Study of the Performance of Emulsified Asphalt Shotcrete in High-Altitude Permafrost Regions

Abstract

To improve the performance of shotcrete in high-altitude and low-temperature environments, emulsified asphalt shotcrete (EASC), which can be used in negative-temperature environments, was prepared by using low-freezing-point emulsified asphalt, calcium aluminate cement, and sodium pyrophosphate as modified materials. The effect of emulsified asphalt on the performance of shotcrete was investigated through concrete spraying and indoor tests. Then, the modification mechanism of emulsified asphalt with respect to EASC was analyzed by combining scanning electron microscopy images and the pore structure characteristics of EASC. The results showed that in a negative-temperature environment, the incorporation of emulsified asphalt delayed the formation of the peak of the cement hydration exotherm, slowed the rate of the cement hydration exotherm, reduced the thermal perturbation of permafrost by EASC, increased the cohesion of the concrete, improved the bond strength between EASC and permafrost, and reduced the rate of rebound. The mechanical strength of the studied EASC decreased upon increasing the amount of emulsified asphalt in the admixture, and its resistance to cracking gradually improved. A content of less than 5% emulsified asphalt could improve the internal pore structure of EASC, thus improving its durability. Increasing the content of emulsified asphalt affected the hydration process of the cement, and the volume content of the capillary pores and macropores increased, which reduced the durability of the EASC.

1. Introduction

Shotcrete is a kind of concrete material that is created by utilizing a pressure gun to spray fine stone concrete on a target surface. This material is widely used in tunnel lining structural supports and in various repair and reinforcement projects; additionally, this material has good economic efficiency and a short construction period, among other characteristics [1,2,3]. Moreover, the combination of reinforcement meshes, anchor rods, and other support tools can significantly improve the protection provided by structures [4]. The main spraying methods used for shotcrete are dry spraying and wet spraying [5]. As wet-sprayed concrete has better homogeneity and durability than dry-sprayed concrete and can effectively reduce rebound rate and dust concentrations, resulting in less environmental pollution, it is a cleaner mixture; thus, global researchers in the field of sprayed concrete technology have focused on develo** the wet-spraying method [6,7,8]. However, because the wet-spraying method requires the mixing of aggregates, cement, and water in the designed proportions before construction, quality cannot be effectively guaranteed under adverse weather conditions, which limits its application in perennial permafrost zones at high altitudes [9].

With the development of Western Development 2.0 and the national defense strategy, human activities and resource protection measures in the high-elevation area of the Qinghai–Tibet Plateau have gradually intensified. In addition, the construction of a transportation network on this plateau has been comprehensively planned, and the construction of tunnels in this plateau’s tundra area has become extremely important. The permafrost regions of China’s plateaus are mainly distributed on the Qinghai–Tibeta Plateau. In addition, these regions are characterized by a high altitude, strong radiation, low year-round temperatures, winter temperatures below −10 °C or even −15 °C, and annual freezing periods lasting 7~8 months. These adverse climatic conditions have created great difficulties in regard to the tunnel-opening operations of shotcrete construction [10]. To solve this problem, Fu Zhaogang et al. [11] added a super-early-strength anti-freezing thickener to shotcrete and applied it with a heating device at the nozzle so that wet-sprayed concrete could be successfully applied in the Badaling Tunnel under negative-temperature conditions in the winter. Zhang Dehua et al. [12] considered four aspects—concrete and additive content selection, raw material heating, concrete mixing and transportation, and concrete maintenance—to construct the Wind Volcano Tunnel to prevent the freezing of the concrete under high-plateau alpine conditions, providing a reference for the design of tunnel ventilation in high-altitude permafrost areas. Yang Anjie et al. [13] introduced the application of wet-sprayed concrete in the Kunlun Mountain Tunnel by regulating the mixing amounts of antifreeze and accelerators, using automatic temperature-controlled spraying machinery and strictly controlling the temperature of the sprayed concrete mix contacting the spraying surface. Wet-sprayed concrete technology has been successfully applied to tunnels in permafrost zones, contributing to the development of this technology in high-altitude multiyear permafrost zones.

When tunnels are constructed in permafrost areas, their perimeters will be tubular heat exchange surfaces that run through the permafrost; this process leads to an effect that is very different from that of roadwork on perennial permafrost [14]. Therefore, the impacts of tunnel construction processes on the thermal environment of the surrounding rocks in permafrost zones should be considered first, especially the impact of shotcrete on thermal stability. In addition, due to the frequent alternation of positive and negative temperatures in tunnels in perennial permafrost regions, the internal moisture area of shotcrete increases s via freezing and thawing; thus, the frost resistance of shotcrete is an important indicator of its durability [15]. Several scholars have carried out in-depth research on the frost resistance of shotcrete. Haldane [16] suggested that the frost resistance of shotcrete is affected by the distribution of pores and is related to the water–cement ratio. Wan et al. [17] reported that the addition of micro/nanobubbled water to shotcrete can improve its pore structure, thus improving its frost resistance. Hu et al. [18] conducted a study on the current research status of shotcrete in cold tunnels. According to this research, the factors influencing shotcrete in cold-zone tunnels were summarized, and the development direction of shotcrete in cold-zone tunnels was predicted. Holter et al. [19], regarding cold zone tunnel linings that are prone to freezing damage, investigated the frost resistance of shotcrete with a water–cement ratio of 0.45–0.47 through experiments and proposed a new functional freeze–thaw cycle test method that could realistically simulate the moisture and freezing gradient conditions in tunnel linings. Kalhori et al. [20] studied the effect of nanomaterials on the frost resistance of shotcrete and reported that both nanoclay and nanosilica could improve the frost resistance of shotcrete and that the effect of nanosilica was better than that of nanoclay.

