Mechanical properties of molybdenum tailings concrete after exposure to high temperature

Abstract

In this paper, 75 concrete prisms were tested under different temperatures (20, 200, 400, 600 and 800 °C) and molybdenum tailings replacement ratio (0, 25, 50, 75, and 100 %). The axial compression failure modes and mechanical properties of molybdenum tailings concrete were studied. The results show that the increase in temperature will aggravate the failure of concrete under axial compression, instead of molybdenum tailing replacement ratio. With the increase of temperature (20–800 °C), the surface color of the concrete becomes lighter, and obvious cracks began to appear from 400 °C, the mass loss ratio and peak strain of molybdenum tailings concrete show an increasing trend, and reach the maximum growth ratio (6.58 % and 34 %) at 800 °C. The peak stress and elastic modulus of molybdenum tailings concrete show a decreasing trend, and reach the maximum reduction at 800 °C (52 % and 71 %). With the increase of replacement ratio, the mass loss ratio of molybdenum tailings concrete increases linearly. The peak stress, peak strain and elastic modulus of molybdenum tailings concrete at all temperatures increase first and then decrease. The 25 % molybdenum tailings content can improve the deterioration of molybdenum tailings concrete after exposure to high temperature. Based on the experimental data, the prediction formulas of peak stress/strain and elastic modulus of molybdenum tailings under different temperature-molybdenum tailings replacement ratio coupling conditions are fitted respectively. The experimental and calculated values of the formula are in good agreement.

1. Introduction

Molybdenum tailings are solid wastes left over from the extraction of metal molybdenum from refined molybdenum ore. The refining degree of molybdenum tailings provided by the current refining technology is only 5 %, resulting in the output of a large number of molybdenum tailings [1]. These molybdenum tailings have been shelved on the ground for a long time, causing serious land occupation and environmental pollution problems [2]. The reserves of molybdenum resources in the world are about 15 million tons, and the reserves of molybdenum resources in China are 8.4 million tons, ranking first in the world [3]. Such huge molybdenum resources have also caused a serious accumulation of molybdenum tailings in China [4]. In 2021, the National Development and Reform Commission of China, together with the Ministry of Science and Technology, the Ministry of Industry and Information Technology and the Ministry of Finance, issued the “Guiding Opinions on the Comprehensive Utilization of Large Solid Wastes in the 14th Five-Year Plan” (2021-381). The comprehensive treatment of tailings has become a hot spot in China.

The accumulation of molybdenum tailings has attracted the attention of many scholars. Some of them consider using molybdenum tailings to prepare concrete, reducing the use of natural sand on the basis of improving the utilization ratio of molybdenum tailings. Cui et al. [5,6] successfully prepared a molybdenum tailings concrete block with a compressive strength of 68.7 MPa for 28 days by subdividing the particle size of molybdenum tailings and using molybdenum tailings sand with a particle size greater than 0.16 mm as fine aggregate. At the same time, the effects of different molybdenum tailings replacement ratios (0, 20, 30, 40, and 50 %) on the mechanical properties of concrete were also studied. Hu et al. [7] prepared molybdenum tailings powder concrete by grinding molybdenum tailings in Yuhang District of Hangzhou and replacing cement, and studied its working performance, mechanical properties and durability. The results show that the specific surface area of molybdenum tailings powder after grinding is 547 m²/kg. When the amount of molybdenum tailings powder replacing cement in concrete is not more than 10 %, the strength of concrete can be improved without affecting the durability, but excessive molybdenum tailings powder admixture will lead to the decline of various properties. Sun et al. [8] adopted two forms of single-doped fly ash and mixed fly ash and silicon powder. The active powder molybdenum tailings concrete was prepared based on the replacement ratio of 50 % molybdenum tailings, and the related mechanical properties were tested. The results show that the compressive strength of molybdenum tailings concrete can be improved by adding fly ash alone. When fly ash and silica fume are mixed, the early strength increases slowly and the later strength increases rapidly.

