, TRI
Liu et al. [41]	Mudstone	100, 300, 450, 600, 750, 900	24	BTS, S-wave, XRD, MIP
Xiao et al. [57]	Coal, mudstone	25, 100, 200, 300, 400, 500	0.083	Micro-CT
Gautam et al. [58]	Sandstone	25, 100, 250, 450, 550, 650, 800, 850, 950	8	UCS
Sirdesai et al. [26]	Sandstone	Rm, 50, 100, 150, 200, 250, 300, 350, 400, 450, 500	120, 240, 360, 480, 600, 700	BTS, porosity
Zhang et al. [59]	Limestone	25, 100, 200, 300, 500, 600, 700, 800	2	XRD, TGA
Sirdesai et al. [60]	Sandstone	25, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000	120	UCS, BTS, FDXM, XRD
Shen et al. [61]	Sandstone	25, 100, 200, 300, 400, 500, 600, 700, 800, 900	0.5	XRD, SEM, TGA
Sirdesai et al. [62]	Sandstone	25, 200, 400, 600, 800, 1000	120	XRD, UCS, BTS, AE, Micro-CT
Meng et al. [63]	Limestone	20, 200, 400, 600, 800	4	UCS
Zhu et al. [15]	Shale	550, 650, 750	0.5, 0.33, 0.17, 0.9	SEM, Micro-CT
Liu et al. [64]	Sandstone	Rm, 200, 400, 600, 800, 1000	None	UCS, P-wave, AE, XRD
Ersoy et al. [34]	Calcarenites, chalks	25, 105, 200, 400, 600, 800, 1000	0.16, 0.33, 0.5, 1, 2, 3	P-wave, BTS, SEM
Orlander et al. [12]	Sandstone	50, 100, 150, 170	None	TRI, P-wave, S-wave
Tripathi et al. [19]	Sandstone	25, 100, 150, 300, 400, 500, 600, 700, 800	24	P-wave, BTS, UCS, SEM, TGA, porosity
Huang et al. [37]	Sandstone	25, 200, 300, 400, 500, 600, 700, 800, 900, 1000	4	XRD, P-wave, BTS, thermal conductivity, porosity, SEM
Vidana Pathiranagei et al. [65]	Sandstone, argillite	25, 100, 200, 300, 400, 500, 600, 700, 800	2	XRD, TGA
Chen et al. [66]	Sandstone	25, 200, 400, 600, 800	2	SHPB, SEM, XRD
Vigroux et al. [67]	Limestone, sandstone	Rm, 200, 400, 600, 800	1	TGA, UCS, BTS, P-wave
Ersoy et al. [35]	Clayey rock	Rm, 200, 400, 600, 800, 1000	2	XRD, TGA, BTS, UCS, P-wave
Meng et al. [28]	Limestone	20, 200, 400, 600, 800	4	TRI
Wang et al. [68]	Sandstone	Rm, 400, 500, 600, 700, 800, 1000	6	UCS, NUM
Xiao et al. [69]	Sandstone	25, 200, 400, 600, 800, 1000	4	NMR, SEM
Shtober-Zisu and Wittenberg [70]	Carbonate rock	880	None	Field test
Vidana Pathiranagei and Gratchev [71]	Sandstone	25, 400, 600, 800	2	TRI
Wang et al. [72]	Sandstone	25, 400, 500, 600, 700, 1000	6	BTS, UCS, NUM
Hao et al. [73]	Sandstone	25, 150, 300, 450, 600, 750, 900	2	NMR, XRD, P-wave, UCS, AE
El Jazouli and Tsangouri [74]	Limestone, sandstone, quartzite, ferrous quartzite, Silestone	1100	2	Three-point bending, P-wave, UCS, AE
Vidana Pathiranagei et al. [75]	Sandstone	25, 400, 600, 800	2	Micro-CT, XRD, SEM, PLI, TGA
Meng et al. [76]	Sandstone	20, 200, 400, 600, 800, 1000	2	SHPB, P-wave, NMR
Ge et al. [31]	Sandstone	25, 100, 200, 300, 400, 500, 600, 700, 800	2	UCS, P-wave
Kang et al. [77]	Shale	25, 200, 300, 400, 500, 600	2	BTS, in situ hydraulic fracturing, and elevated temperature steam fracturing of metre-scale oil shale
Zhang et al. [78]	Sandstone	25, 200, 400, 600, 800	3	UCS, NMR
Wang et al. [79]	Sandstone	25, 100, 200, 400, 600, 800, 1000, 1200	2	TGA, NMR, P-wave, UCS, BTS
Guo et al. [80]	Sandstone	−30, 150	2	TRI, NUM, AE
Shen et al. [81]	Sandstone	25, 200, 400, 600, 800	4	NMR, XRD, UCS
Zhou et al. [14]	Shale	25, 50, 100, 200, 300, 400	None	UCS, BTS, TRI, NUM
Zhao et al. [82]	Sandstone	500	None	NMR
Daoudi et al. [22]	Limestone	Rm, 650, 850	2	Modify test
Guo et al. [83]	Shale	899, 1243, 1599	None	NUM
Bi et al. [84]	Sandstone	Rm, 100, 200, 300, 400, 500, 600, 700	3	Permeability, NUM
Shen et al. [85]	Sandstone	25, 200, 400, 600, 800	4	UCS, P-wave

