Experimental investigation into the effects of composition and microstructure on the tensile properties and failure characteristics of different gypsum rocks

The present work investigated the differences in the composition and internal microstructure of four types gypsum rock—fiber gypsum, transparent gypsum, alabaster, and ordinary gypsum by X-ray fluorescence spectrometry, X-ray diffraction, scanning electron microscope and Brazilian split test, and analyzed its effects on the tensile strength and fracture characteristics of gypsum rock. For alabaster, fiber gypsum, transparent gypsum, and ordinary gypsum, CaSO4·2H2O is the main component with 72.78%, 72.72%, 72.57%, and 71.51% content, and tensile strength of 1.79, 2.22, 3.22, and 4.35 MPa, respectively. In addition, the fracture line is arc-shaped, vertical, and zigzag for fiber gypsum, ordinary and transparent gypsums, and alabaster, respectively. On the microscopic level, fiber gypsum has an evident striated structure while the gradual increased pore development for alabaster, transparent gypsum, and ordinary gypsum. Gypsum rock has an obvious layered crystal structure with the increase of CaSO4·2H2O, contributing to the phenomenon with a larger grain size and lower tensile strength. In addition, the number of particles for alabaster, transparent gypsum, and ordinary gypsum increased in turn, while their particle size decreased uniformly, indicating that the lower CaSO4·2H2O content, the more sufficient energy accumulation and release. This paper can provide a theoretical basis for the analysis of the mechanical properties of rocks with different mineral composition and contribute to the design for different ore grades mining.

Gypsum has long been considered an important raw material for engineering construction. Based on its microstructure and component, gypsum rocks can be classified into several kinds. For the mining of underground gypsum, room and pillar mining method is commonly adopted to obtain by "retaining pillars and mining room" to treat goaf [1][2][3][4][5][6] . After ore extraction, original rock stress redistributes, thereby the mechanical properties of the retained pillars are closely related to the stability of the goaf. Existing research shows that the tensile strength of a rock is significantly lower than its compressive strength. Thus, the tensile performance of a rock is a critical factor in evaluating its stability in an engineering system 7,8 . In addition, the mineral composition and microstructure characteristics of a rock directly affect its mechanical properties [9][10][11][12] , thereby making them key factors in rock damage and fracture.
There are direct and indirect methods often used in measuring tensile strength. To avoid the irregularity of tensile stress, indirect tensile method or Brazilian disc split test is often used locally and abroad for rock materials [13][14][15][16][17][18] . This method investigates the influence of different loading rates on the tensile strength of a rock in a micro view and energy evolution [19][20][21] . Rafiei Renani and Martin 22 studied the effect of rock size on its tensile strength by strength statistical theory and fracture mechanics. Masoumi et al. 23 found that all rock types follow a Brazilian disc samples. The Brazilian split test samples were obtained as follows. The four types gypsum rocks were obtained on site and processed into samples sized Φ 50 × 25 mm by coring, cutting, grinding, and other processes. To reduce the deviations of the test results, samples with obvious defects were removed after macroscopic analysis. Sonic velocity measurement analysis was conducted and the rock samples with similar wave velocities were selected as the test samples for each group. The longitudinal section characteristics of the gypsum rock samples are shown in Fig. 2.
As shown in Fig. 2, the experimental samples include three kinds of high-purity gypsum rocks-fiber gypsum, transparent gypsum, and alabaster, low-ore grade transparent gypsum rock, referred to as ordinary gypsum, was selected for the comparative analysis. In addition, fiber gypsum has a translucent characteristic with compact striped fiber structure distributed vertically along the axial direction. Transparent gypsum has a dense and translucent appearance with dark color on some areas, and with obvious crystalline particles. For alabaster, there are large white crystal particles distributed with a low compactness visible to the naked eye, thereby presenting its white color. Ordinary gypsum has high density and dark gray appearance with white spots and a clastic feature similar to that of transparent gypsum.