The losses caused by fires in China are quite serious. According to statistics, the direct economic losses caused by fires in China each year reached 34 million dollars in the 1970s. In the eighties reached 43 million dollars; in the 1990s, it reached 110 million dollars, accounting for 3‰of GDP. Fire is also a frequent building disaster. After it appears in concrete structure buildings, concrete materials will be damaged to varying degrees, resulting in a rapid decline in the durability and mechanical properties of concrete materials which have been comprehensively studied by tests and simulations [[9], [10], [11], [12], [13], [14], [15], [16]]. The corresponding stress-strain relationship models have also been proposed. In contrast, up to date, several studies have been conducted on the mechanical properties of molybdenum tailings concrete, but the effect of high temperature has never been considered in these studies which is rather important for the practical application of molybdenum tailings concrete.

In order to analyze the axial compression mechanical properties of molybdenum tailings concrete after exposure to high temperature, the heating temperature (20, 200, 400, 600 and 800 °C) and the replacement ratio of molybdenum tailings (0, 25, 50, 75, and 100 %) were used as parameters and the working conditions were coupled. Through the test of 75 prisms of molybdenum tailings concrete after exposure to high temperature, the mass loss ratio and axial compression mechanical properties (peak stress, peak strain and elastic modulus) of molybdenum tailings concrete under the coupling state of temperature and replacement ratio were studied respectively and the corresponding prediction methods were also proposed.

2. Specimen design

2.1. Mixture proportion

The prism specimens of molybdenum tailings concrete with different replacement ratios of molybdenum tailings (0, 25, 50, 75, and 100 %) were prepared by replacing natural sand in ordinary concrete components with molybdenum tailings. In order to study the influence of heating temperature-molybdenum tailings replacement ratio coupling condition on the axial compression mechanical properties of molybdenum tailings concrete, the effects of different heating temperatures (20, 200, 400, 600, and 800 °C) were considered. The coupling parameters of heating temperature-molybdenum tailings replacement ratio include 25 kinds (combination number of 5 replacement ratios and 5 temperatures). The design grade of concrete is C45. According to JGJ 55–2011 ‘Design Specification for Mix Proportion of Ordinary Concrete’ [17], the mix proportions with molybdenum tailings replacement ratios of 0, 25, 50, 75 and 100 % are shown in Table 1.

Table 1.

Molybdenum tailings concrete parameters and mix design.

Temperature-H	20 °C, 200 °C, 400 °C, 600 °C, 800 °C
Replacement ratio-R	0 %, 25 %, 50 %, 75 %, 100 %
Replacement ratio (%)	Material consumption (kg/m3)						Water-cement ratio
	Cement	Water	Sand	Molybdenum tailings	Gravel	Water reducer
0	461	175	670	0	1 090	4.61	0.38
25	461	175	502.5	167.5	1 090	4.61	0.38
50	461	175	335	335	1 090	4.61	0.38
75	461	175	167.5	502.5	1 090	4.61	0.38
100	461	175	0	670	1 090	4.61	0.38

According to the provisions of GB/T 50081-2019 “Standard for Testing Methods of Physical and Mechanical Properties of Concrete” [18], three parallel specimens should be made for each parameter. Therefore, 75 molybdenum tailings concrete prisms with a size of 150mm × 150mm × 300 mm were made for the 25 parameters involved in this paper to determine the mass loss ratio, peak stress, peak strain and elastic modulus of concrete after exposure to high temperature.

However, when measuring the elastic modulus, the test equipment needs to input the bearing capacity of the concrete test block in advance. Therefore, for the 25 concrete parameters under the influence of different heating temperatures and molybdenum tailings replacement rates, 75 additional prismatic concrete test blocks need to be prepared to determine the bearing capacity of the concrete specimen in advance.

The ingredients for Molybdenum tailings concrete are weighed and mixed in strict accordance with the above mix proportion during the production process and the corresponding provisions in the ‘Concrete Physical and Mechanical Properties Test Method Standard’ [18]. The specimens are demoulded and placed in a concrete curing box. They are cured for 4 weeks under standard conditions (temperature 20 °C, humidity >90 %).