(PS: AE = acoustic emission, BTS = Brazilian tensile strength tests, FDXM = four-dimensional X-ray microscopy, MIP = mercury intrusion porosimetry, NMR = nuclear magnetic resonance spectroscopy, NUM = numerical simulation, PLI = point load index test, SEM = scanning electron microscope, SHPB = split Hopkinson pressure bar, TGA = thermogravimetric analysis, TRI = triaxial compression test, UCS = unconfined compression test, XRD = X-ray diffraction).

, the meta-analysis included 46 articles that provide comprehensive research on rock engineering properties under elevated temperatures. These factors include rock types, temperature ranges, heating durations, and experimental methods. It is noted that only studies directly related to rock thermal properties were included. As a result, this analysis included 13 different rock types, 33 distinct target temperatures, and 28 experimental methods.

3.2. Rock Types

Figure 3 offers valuable insights into current research priorities and the geological conditions being targeted. Sandstone is the most frequently investigated rock, appearing in 31 studies, followed by limestone (10 studies) and shale (4 studies). Sandstone, a common surface rock widely used in construction, coal mining, and as a building material [25,75,76,86,87], has been extensively studied and is often selected for experimental setups due to its prevalence. Limestone is primarily used in construction and the restoration of historical sites [46,59,87]. Shale is closely associated with petroleum extraction technologies, including microwave extraction and high-pressure steam techniques [14,15,88,89]. Less commonly studied rocks include calcarenite, carbonate, quartzite, and siltstone [41,57]. Although research on these rocks remains limited [14,74,90,91], understanding their behaviour after elevated-temperature treatment is of considerable significance.

3.3. Temperature Range and Heating Duration

Temperature (Figure 4) and heating duration (Figure 5) are critical parameters for studying the elevated-temperature behaviour of rocks. The analysis indicates that commonly investigated temperatures are 200 °C, 400 °C, 600 °C, and 800 °C [29,51]. Research within the range of 200 °C to 600 °C is essential for identifying strength thresholds in sedimentary rocks, while 800 °C is often used to examine the effects of extreme heat on rock properties. For example, a significant strength variation in sandstone exhibits a primary threshold between 400 °C and 550 °C. It is noted that investigations at ultra-high temperatures, such as 899 °C, 1243 °C, and 1599 °C, are conducted using specialized heating techniques [83].

Figure 5 indicates that the most common heating duration is approximately 2 h. Previous studies suggest that 2 h of heating is sufficient to initiate temperature-related changes in rock strength [34,35,65]. Sirdesai et al. [26] have examined the effects of prolonged heating on rock properties, with durations extending to several hundred hours.

To minimize the impact of heat shock on rock properties, a relatively low heating rate of 10 °C/min is commonly adopted [92]. Research by Becattini et al. [20], Li et al. [93], and Rossi et al. [94] demonstrates that variations in heating rates can significantly influence the heat expansion coefficient and the development of microcracks in rocks.

It is important to note that while common heating conditions are reported in previous studies, significant variability in the heating duration and rate exists across studies. The normalized trends and thresholds presented in the subsequent sections pool data across different thermal histories, which introduces scatter and should be considered when interpreting the results. The identification thresholds may shift depending on the specific heating conditions and testing protocols.