SEM samples. Four types of gypsum rock blocks were crushed to obtain rectangular test samples with a side length less than 1 cm and one untreated main plane (length × width), and each group had at least two finished rock samples. The main plane was considered as the observation surface, meanwhile, the debris and surface residue attached to the surface was removed by a suction balloon and in high-purity alcohol immersion, respectively. Finally, gold powder was sprayed on the sample to increase the conductivity.
Material composition analysis equipment. XRD tests were performed by the D/max 2500 PC XRD device (radiation, 2 h = 3°-90°) with an operating voltage of 40 kV, emission current of 40 mA, and step size of 0.02. The XRF element detection equipment is PANalytical Axios, Range of measurable elements: 11 Na-92 U. The elementary analysis test equipment is Elementar vario PYRO which can be used to analyze the isotopic ratio of O, H and C, N, S.   [40][41][42] , and among which the more commonly used is shown in Eqs. (1) where σ t is the tensile strength of the rock, P is the failure load, D is the diameter of the Brazilian disc sample, t is the height of the Brazilian disc sample. Equation (1) is commonly used because of its simplicity. However, the test materials should meet two criteria: (1) homogeneity and isotropy, and (2) macro failure crack at the center of the disk. In this study, only the striped structure of fiber gypsum rocks was characterized by bedding. In addition, this test aims to compare the different laws on the tensile strength of the four kinds gypsum rocks. Therefore, Eq. (1) could satisfy the research needs.

Results and analysis
Mineral composition. XRF element analysis and XRD were carried out to investigate the differences in the composition of different gypsum rocks and the influence of composition on the tensile strength.
XRF element analysis. XRF was performed on each group of samples to obtain the elemental composition and content of the gypsum rocks. In addition, it was determined that the composition of the rock included components in the form of oxides, and the element contents are shown in Table 1. Table 1 shows that the number of elements in ordinary gypsum rock is higher than that in the other three gypsum rocks; however, the four types of gypsum rock comprise the same main elements, which include, in decreasing order, O, Ca and S. The contents of O, Ca, and S in ordinary gypsum are 45.99%, 31.24%, and 22.00%, respectively, while there is little Fe and P in ordinary gypsum rock, which may contribute to the dark characteristics of the rock. SiO 2 is present in ordinary gypsum, transparent gypsum and alabaster (in order of decreasing content), but it is not found in fiber gypsum because of the different places of origin. Moreover, because the content of S, which has a relative atomic mass of 64, is lower in all gypsum rocks than that of Ca, which has a relative atomic mass of 40, so there may be other composition that contains Ca in the rock. The four kinds of gypsum rocks contain little SrO, which means that all the gypsum rocks may have a small amount of calcite (usually containing Sr).

XRD phase identification.
To explore the differences in material composition of the four types of gypsum rocks, material analysis was carried out through XRD. The characteristic patterns are shown in Fig. 3.
The XRD curves of the four types of gypsum rocks are relatively similar with strong first four diffraction peaks and minimal difference in the diffraction angles of the main peaks. The detection results showed CaSO 4 ·2H 2 O is the main component of gypsum rocks. The main difference for the curves is the intensity of the first diffraction peaks, which indicates variation in the crystalline-preferred crystal orientation 43 . The peak intensity of the first www.nature.com/scientificreports/ diffraction peak is higher for transparent gypsum than for ordinary gypsum. Therefore, to explore the impact of the ore grade of the gypsum rock on the intensity of the first diffraction peak, XRD experiments were also performed on different grades of transparent gypsum rock, as shown in Fig. 4, and the statistical results for different gypsum rocks processed by MDI Jade 6.0 are shown in Table 2.