2.2. Materials

The test specimens were prepared from ordinary river sand, gravel, molybdenum tailings, ordinary Portland cement and a small number of additives. The selected molybdenum tailings were the molybdenum tailings abandoned after smelting and processing by Shaanxi Jinduicheng Molybdenum Co., Ltd.; river sand was ordinary natural river sand sold in the market; the coarse aggregate was taken from the ordinary gravel in the Qinling Mountains of Shaanxi Province, China, and the stone with too large particle size was removed by screening, and the grading of the used gravel was 5–16 mm. The water used in the configuration specimen was ordinary tap water. Since only the fine aggregate is the focus of this study, the physicochemical information of these ingredients was not tested.

According to the relevant standards of GB/T 14684-2022 ‘sand for construction’ [19], the basic physical properties of molybdenum tailings and river sand were measured. The parameters are shown in Table 2. After comparison, it can be found that molybdenum tailings belong to very fine sand, while river sand belongs to medium sand or coarse sand. The bulk density and alkali-aggregate reaction of the two are similar. That is the reason that the mass replacement ratio of fine aggregate was used in this study which would be convenient for practical application.

Table 2.

Physical properties.

Variety	Fineness modulus	Apparent density (g/m3)	Bulk density (g/m3)	Alkali-aggregate reaction
River sand	3.20	2.64	1.45	0.05
Molybdenum tailings	1.20	2.57	1.39	0.07

The chemical composition of molybdenum tailings and river sand used in the test was obtained by XRF fluorescence spectroscopy, as shown in Table 3. It can be found that the main component of molybdenum tailings is SiO₂, accounting for 78.73 % of the total. For natural river sand, SiO₂ is the main component material, accounting for 78.73 % of the total. It can be seen that the main composition of molybdenum tailings and natural river sand is the same, and the composition of secondary trace elements is also similar.

Table 3.

Composition of molybdenum tailings/%.

Composition	SiO2	Al2O3	K2O	Fe2O3	CaO	SO3	BaO	MgO	SrO	TiO2	MnO	Loss
Molybdenum tailings	78.73	6.12	4.24	3.15	3.03	1.77	0.93	0.81	0.71	0.25	0.09	0.17
River sand	71.34	8.89	1.66	5.73	5.49	0.19	–	1.16	–	–	–	5.54

3. Test program

3.1. High temperature test

The high temperature test before the axial compression test aimed to investigate the high temperature damage of the molybdenum tailing concrete after fire accident. 20, 200, 400, 600, and 800 °C are the aim temperatures. Room temperature, which may be heated without a high-temperature furnace, is represented by 20 °C. As shown in Fig. 1, the high temperature apparatus utilized in the test is a box-type resistance furnace made by Luoyang Liyu Kiln Co., Ltd. called the LYL-17LBT. The furnace chamber is 500 mm by 500 mm by 500 mm, has a maximum temperature of 1200 °C, and uses 18 kW of power.

Fig. 1.

High temperature furnace.

After 28 days of curing in the concrete standard curing box under standard conditions (temperature 20 °C, humidity >90 %), the specimens were removed. They were weighed on an electronic balance after 24 h naturally drying, and the high temperature test was then performed. For each test, six specimens were collected, and they were divided equally among the high-temperature furnace. In order to prevent the specimens from coming into direct touch with the resistance wire, a high temperature resistant stone plate was positioned at the bottom of the high temperature furnace.

The target temperature of the high temperature furnace is set at 200, 400, 600, and 800 °C, and the heating ratio is 10 °C/min. The resistance furnace is kept at a constant temperature for 3 h after reaching the set temperature to ensure that the concrete specimen fully reaches the target temperature, and then the furnace door is opened so that the specimen is completely exposed until completely cooled. This is done in order to guarantee that the concrete specimen's internal temperature also reaches the target temperature. After the high temperature exposure was completed, the mass loss ratio was calculated again, and the distribution and number of cracks on the surface of the specimen were observed, and the damage was analyzed.

3.2. Axial compression test

After cooling the specimens after high temperature treatment, the axial compression test was carried out. The purpose of the axial compression test is to determine the peak stress, peak strain and elastic modulus of the specimen. The equipment used is YAW-2000D microcomputer electro-hydraulic servo press produced by Hebei Sanyu Testing Machine Co., Ltd. When measuring the elastic modulus, the elastic modulus measuring fixture should be connected in advance, as shown in Fig. 2(a).

Fig. 2.