3.4. Experimental Methods for Studied Rocks’ Properties

Figure 6 summarizes the most common laboratory tests used to examine rock properties. Among these, unconfined compressive strength (UCS), Brazilian tensile strength (BTS), point load index (PLI), Mohs hardness, and triaxial compressive strength (TRI) are the most widely applied methods. Their high citation frequency in the literature indicates that these tests are the primary focus of research in this field. In contrast, although TRI is considered an important method, its application remains limited due to significantly higher equipment and operational costs.

Other commonly used tests include mercury intrusion porosimetry (MIP), micro-computed tomography (Micro-CT), and nuclear magnetic resonance (NMR), which are primarily employed to determine rock porosity and density [4,11,43,54,81]. Micro-CT, in particular, enables the direct visualization of the three-dimensional distribution and interface features of internal pores, providing valuable insights into crack evolution under temperature effects. However, porosity measurements from [15,75] obtained using Micro-CT can differ from those obtained using MIP [40,41], particularly in capturing sub-resolution micropores, depending on the segmentation thresholds. NMR, which measures fluid in pores, often shows good agreement with MIP for porosity in sedimentary rocks such as sandstone after temperature treatment [73,81], as both techniques are sensitive to connected pore volumes. Digital image analysis, on the other hand, relies on high-resolution microscopy or CT images and facilitates quantitative and visual descriptions of pore networks through image segmentation and two- or three-dimensional reconstruction.

Additionally, ultrasonic P-wave velocity testing is widely used to measure the longitudinal wave velocity of sedimentary rocks after heating and to infer changes in rock strength. Several studies have demonstrated a correlation between P-wave velocity and mechanical strength, highlighting its role in monitoring thermal damage and assessing engineering stability. Overall, each method has distinct advantages and limitations, and their combined application can provide a more comprehensive understanding of the damage and evolution mechanisms in sedimentary rocks under elevated-temperature conditions [49,95].

Other frequently employed techniques include X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD is primarily used to analyze changes in mineral composition, while SEM focuses on the morphology and microstructural features of minerals.

3.5. Effect of Temperature on Rock Mineral Properties

Rocks generally consist of minerals, pores, and fractures. Mineralogical transformations at elevated temperatures are a critical factor influencing their engineering properties. In sedimentary rocks, elevated-temperature mineral changes typically occur in two distinct stages, with a widely accepted threshold range of 450 °C to 600 °C.

Stage 1, occurring below this threshold, involves processes such as sintering, cementation, and fusion among clay minerals, as well as the decomposition of secondary (non-primary) minerals [35,40]. During this stage, changes in rock hardness are challenging to predict and may vary—increasing, decreasing, or remaining relatively stable.

Stage 2, beyond the threshold, is characterized by transformations in primary strength-governing minerals such as quartz [36,41].

Table 3. Summary of changes in common minerals at different temperatures.

Mineral	Temperature (°C)	Type of Change	Description of Change
Quartz	~573	α–β transition	Crystal structure transformation accompanied by volume expansion, leading to microcrack initiation and the enlargement of pore spaces.
	~870	Transition to hexagonal β-tridymite	Loss of crystallinity and weakening of the crystal framework.
Clay minerals	100–200	Dehydration	Removal of pore water, resulting in a slight loosening of the microstructure.
	400–600	Dehydroxylation	Breakdown of hydroxyl bonds and formation/propagation of microcracks.
Calcite (CaCO3) data	~620	Onset of calcination	Beginning of decomposition into quicklime (CaO) with the release of CO2.
	~820	Peak of calcination	Major conversion of calcite into quicklime; significant reduction in mass and density.
Other minerals	~650	Initial mineral reactions and melting	The formation of microcracks accompanies early synthesis reactions and the onset of partial melting.
	~1000	Crack penetration through crystals	Cracks propagate through grains, reducing crystal integrity and grain bonding strength.
	~1200	Partial melting	Local melting occurs, increasing pore connectivity and overall porosity.

summarizes the common mineral types and temperature-related changes in sandstone and limestone. These changes result in noticeable voids and a significant reduction in rock strength.