As shown in Fig. 4, the XRD peak curves are similar for the four different grades of transparent gypsum rocks. CaSO 4 ·2H 2 O is noted as the main component. The peak intensity of the first diffraction peaks increased with increasing CaSO 4 ·2H 2 O content, while the other three main diffraction peaks exhibited similar changes. Thus, it can be concluded that the CaSO 4 ·2H 2 O content affects the diffraction intensity in the XRD curves. Table 2 shows the results from the MDI Jade 6.0 software which was used to process and analyze the experimental data. Based on the XRF element analysis results, gypsum. syn which has a molecular formula of CaSO 4 ·2H 2 O, was found to be the main component in the four types of gypsum rocks. In addition, the gypsum rocks had a stable crystal face spacing of approximately 7.60 nm with obvious differences in the crystal sizes for each type. The crystal size was at the micron level, which is significantly larger than the crystal face spacing.
Because XRF cannot measure the content of elements smaller than Na, the XRD result shows that all the gypsum rocks have a main composition of CaSO 4 ·2H 2 O; however, the presence of carbonates, such as CaCO 3 ,  www.nature.com/scientificreports/ cannot be ruled out. Thus, to finally obtain the content of CaSO 4 ·2H 2 O for each kind of gypsum rock, elemental analysis was carried out by an element analyzer to obtain the content of C, and the test results are shown Fig. 5. Figure 5 shows that the C contents in the four gypsum samples were less than 3‰, which indicates that there may be few carbonates in any of the gypsum rocks and that they do not have a significant impact on the composition of the rock. It is worth noting that the C contents in ordinary gypsum, transparent gypsum, fiber gypsum and alabaster decrease in turn, which is the same trend as was observed for the S content of the rocks. In addition, the contents of N in the rock were all less than 0.25‰, which should not be analyzed in the later discussion of gypsum rock compositions. Through elemental analysis, we obtained the result that there is little C in each type of gypsum rock. Based on the XRF results, we obtained the content of S for each gypsum rock; therefore, we can conclude that the CaSO 4 ·2H 2 O content by Eq. (2) of the four gypsum rocks is as follows: fiber gypsum (72.72%), transparent gypsum (72.57%), alabaster (72.78%), and ordinary gypsum (71.51%). The results show that there is little difference in the contents of CaSO 4 ·2H 2 O in the four kinds of gypsum rock and that the dark characteristics of ordinary gypsum and transparent gypsum rock may be caused by the low contents of other elements.
Tensile test results. The results of the Brazilian split test of the four kinds gypsum rocks are shown in Table 3 and the difference of tensile strength and density are shown in Fig. 6. Figure 6 shows that the relatively large tensile strength of transparent gypsum can be attributed to its relatively compact internal structure and small grain size. Meanwhile, the striped surface with sliver and layered structures in the radial direction of the fiber gypsum are easily separated under tensile stress, leading to its lower tensile   www.nature.com/scientificreports/  www.nature.com/scientificreports/ strength. The large internal particles in alabaster contributed to the stress concentration under uniaxial loading, causing premature failure of the rock sample. In comparison to transparent gypsum, ordinary gypsum has a similar structure, better compactness, and internal cementation, leading to a slight improvement in the overall tensile strength. These structure differences were further studied by SEM. From Table 3 and Fig. 6, the density variations of the different gypsum rocks are similar to their tensile strength. The density of ordinary gypsum, transparent gypsum, fiber gypsum, and alabaster declines in turn. With a similar rock composition, the changes in density reflect the difference in the internal structure and compactness of each gypsum rocks.

Failure characteristics of gypsum rock samples. Macroscopic failure characteristics. The indirect
tensile failure characteristics with the fracture lines of the four types gypsum rock are shown in Fig. 7. The fracture line of fiber gypsum is relatively smooth traversing through the entire specimen with an arc shape, indicating the extension of the fracture line along its fibrous arcuate structure. Transparent and ordinary gypsums have a similar fracture line that distributed along the center of the specimen without evident inflections. The fracture line of alabaster is zigzag, extending along the edge of large crystal particles, and traversing through the crystal particles in some areas.