Arrangement of axial compression test.

Since the axial compressive strength of the specimen should be known before measuring the elastic modulus, a press should be used to pre-load 75 molybdenum tailings concrete prisms under the coupling effect of different temperatures and different replacement ratios. The pressure step was consistent with the axial compression test, but the cyclic loading step was omitted. To determine the specimen's maximum bearing capacity with various parameters, it was directly loaded until it failed at a loading ratio of 0.5 MPa/s.

According to GB/T 50081-2019 “Standard for test methods of physical and mechanical properties of concrete” [17], the load loading speed was loaded at a speed of 0.3 MPa/s to the initial load value F₀ with a reference stress of 0.5 MPa, and the constant load was maintained for 60s. After the reading was recorded within 30s, it was immediately loaded to the load value Fa with a stress of 1/3 axial compressive strength which was obtained by the abovementioned pre-loading test, holding for 60s, and then unloading at a speed of 0.3 MPa/s to the reference stress 0.5 MPa, constant load 60s. After repeated twice preloading, the load was applied to Fa at the base stress of 0.5 MPa-F₀ for 60s. After that, the elastic modulus device was unloaded, the load was held for 60s, and the reading was recorded within 30s. The loading speed was 0.5 MPa/s, and the displacement control was changed after the load reached the peak stress F_max. The loading speed was 0.5 mm/min, and the test was completed when the load was loaded to 5 % of the peak stress. The loading system is shown in Fig. 2(b).

4. Test results

4.1. High temperature test

The characterization in Fig. 3(a)∼(e) of concrete specimens made from molybdenum tailings with a 100 % replacement ratio after heating can be compared, and it can be found that under the same replacement ratio, the specimens will change significantly with the increase of temperature. The concrete prism is green gray at room temperature; When the heating temperature is 200 °C, the color of the specimen begins to lighten; At 400 °C, minor cracks appeared on the outer surface. Under the heating condition of 400 °C, the hydration products such as hydrated calcium silicate and hydrated calcium aluminate inside the concrete slurry begin to decompose, which leads to the further expansion of the pores inside the concrete, and the free water inside the concrete is precipitated from the cracks on the surface of the concrete. At 600 °C, the color of the specimen turns to gray-white, and a large number of pores and microcracks appear on its surface. At 800 °C, the color of the specimen further turns white and the number of pores and cracks on the surface of the specimen increases. At the same time, the width of the cracks increases sharply. The surface of the test piece is wrinkled and scattered, with a tendency to peel off. On the contrary, the apparent features upon exposure to high temperatures are not significantly affected by the varied replacement ratios of molybdenum tailings.

Fig. 3.

Surface feature of specimens after exposure to high temperature.

4.2. Axial compression test

The failure mode of axial compression specimens of molybdenum tailings concrete is not unaffected by an increase in molybdenum tailing replacement ratio at different temperatures, whereas an increase in temperature will worsen the specimen's damage. As a result, the failure mode and mechanism of the specimen are examined when the heating temperature is 20, 200, 400, 600, and 800 °C, using the failure of the specimen with a 100 % replacement ratio of molybdenum tailings as an example, as shown in Fig. 4(a)∼(e).

Fig. 4.

Failure mode of specimens after axial compression.

When the temperature is 20 °C, some microcracks appear in the specimen after applying a certain load. As the load further increases, the specimen enters the plastic stage, and the micro-cracks gradually increase and extend along the vertical direction. When the load is added to the peak load, the crack develops rapidly and one or more parallel longitudinal cracks appear. Subsequently, the width of the longitudinal crack increased sharply, and began to spread to the middle of the molybdenum tailings to form oblique cracks. When the ultimate compressive strain of the concrete was reached, the concrete was crushed, part of the concrete skin was peeled off, and the internal molybdenum tailings concrete showed two opposite cone failure surfaces. This is caused by the strong constraint and vertical axial force of the press on the upper and lower ends of the molybdenum tailings concrete prism.

After being exposed to high temperature, the specimens' failure process and morphology are similar to those at room temperature. The difference is that the specimens' longitudinal and oblique cracks are larger and more numerous after being exposed to high temperature. The specimen becomes looser as the heating temperature rises. After heating at 800 °C, the outer concrete of the specimen is obviously lost when it is destroyed. The main reason for this phenomenon is that the high temperature deteriorates the molybdenum tailings concrete prism, resulting in a large number of cracks in the concrete prism before loading.