3.6. Effect of Temperature on Engineering Properties of Rock

Under elevated temperatures, the formation of micro-pores and fractures, as well as chemical transformations in minerals, can significantly alter the physical and mechanical properties of rocks. The following sections examine the effect of temperature on the engineering properties of rocks. To enable comparison across studies with different initial rock properties, laboratory data were normalized. All rock property values (e.g., density, UCS, BTS) from the same study obtained at elevated temperatures were normalized to room temperature (25 °C). Normalized values of 1 indicate no change, and values < 1 indicate a decrease. This normalization facilitates the identification of general patterns but does not eliminate the inherent variability caused by differences in mineralogy, porosity, and fabric across rock samples from different geological environments. Hence, the normalized data presented should be interpreted as illustrating qualitative trends rather than precise quantitative relationships.

3.6.1. Density and Porosity

Figure 7 illustrates the variation in normalized density for limestone and mudstone. As the temperature increases, the normalized density values generally decrease [35,41,55]. For example, Ugur, Sengun, Demirdag, and Altindag [54] observed a monotonically decreasing density with increasing temperature for the mudstone specimen. In contrast, the density of the limestone specimen increased from room temperature to 200 °C, then decreased with further increases in temperature [35,54,55]. This initial increase is typically attributed to differential thermal expansion of the mineral composition, which can open existing microstructural defects or form new microcracks at grain boundaries or within grains [60,64,70,72].

Figure 8 presents the normalized porosity for sandstone, limestone, and mudstone. Figure 8a shows the variation in porosity for sandstone, where porosity initially increases slightly up to about 200 °C and then decreases beyond this temperature. This initial increase is typically attributed to the thermal expansion of minerals and the closure of microcracks [60,64,71,73].

Figure 8b,c illustrate the changes in porosity with temperature for limestone and mudstone. In these rocks, porosity generally increases with temperature, particularly at elevated temperatures [40,41,54,55,57,96]. This increase is primarily attributed to the formation of microcracks as the temperature rises.

Figure 8. Normalized porosity for heated rocks: (a) sandstone [37,40,73,78,97]; (b) limestone [54,55,96]; (c) mudstone [40,41,57].

Figure 8. Normalized porosity for heated rocks: (a) sandstone [37,40,73,78,97]; (b) limestone [54,55,96]; (c) mudstone [40,41,57].

3.6.2. Permeability

Research on the elevated temperature permeability of rocks is limited, despite its importance in controlling the movement of air and water through rock formations. Increased permeability at elevated temperatures can exacerbate or facilitate air exchange during underground fires, influencing the effectiveness of firefighting measures for subsurface coal fires.

Rock permeability is closely linked to porosity and the distribution of voids [84]. Previous studies indicate that permeability tends to increase at approximately 200 °C, a trend generally associated with increasing porosity and microcrack connectivity [41,92]. Furthermore, the cooling method applied after heating plays a critical role in permeability changes. Heating shocks caused by rapid low-temperature cooling can lead to substantial increases in permeability [50,98].

3.6.3. P-Wave Velocity

Figure 9 shows the variation in normalized P-wave velocity with temperature for sandstone and limestone. Regardless of rock type, P-wave velocity decreases gradually at first and then drops sharply as temperature rises [19,35,54,99]. This trend is attributed to several geological factors, including differences in thermal expansion among mineral grains, mineral phase transitions that induce microcracks, and increased porosity, all of which reduce rock stiffness and slow seismic wave propagation. In addition, the correlation between P-wave velocity and mechanical strength is often reported in individual studies for particular rock types. However, these relationships are usually empirical, and their applicability may be influenced by factors such as anisotropy, saturation state, and measurement procedures.

In practical applications, particularly in complex geological and geotechnical terrains, the reliability of P-wave velocity as an indicator of temperature damage can be enhanced via several methods. For example, in anisotropic rocks such as shale and laminated sandstones, velocity measurements must be taken in different directions relative to the bedding plane to develop directional velocity and strength correlations [14,74]. Saturation effects can be mitigated by standardizing moisture conditions prior to testing or by employing saturated-to-dry velocity ratios as a damage index, as water-filling pores can temporarily cause the measured P-wave velocity to increase even in cracked rocks [50,73]. Furthermore, integrating P-wave velocity with non-destructive methods, including unclear magnetic resonance (NMR) and electrical resistivity, can help decouple the impact of porosity, crack geometry, and fluid content, leading to robust multi-variable damage models for field evaluation [4,73,81].