Microscopic failure characteristics. Figure 8 shows the micromorphological characteristics of the four types of gypsum rocks at a magnification of 2000. Even with the striped structure of fiber gypsum, the connections between fringes are relatively compact with several clastic particles on the surface. Large particles can be observed in transparent gypsum but micro clastic particles still dominated the structure; moreover, the degree of cementation between the particles is relatively low with notable pores in some areas. Alabaster has a sturdy surface with undamaged veined skeleton and without pores, indicating characterization by local failure. The particle size of ordinary gypsum is relatively small with good homogeneity and developed pores.

Discussions Correlation analysis of gypsum rock components and tensile strength.
According to the analysis of the material composition and Brazilian split test, there may be a relationship between the CaSO 4 ·2H 2 O content and tensile strength of the gypsum rocks. Particularly, the higher the CaSO 4 ·2H 2 O content is, the lower the tensile strength. Thus, the relationship between the CaSO 4 ·2H 2 O content and tensile strength of different gypsum rocks was determined by the test results, as shown in Fig. 9. Figure 9 shows that the higher the Correlation analysis of the tensile strength and microstructure. Molecular structure and grain characteristics. Rock is a complex mineral aggregate with its own particular internal material composition and spatial structure 44 . From the diagenetic conditions and grain size of the microstructure, the main material composition of the gypsum rocks was found to be CaSO 4 ·2H 2 O. The effect of the microstructure on the tensile www.nature.com/scientificreports/ strength of the gypsum rocks and its mechanisms were analyzed based on the experimental results. Figure 10 shows the CaSO 4 ·2H 2 O molecular space arrangement and the microstructure of gypsum rock. From Fig. 10a that the CaSO 4 ·2H 2 O molecules are arranged in layers with crystal water connected and distributed in the middle by weak chemical bonds 45 . The layered structure formed in the gypsum is shown in Fig. 10b. When load is applied, a tensile stress perpendicular to the layer direction is generated, leading to tensile failure. With a higher CaSO 4 ·2H 2 O content, there is a more evident concentrate stress phenomenon, which makes the rock sample more prone to damage and decreases the tensile strength on a macro level.
Gypsum rock has a complete crystal structure with different grain size for each type gypsum rock. Many scholars noted the inverse correlation between the strength of nonporous rocks and the size of crystal particles [46][47][48][49] . From Table 2, the grain sizes of ordinary gypsum, transparent gypsum, fiber gypsum, and alabaster are 691, 896, 929, and 990 nm, respectively. In addition, the tensile strength of gypsum is inversely proportional to the crystal size except for fiber gypsum due to its striated structure.
Particle granularity of fractured rock samples. To further explore the particle characteristics after failure of the four kinds of gypsum rocks, the SEM images were analyzed by Image Pro Plus and the diameters of the failure particles were measured and counted. The results are shown in Fig. 11. The fracture of fiber gypsum rock was along its fiber interlayer without obvious particle characteristics and only some clastic particles adhered to its surface. Thus, the particle size statistics was only applicable for the other three kind gypsum rocks.
It can be seen from Fig. 11 that the three gypsum rocks have different particle size distribution characteristics. Under the same statistical area, the particle diameter of alabaster is below 2 mm with few particles in the range of 4-8 mm. From the SEM image in Fig. 8b, there are large particles in transparent gypsum rock, differing from the statistical analysis, in which the main fractured particle diameter range is less than 2 mm. The particle size of ordinary gypsum rock is relatively uniform with a main distribution range below 2 mm and all particle diameters are less than 6 mm.
Under the same statistical area, the total number of statistical particles can reflect the fracture degree of gypsum rock. The number of statistical particles of alabaster, transparent gypsum, and ordinary gypsum are approximately 3000, 4000, and 5000, respectively. The result indicates the greater fracture degree and more thorough energy release for ordinary gypsum rock under equal loading condition. Compared with the SEM images, alabaster has significantly less particles in micro failure, further verifying the minimal change of the overall structure of alabaster during fracture. In addition, there is a linear relationship between the tensile strength of the group samples and the statistical number of particles after failure within the same statistical range, which appears the greater the tensile strength, the more the number of fracture particles.