5. Result analyses

The mass loss ratio, peak stress, peak strain and elastic modulus of molybdenum tailings concrete under 25 different heating temperatures (20, 200, 400, 600 and 800 °C)-different replacement ratios of molybdenum tailings (0, 25, 50, 75 and 100 %) are shown in Table 4. The mass loss ratio can be calculated by Formula (1), and the peak stress, peak strain and elastic modulus are read directly by the press. The data in the table are the average value of the three parallel specimens. Since 20 °C is the temperature at room temperature, the molybdenum tailings do not overflow, so the specimens at room temperature think that the loss ratio is 0 %.

$$m_s=\left(m_0-m_1\right)/m_0$$ (1)

in the formula, m_s represents the mass loss ratio, m₀ represents the mass of the specimen at room temperature, and m₁ represents the mass of the specimen after exposure to high temperature.

Table 4.

Test data.

Specimen types (oC-%)	Mass loss ratio (%)	Peak stress (MPa)	Peak strain (ε)	Elastic modulus (GPa)
20–0	0	42.8	1 339	33.91
20–25	0	43.6	1 513	41.02
20–50	0	41.15	1 450	38.22
20–75	0	39.22	1 439	36.52
20–100	0	37.86	1 372	33.48
200–0	0.07	40.62	1 604	30.61
200–25	0.10	40.8	1 872	37.46
200–50	0.18	37.28	1 857	33.9
200–75	0.27	37.01	1 750	31.4
200–100	0.31	35.3	1 644	29.5
400–0	2.28	35.95	2 278	26.35
400–25	2.78	36.5	2 397	30.25
400–50	3.31	33.68	2 356	28.78
400–75	3.73	31.51	2 269	25.71
400–100	3.98	30.08	2 254	22.03
600–0	4.61	31.14	2 660	20.63
600–25	4.98	32.88	3 021	22.45
600–50	5.21	29.68	2 999	22.09
600–75	5.45	26.25	2 911	19.67
600–100	5.57	24.18	2 827	18.44
800–0	5.38	26.2	4 028	8.87
800–25	5.75	28.0	5 946	12.75
800–50	5.98	24.34	5 502	12.08
800–75	6.26	22.88	4 930	11.65
800–100	6.58	18.17	4 322	9.75

5.1. Mass loss ratio

It can be found from Fig. 5 that when the temperature is 200 °C, the mass loss ratios of the specimens at 0–100 % replacement ratios are 0.07, 0.10, 0.18, 0.27 and 0.31 %, respectively. The mass loss ratio at all replacement ratios is less than 1 %. At this time, the internal products of the molybdenum tailings concrete specimens have not yet begun to decompose, and the water content has not changed significantly. When the temperature is 400 °C, the mass loss ratios at 0–100 % replacement ratios are 2.28, 2.78, 3.31, 3.73 and 3.98 %, respectively. The hydration products inside the specimen begin to decompose, and free water emerges in the form of water vapor, and the mass loss ratio increases rapidly. When the temperature is 600 °C, the mass loss ratios at 0–100 % replacement ratio are 4.61, 4.98, 5.72, 5.45 and 5.57 %, respectively, and the damage of the specimen is aggravated. When the temperature is 800 °C, the mass loss ratios at 0–100 % replacement ratios are 5.38, 5.57, 5.98, 6.26, and 6.58 %, respectively, and the mass loss ratio is 5.38-6.58 %, the damage range is large. The aforementioned results indicate that as temperatures rise, concrete mass loss will progressively degrade since concrete deterioration is temperature-dependent. Additionally, the mass loss ratio will rise along with the replacement ratio of molybdenum tailings. This is due to molybdenum tailings' low fineness modulus. More water is involved in moistening the aggregate when the replacement ratio rises since the specific surface area also grows. As a result, after being exposed to high temperatures, the quantity of dehydration rises in proportion to the amount of molybdenum tailings present.

Fig. 5.