3.6.4. Unconfined Compressive Strength (UCS)

Figure 10 illustrates the variation in normalized UCS with temperature for sandstone, limestone, and mudstone. For sandstone specimens (Figure 10a), the UCS initially increases slightly, then decreases at moderate temperatures. Beyond the threshold temperature, UCS declines sharply as temperatures rise from 500 °C to 700 °C [51,62].

For example, sandstone strength reduction occurs between 400 °C and 600 °C, mainly due to the α-β quartz transition around 573 °C [36]. The transformation is accompanied by the volumetric expansion of quartz grains, which develop internal stresses [40,51]. These stresses induce widespread intergranular and intragranular microcracks, weakening cementation bonds and increasing porosity and crack connectivity [19,37,60,64]. Concurrently, clay minerals undergo dehydroxylation in this range, releasing structural-bound waters and further weakening the matrix [35,40]. The combined effects of quartz expansion and clay weakening lead to irreversible degradation of the rock’s load-bearing framework, marked by decreases in UCS, tensile strength, and elastic modulus [26,37,41,51,62].

In high-temperature environments such as underground coal fires [16,19], geothermal reservoirs, and nuclear waste storage systems [4,20], sustained exposure in this crucial range can result in progressive damage accumulation. Microcracks initiated during heating can propagate under in situ stresses, accelerating time-dependent deformation and possibly leading to delayed failure [31,33]. For sandstone-dominated projects like tunnels, the 400–600 °C temperature threshold represents a stable limit; engineering designs should avoid sustained exposure beyond 400 °C or ensure that additional monitoring supports are provided to account for strength reduction, increased porosity and permeability, and decreased long-term durability [19,29,50].

The threshold temperatures reported in the literature vary considerably. In this synthesis, threshold refers to the temperature at which a marked change in the trend of a mechanical property is observed, usually corresponding to underlying mineralogical transformations. This is not the same as the mineralogical threshold, which refers to a particular phase transition temperature of rock mineral compositions. Figure 10b summarizes these thresholds against the percentage of studies in which they were reported. For example, the mechanical property threshold of 300 °C was noted in approximately 30% of the studies analyzed, indicating where significant changes in UCS were first reported, usually preceding significant mineralogical transformations. Overall, about 30% of studies indicate that significant changes occur after exceeding 300–400 °C.

For limestone specimens, the variation in normalized UCS with temperature is shown in Figure 10c,d. Results indicate that UCS generally decreases gradually as temperature increases. In contrast, mudstone, which was studied by Liu, Yuan, Sieffert, Fityus, and Buzzi [40], showed a different trend: its normalized UCS increased with temperature up to about 900 °C, then dropped sharply at 1200 °C. The initial increase in UCS for mudstone is likely due to dehydration and densification, which harden the specimen between 100 °C and 900 °C. The subsequent sharp decrease can be attributed to partial melting, vitrification, and microcrack formation, all of which significantly reduce stiffness and strength. It should be noted that the normalized UCS data and thresholds synthesized in this section may not directly apply to high-stress geological conditions.

3.6.5. Elastic Modulus (E)

The modulus of elasticity (E) measures rock stiffness and is derived from stress–strain curves. Figure 11 presents the variation in normalized E with temperature for different rock types. For sandstone (Figure 11a), normalized E exhibits trends similar to those of normalized UCS, showing more complex behaviour than in limestone and mudstone. Two distinct patterns can be observed: (a) increase followed by decrease: E initially rises and then declines with temperature, with the transition occurring around 400 °C [19,47,58,72,73]. (b) Monotonic decrease: E decreases continuously as the temperature increases [73,78,85]. These two different trends observed may be related to initial rock behaviours: (a) an increase then decrease is common in sandstones with rich clay cement or higher porosity, where initial stiffness may occur due to water dissipation, and (b) monotonic decrease is often noticed in well-cemented, quartz-rich sandstones. This difference requires further controlled comparative experiments.

For limestone, normalized E follows a pattern similar to UCS: a gradual decrease at moderate temperatures, followed by an accelerated decline beyond approximately 400 °C (Figure 11b). Mudstone behaves differently, with normalized E increasing steadily as the temperature rises, except for a slight dip at 600 °C, mirroring the UCS trend.

3.6.6. Poisson’s Ratio

Poisson’s ratio, which reflects the lateral strain response of rocks, has also been investigated. Studies suggest that changes in Poisson’s ratio tend to be inversely related to variations in Young’s modulus [97]. For instance, Rathnaweera et al. [100] reported that the Poisson’s ratio of Hawkesbury sandstone decreased as the temperature increased from 25 °C to 1000 °C.