Microstructural characteristics of fractured rock samples. According to the analysis in Figs. 7 and 8, there are large differences in the microstructure of gypsum rocks, which influences their tensile strength more evidently after compression. Although the fiber striated structure of fiber gypsum is relatively compact, it is easily separated under tensile stress. In addition, this arc-shaped striation structure distribution peeled and stretched along the interlayer, leading to an arc-shaped fracture line during rock fracture. Alabaster has large blocky characteristics that cause easy stress concentration and failure under loading conditions, which contributes to the lowest tensile strength. The stress propagation path in the rock samples usually traverses the direction of the least work. Therefore, as the fracture line extends, large particles are bypassed, causing the zigzag fracture line and uneven particle size distribution of the damaged particles. Rock samples with less destroyed vein skeleton retained a good integrity. Transparent gypsum has a relatively uniform particle size distribution after failure, proving the www.nature.com/scientificreports/ more sufficient and higher energy release; thus, tensile strength was relatively high. Therefore, based on the microstructure analysis of the gypsum rocks, the strength of ordinary gypsum, transparent gypsum, fiber gypsum, and alabaster decreased during fracture, which is consistent with the experimental results. Rock failure is the accumulation and release of internal energy. The work done by an external force causes the complete breakage of the rock mass into smaller particles. As the space between the particles expand after energy release, the external force exerts more work, thereby internal pores develop at a higher degree [50][51][52] . In addition, there is more energy stored in the rock, more obvious micro failure characteristics, and larger tensile strength. This is evident in the increased number of micro particles of alabaster, transparent gypsum, and ordinary gypsum in the same statistical area and the full development of pore space, confirming the increased tensile strength of gypsum rocks.

Conclusions
In this paper, the content and microstructure of four kinds of gypsum rock-transparent gypsum, ordinary gypsum, fiber gypsum, and alabaster-were analyzed by XRF, XRD, SEM, and Brazilian split experiments. The influence of the composition and microstructure on the tensile strength and failure characteristics of the gypsum rocks and their mechanisms were investigated. The specific conclusions are as follows.
The main components of alabaster, fiber gypsum, transparent gypsum, and ordinary gypsum is found to be calcium sulfate dihydrate (CaSO 4 ·2H 2 O) with a calculated content 72.78%, 72.72%, 72.57%, and 71.51%, respectively, and the grain size decreasing accordingly. For transparent gypsum rocks, the higher the CaSO 4 ·2H 2 O content, the greater the peak value of its first main diffraction peak. Particle diameter/μm Particle counts/10 3 Figure 11. Statistics of the fracture particles of gypsum rocks based on the scanning electron microscopy images: (a) alabaster, (b) transparent gypsum, and (c) ordinary gypsum. The size of the ball is consistent with the diameter of the fracture particle. www.nature.com/scientificreports/ The tensile strength and density of alabaster, fiber gypsum, transparent gypsum, and ordinary gypsum decrease with the increase of CaSO 4 ·2H 2 O content. In addition, there may be a negative correlation between the tensile strength and the content of CaSO 4 ·2H 2 O, that is the higher the CaSO 4 ·2H 2 O content, the lower the tensile strength of gypsum rock.
The four types of gypsum rocks have different fracture structural characteristics. In a macroscopic scale, fiber gypsum fracture line exhibits an arc shape that traverses the specimen while that of ordinary and transparent gypsums vertically traverses the center of the specimen. Alabaster has a zigzag line fracture with intergranular or trans-granular distribution. In contrast, in a microscopic scale, a stripe structure is observed for fiber gypsum, the pore development degree of alabaster, transparent gypsum, and ordinary gypsum gradually increases and the diameter of the destroyed particles decreases accordingly and uniformly.
Based on the molecular arrangement, the higher the CaSO 4 ·2H 2 O content, the more evident the layered characteristics of the gypsum rock, and the larger the grain size, thereby the lower the tensile strength. The number of destroyed particles of alabaster, transparent gypsum, and ordinary gypsum is directly proportional to its tensile strength. Particularly, a larger tensile strength has a smaller particle diameter distribution interval.