Mass loss ratio of molybdenum tailings concrete after exposure to high temperature.

5.2. Peak stress

It can be found from Fig. 6(a) that the peak stress of molybdenum tailings concrete after heating to different temperatures increases first and then decreases with the increase of replacement ratio, and the inflection point is 25 % replacement ratio. Based on the zero replacement ratio, the peak stress after heating to different temperatures increased by 1.8, 0.4, 1.5 and 0.5 % at 25 % replacement ratio, and the peak stress increased slightly. When the replacement ratio is greater than 25 %, the peak stress decreases with the increase of replacement ratio. This is because the inappropriate amount of molybdenum tailings (25–100 %) will occupy the position of the original coarse aggregate, thus breaking the tight state, resulting in a decrease in the peak stress of concrete.

Fig. 6.

Peak stress relationship curve under temperature-replacement ratio coupling.

It can be found from Fig. 6(b) that the peak stress of different replacement ratios decreases with the increase of heating temperature. When the heating temperature is higher than 200 °C, the decrease of peak stress increases, and further increases at 600 °C. Compared with the peak stress of the specimen at 800 °C and 0 % replacement ratio, the minimum value (52 %) was reached at 800 °C and 100 % replacement ratio. This is because the structural degradation within the concrete material causes the peak stress to drop as the temperature rises, the hydration reaction product Ca(OH)₂ inside the concrete gradually decomposes, and the internal deterioration of the specimen becomes more severe.

5.3. Peak strain

As shown in Fig. 7(a), with the increase of replacement ratio, the peak strain of molybdenum tailings concretes at all temperatures increased first and then decreased. In the range of 0∼25 %, the peak strain increases significantly; in the range of 25 ∼100 %, the curve shows a downward trend as a whole, the slope is relatively gentle, the change range of peak strain tends to be stable, and the replacement ratio of 25 % molybdenum tailings is the inflection point. This is because the excessive incorporation of molybdenum tailings has changed the dense state inside the original specimen, making the concrete brittle.

Fig. 7.

Peak strain relationship curve under temperature-replacement ratio coupling.

As shown in Fig. 7(b), the peak strain increases with increasing temperature. Between 20°C and 600 °C, the peak strain increases linearly with the increase of temperature. At 600–800 °C, the slope increases obviously, and the peak strain increases greatly. When the replacement ratio is 25 % at 800 °C, the peak strain reaches the maximum 0.006. This is because with the increase of temperature, the hydration reaction product Ca(OH)₂ in concrete gradually decomposes, the bonding force between mortar slurry and aggregate is weakened, the plasticity of concrete is enhanced, and the peak strain is increased.

5.4. Elastic modulus

As shown in Fig. 8(a), with the increase of replacement ratio, the elastic modulus of molybdenum tailings concrete at all temperatures increases first and then decreases, which is consistent with the peak stress law. In the range of 0-25 %, the growth slope increases rapidly, and the elastic modulus increases obviously. In the range of 25-75 %, the curve decreases greatly; in the range of 75 -100 %, the slope of the curve becomes gentle, the change of elastic modulus tends to be stable, and the replacement ratio of 25 % molybdenum tailings is the inflection point.

Fig. 8.

Relationship curve of elastic modulus under temperature-replacement ratio coupling.

As shown in Fig. 8(b), the elastic modulus of concrete made from molybdenum tailings with all replacement ratios showed a linear downward trend as temperature rose. The slope sharply increased by about 600 °C. The elastic modulus of concrete made with molybdenum tailings dropped by 71 % when compared to specimens made with molybdenum tailings at 800 °C and 0 % replacement ratio.