3.6.7. Brazilian Tensile Strength (BTS)

The Brazilian tensile strength (BTS) is an indirect measure of tensile strength, determined using Brazilian disc specimens loaded in a Brazilian testing apparatus [101]. Figure 12 illustrates the variation in normalized BTS with temperature for different sedimentary rocks. For sandstone specimens (Figure 12a), two distinct trends are observed: (a) initial increase followed by rapid decrease: BTS rises slightly at moderate temperatures (from room temperature to 200 °C [19,26,37] and up to 400 °C [62,67], then declines sharply as temperature continues to increase. The transition typically occurs between 200 °C and 500 °C. (b) Monotonic decrease: some sandstone specimens show a linear reduction in BTS with increasing temperature [26].

Similar patterns are observed for limestone and mudstone (Figure 12c–e), where BTS initially increases and then decreases with increasing temperature [35,41]. The transition temperature for these rocks is approximately 400 °C; it refers to the temperature at which the normalized BTS reaches a maximum before declining. This applies to studies where such a peak is identifiable; however, in some limestone specimens, BTS decreases continuously without a precise transition temperature [55,67].

3.6.8. Point Load Index (PLI) and Other Tests

Over the past decade, several researchers have employed indirect testing methods to assess thermal damage in sedimentary rocks. These methods include Point Load Index (PLI), colour analysis, abrasion resistance, Mohs hardness, P-wave velocity, and three-point bending tests [42,54,55,74,75].

These techniques offer notable economic advantages, requiring minimal sample preparation, simple equipment, and straightforward procedures. Experimental results have demonstrated reasonable accuracy, with some tests effectively predicting changes in the engineering properties of sedimentary rocks after exposure to elevated temperatures. For example, PLI and Schmidt hammer rebound hardness tests exhibit a clear positive correlation with uniaxial compressive strength (UCS) and show significant changes near the threshold temperature, effectively reflecting variations in rock properties under thermal conditions. However, these methods also have limitations. Their applicability may be limited, and their accuracy is generally lower than that of direct testing methods.

4. Discussion on Rock Behaviour at Different Temperatures

This section provides a synthesis and comprehensive analysis of the experimental results trends discussed in Section 3.4. It presents the observed changes in engineering properties as a function of underlying microstructural evolution and mineralogical transformations, which explain the distinct characteristics of sedimentary rocks under elevated-temperature conditions. Sedimentary rocks are typically composed of diverse mineral constituents. For example, in rocks rich in quartz and kaolinite, elevated temperature damage behaviour can generally be divided into three stages:

(1)

Room temperature up to 400 °C: mineral structures remain largely stable during this stage. Free water and some bound water within the rock begin to vaporize. Between 100 °C and 200 °C, the thermal expansion of mineral grains may close pre-existing microcracks but can also introduce new ones. Due to differences in thermal expansion coefficients among minerals, stress mismatches at grain boundaries promote the propagation of initial defects.

(2)

Temperatures of 400 °C to 600 °C: quartz undergoes an α–β phase transition, causing significant expansion and the formation of cracks within grains and the surrounding matrix. Kaolinite loses structural water, reducing lattice stability and making this a critical stage for strength degradation. Other minerals, such as siderite, muscovite, and illite, also exhibit notable thermal reactions or transformations during this phase.

(3)

Temperature above 600 °C: this stage is characterized by pronounced thermal damage. The continued expansion of mineral particles induces numerous new fractures and voids. Minerals such as calcite and feldspar undergo significant thermal decomposition or phase changes, leading to disintegration and separation from adjacent grains, thereby further accelerating crack development and connectivity.

Although mineralogical transformations and crack development at elevated temperatures share certain similarities across sedimentary rocks, variations in mineral composition can yield different engineering properties when exposed to elevated temperatures.

4.1. Sandstone

The temperature degradation of quartz- and clay-rich sandstone follows a phase progression caused by mineral transformations, as presented in Figure 13. At temperatures up to 400 °C, dehydration and minor grain expansion occur, and sometimes close the initial microcracks. The primary damage thresholds are between 400 °C and 600 °C, where the α–β quartz transition at 573 °C induces volumetric expansion, leading to widespread microcracks and increased porosity. This explains the sharp decrease in UCS and P-wave velocity observed in Section 3.6.3 and Section 3.6.4. Beyond 600 °C, quartz recrystallizes (~870 °C), and its partial melting above 900 °C significantly weakens the rock microstructure and improves pore connectivity, leading to a complete loss of cohesion.