6. Prediction models of compression mechanical properties

In the construction of concrete structures, peak stress, peak strain, and elastic modulus are crucial design pillars. Based on the test results, the peak stress, peak strain and elastic modulus prediction models of molybdenum tailings concrete under high temperature were fitted with heating temperature T and molybdenum tailings replacement ratio R as independent variables. The results are shown in Formula (2).

$$f(T,R)=\{\begin{array}{l}\frac{\sigma \left(T,R\right)}{\sigma \left(\mathrm{20,0}\right)}=1.013-\frac{3.444}{10000}T-\frac{1.536}{10000000}{T}^2+0.016R-\frac{9.553}{100000}TR-0.141R^2\\ \frac{\epsilon \left(T,R\right)}{\epsilon \left(\mathrm{20,0}\right)}=0.985-\frac{7.185}{10000}T+\frac{4.752}{1000000}{T}^2+1.328R-\frac{8.802}{100000}TR-1.318R^2\\ \frac{E\left(T,R\right)}{E\left(\mathrm{20,0}\right)}=1.081-\frac{5.373}{10000}T-\frac{5.554}{10000000}{T}^2+0.373R+\frac{8.158}{100000}TR-0.485R^2\end{array}$$ (2)

In the formula, σ(T,R)、 $\epsilon$ (T,R)and E (T,R)are the peak stress, peak strain and elastic modulus of molybdenum tailings concrete under the coupling of different replacement ratios and heating temperatures, respectively. σ(20,0)、 $\epsilon$ (20,0) and E (20,0) are the peak stress, peak strain and elastic modulus of molybdenum tailings concrete under the coupling of normal temperature (20 °C) and replacement ratio, respectively. T is the maximum heating temperature of concrete; R is the replacement ratio of molybdenum tailings (0–1).

Formula (2) is used to calculate the peak stress, peak strain, and elastic modulus of molybdenum tailings concrete under the influence of the 25 kinds of temperature-replacement ratio coupling that were tested. The results of the test and calculation were then compared. The results are shown in Fig. 9. It can be seen from the figure that the average X of the ratio of the calculated value of peak stress/strain and elastic modulus to the experimental value is 1.001, 1.004 and 1.003, respectively, and the standard deviation s is 0.029, 0.113 and 0.064, respectively. The above results show that the formula is in good agreement.

Fig. 9.

Formula fitting comparison.

Fig. 10 shows the peak stress, peak strain and elastic modulus prediction model and test value comparison results under the influence of heating temperature H and molybdenum tailings replacement ratio R. The diagram shows that the test trend and the prediction formula's trend with changes in the heating temperature H and the replacement ratio of molybdenum tailings R are essentially the same, demonstrating that the prediction formula fitted in this section can more accurately reflect the coupling effect of H–R [[20], [21], [22]].

Fig. 10.

Comparison of mechanical properties prediction model and test trend.

7. Conclusions

The performance of molybdenum tailings concretes under the influence of 25 heating temperatures -replacement ratios coupling conditions was studied. The following conclusions are drawn through research:

• (1)The change in molybdenum tailings replacement ratio will not affect the apparent feature and failure mode of molybdenum tailings concrete after exposure to high temperature, but the increase in temperature will aggravate the failure of concrete under axial compression. With the increase in temperature (20–800 °C), the surface color of specimens gradually becomes lighter, and obvious cracks begin to appear from 400 °C.
• (2)Under 200 °C, the mass loss ratio of molybdenum tailings concrete under all replacement ratios is less than 1 %, and the growth ratio begins to increase after 400 °C. At 800 °C and 100 % replacement ratio, the mass loss ratio of the specimen reaches a maximum of 6.58 %. As the temperature increases from 20 °C to 800 °C, the peak stress and elastic modulus of molybdenum tailings concrete at all replacement ratios show a decreasing trend. The peak strain shows an increasing trend, and the maximum increase is 344 % at 800 °C and 25 % replacement ratio.
• (3)With the increase in the replacement ratio of molybdenum tailings from 0 to 100 %, the peak stress, peak strain and elastic modulus of molybdenum tailings concrete at all temperatures increase first and then decrease. The 25 % molybdenum tailings content can improve most of the mechanical properties of molybdenum tailings concrete after exposure to high temperature.
• (4)Based on the experimental data, the prediction formulas of the axial compression mechanical properties (peak stress, peak strain and elastic modulus) of molybdenum tailings concrete are proposed by considering the heating temperature T and the replacement ratio R as independent variables. The calculated results are in good agreement with the experimental values.

CRediT authorship contribution statement

Man Xu: Formal analysis. Hanzhao Zhang: Data curation, Writing - original draft. Jian Yuan: Conceptualization, Writing - review & editing. Suhui Yu: Project administration, Resources.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.