4.2. Limestone

Limestone’s behaviour is controlled by the calcination of calcite, differing from the quartz-driven behaviour of the sandstone, as illustrated in Figure 14. While the quartz mineral grains in the sandstone undergo an abrupt, expansive phase transition at 573 °C, limestone undergoes a progressive, endothermic decomposition reaction: between 600 °C and 800 °C, CaCO₃ decomposes into CO₂ [39,59,96]—this primary difference in mechanism results in distinct temperature-damage patterns. The α-β quartz transition in sandstone is a rapid, reversible crystalline rearrangement that involves significant volumetric expansion, leading to immediate and increasing microcracks [36,40]. Conversely, calcite calcination is a progressive, irreversible chemical decomposition. It does not involve a rapid volume change in the solid crystal structure, but rather a continuous mass loss and a reduction in density to release CO₂, coupled with the formation of microcracks from gas pressure and the weakening of the remaining CaO matrix [39,59].

This mechanism difference underpins the contrasting engineering properties trend: sandstone shows a sharp, threshold-based failure in UCS and elastic modulus between 400 and 600 °C, while limestone exhibits a pronounced, gradual, continuous reduction in strength and stiffness, significantly accelerating only after sustained exposure above approximately 600 °C [35,46,59]. The decomposition results in the formation of microcracks that deteriorate strength and microstructural integrity, leading to reduced density and mass loss. This mechanism underpins the gradual, continuous decrease in UCS and E in limestone, as explained in Section 3.6.4 and Section 3.6.5, in contrast to the threshold-based collapse in sandstone.

4.3. Mudstone

In iron-bearing mudstone, elevated-temperature damage proceeds through successive phases associated with their clay and oxide mineralogy, as shown in Figure 15. Dehydration (up to 200 °C) and dehydroxylation of clays (from 400 °C to 600 °C) release water and initiate cracking. The distinctive temporary strengthening sometimes observed at intermediate or moderate temperatures (Section 3.6.4) could be attributed to the sintering and cementing action of Fe oxides above 600 °C. However, extreme temperatures, these transformations, and the formation of hematite at 900 °C ultimately result in mass loss, a decrease in density, and the weakening of the mudstone structural framework.

4.4. Shale

The shale shows mineralogical processes similar to those in mudstone; however, its response is primarily shaped by its laminated fabric. While the mineral changes are similar to those in mudstone, temperature damage propagates along weak bedding planes (Figure 16). Shale’s thermal degradation is dominated by its laminated, anisotropic fabric. While mineral changes mirror those in mudstone, damage propagates preferentially along weak bedding planes (Figure 16). At 200 °C and above, dehydration and differential temperature expansion occur between clay-rich minerals. Between 400 and 600 °C, dehydroxylation of clay minerals further weakens laminations, driving crack growth along bedding. At temperatures above 600 °C, cracks coalesce, leading to macroscopic delamination. This anisotropy means that shale’s in situ thermal stability depends more on bedding-plane strength than on bulk compressive strength, representing a critical gap in current laboratory characterization.

4.5. Limitations

The existing studies have identified key factors and processes that occur at elevated temperatures and their influence on rock properties. However, several limitations remain, reflecting more profound contradictions within the literature:

• The basic discontinuous, inhomogeneous, anisotropic, and non-elastic (DIANE) nature of rocks [88] results in inconsistent elevated temperature responses, even within the same rock type. This intrinsic variability arising from fracture, coupled with the normalization issue noted in Section 3.5, is a key source of conflicting findings for threshold temperatures and property correlations.
• The research is heavily skewed towards sandstone, with limited attention paid to limestone, mudstone, and shale. This bias neglects sedimentary rocks such as conglomerates and siltstones, whose behaviour at elevated temperatures may differ significantly, thereby limiting the generalizability of the current research and models.
• Most studies rely on laboratory experiments, particularly unconfined tests to evaluate elevated-temperature effects, often neglecting the role of confining pressure and the pore fluid present in deep geological settings—a critical parameter influencing rock strength. This shows a significant gap between laboratory experimental data and field conditions, particularly for geothermal and nuclear disposal projects. Triaxial testing may provide a more realistic simulation of the high confining pressures and stress conditions encountered in situ.
• The existing literature primarily examines controlled-temperature exposure rather than real-fire scenarios involving open flames, rapid heating and cooling, and oxidative atmospheres. The combined effects of temperature and flame impingement remain poorly understood. Therefore, investigations into the combined effects of elevated temperatures and open flames on rock engineering properties are significant.
• The variations in heating rates, cooling conditions, sample sizes, and testing procedures introduce substantial scatter and hinder the development of robust predictive models.

5. Future Research Recommendations

Based on the comprehensive review of studies on temperature-treated sedimentary rocks over the past decade [10,11,12,13,14,15,16,19,22,23,24,25,26,27,28,30,31,32,33,34,35,37,38,39,40,41,42,43,46,47,48,49,50,51,54,55,56,57,58,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,89,96,97,98,99,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136], the following recommendations for future research are proposed:

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Correlation Between Engineering Properties and Temperature: future investigations should aim to establish robust correlations between engineering properties and thermal exposure. Developing predictive models and correlation formulas will enhance understanding of how temperature variations influence rock behaviour under thermal stress.

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Mechanistic Formulation of Thermal Damage: research should focus on elucidating the mechanisms underlying thermal damage in rocks and on developing predictive formulas. Such formulations will be instrumental in assessing failure processes at elevated temperatures and informing engineering design.

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Future research could explore the role of AI and machine learning in modelling rock behaviour at elevated temperatures. Future studies should aim to develop advanced testing methods and damage models, while also validating their safety and applicability in real-world engineering scenarios.

Addressing these research directions will improve safety and efficiency in underground engineering projects and support informed decision-making in design and policy development.

6. Conclusions

This systematic review synthesizes the current knowledge on the effects of elevated temperatures on the engineering properties of sedimentary rocks. The main conclusions, and their implications for geotechnical and engineering practice, are as follows:

• The review reveals that previous studies focus on sandstone, with limestone and shale receiving less research attention. Experimentally, UCS and BTS tests are the most prevalent. This research bias may limit the applicability of the existing models to a wide range of sedimentary rocks, such as siltstones and conglomerates encountered in real projects. In addition, the widespread use of unconfined compressive tests may fail to replicate in situ stress conditions, potentially overestimating rock strength in deep geological settings, including nuclear waste reservoirs and geothermal energy systems. It is suggested that engineers apply experimentally derived strength parameters with correction factors for confining pressure, especially in underground-related projects.
• A consistent and critical transition zone between 400 °C and 600 °C governs significant strength loss. In quartz-rich sandstone and clay-rich mudstone, this is triggered by α-β quartz transitions and clay dehydroxylation, resulting in a precipitous decrease in strength and stiffness, whereas limestone exhibits progressive deterioration initiated at 600 °C due to calcite calcination. This threshold is important for risk assessment and design in high-temperature environments. For projects in sandstone-dominated terrains, designs should account for potential strength loss within a 400–600 °C range. For projects involving limestone, designs should account for progressive, long-term strength decrease under sustained temperature stress.
• The sedimentary rocks, shale, and other laminated rocks exhibit fabric-controlled temperature damage, in which degradation propagates along weak bedding planes rather than within the rocks. Simultaneously, the correlation between P-wave decrement has been established. The stability of laminated rocks under high-temperature stress depends more on bedding-plane strength than on rock compressive strength. This necessitates tailored testing protocols for projects such as shale gas extraction and nuclear waste disposal in shale formations. Additionally, P-wave velocity measurements in structures and rocks enable rapid mapping of heat-affected areas and the estimation of residual strength.
• The review consolidates normalized data trends for key properties such as BTS, UCS, porosity, and elastic modulus across different temperatures, providing a valuable dataset. However, significant research gaps remain, particularly regarding the effects of confining pressure, real-fire scenarios, and standardized testing protocols. The compiled normalized data and identified threshold temperatures serve as important references for calibrating the thermomechanical constitutive models used in numerical simulations, thereby enhancing the predictive accuracy of rock behaviour under temperature-stress conditions. To bridge the gap between experimental and field conditions, future studies and practice should prioritize triaxial compression testing under confining pressures and develop sedimentary rock models that account for the anisotropic and discontinuous nature of these rocks.