Although some results have been achieved regarding the application of and research on wet-sprayed concrete in highland permafrost areas, there are still many difficulties in tunnel lining construction under these conditions. Minimizing the thermal disturbance of permafrost by concrete, ensuring the solidity of the bond between the concrete and permafrost surrounding rock, and improving the durability of concrete in a negative-temperature environment are all urgent problems that need to be solved. On this basis, flexible emulsified asphalt is often added to shotcrete to reduce the thermal disturbance of permafrost, increase viscosity, reduce rebound, and improve durability. The purpose of this study is to investigate the effect of emulsified asphalt on the properties of shotcrete in a negative-temperature environment. The results obtained can provide a theoretical basis and practical guidance for the design and construction of shotcrete in shallow buried tunnels in high-altitude permafrost areas.

2. Materials and Methods

2.1. Raw Materials

The negative-temperature cement used in this study consisted of a mixture of Portland cement and calcium aluminate cement, both of which were produced by Bei**g Construction Engineering Group Co., Ltd. (Bei**g. China) An X-ray fluorescence (XRF) spectrometer (Rigaku ZSX Primus III+, Hitachi, Tokyo, Japan) was used to analyze the chemical compositions of the two types of cement. An X-ray diffractometer (XRD) (Ultima IV, Hitachi, Japan) was used to conduct a continuous in situ test on the two types of cement; the scanning angle range was 5° to 65°, and the scanning speed was 5°/min. The chemical compositions and XRD patterns of the two types of cement are shown in

Table 1. Chemical composition of the cement.

Type of Cement	Mass fraction (wt.%)
	SiO2	CaO	Al2O3	SO3	MgO	Fe2O3	KO2	Na2O	TiO2	LOI
Portland cement	21.50	64.20	4.14	2.89	2.57	2.40	0.84	0.67	0.32	0.40
Calcium aluminate cement	0.68	26.80	71.30	0.04	0.46	0.09	0.02	0.37	0.01	0.70

and Figure 1, respectively. In abbreviated notation, the different compounds were designated as C₃S, C₂S, C₃A, C₄AF, CA, and CA₂, where C stands for calcium oxide (lime), S stands for silica, A stands for alumina, and F stands for iron. The emulsified asphalt was anionic with 50% solids, and it was provided by Chongqing CNOOC Modern Transportation Oil Material Co., Ltd. (Chongqing, China) Its technical parameters are shown in

Table 2. Technical parameters of emulsified asphalt.

Technical Parameters		Test Index
Demulsification speed		Rapid setting
Angler viscosity (Tested at 25 °C)		8
Properties of evaporation residue	Penetration at 25 °C (0.1 mm)	66.7
	Softening point (°C)	54.0
	Ductility at 15 °C (cm)	>150

. Sodium pyrophosphate, sodium nitrite, and calcium acetate produced by Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China), were used as AR reagents.

The fine aggregate used for the preparation of the shotcrete was river sand with a fineness modulus of 2.8. The coarse aggregate was continuous graded gravel with a maximum nominal size of 5~10 mm. It was provided by Bei**g Construction Engineering Group Co., Ltd. The water reducer used was polycarboxylic acid, a high-performance water reducer produced by Jiangsu Subot New Material Co., Ltd. (Nan**g, Jiangsu, China). The aqueous solution used in this study was a 20 wt.% sodium nitrite solution including tap water.

2.2. Negative-Temperature Shotcrete Proportioning

2.2.1. Negative-Temperature Binder Ratio

Based on the existing research results obtained by our group [21], the mass ratio of silicate cement/calcium aluminate cement used was 7:3. This material was externally mixed with 3% of the total cement mass of sodium pyrophosphate to create the negative-temperature cement used in this study, and its initial solidification time at −5 °C was 90 min. Low-freezing-point emulsified asphalt was prepared by adding 8% calcium acetate by mass to emulsified asphalt, which was externally mixed. Low-freezing-point emulsified asphalt was not frozen and had good fluidity at −5 °C; an optical microscopy photo of it is shown in Figure 2.

2.2.2. Shotcrete Test Ratio

To investigate the effect of emulsified asphalt on the performance of EASC, emulsified asphalt was mixed into concrete at cement mass ratios of 0%, 2.5%, 5%, 7.5%, and 10%. A workability test was performed on concrete specimens with different mixing ratios. Then, the mixing ratios of the EASC test specimens with different emulsified asphalt admixtures were determined, revealing that the water/cement ratio was 0.4 and the sand ratio was 50%. The test mixes are shown in

Table 3. Base mixing ratio of shotcrete (kg/m³).

Groups	Cement	Emulsified Asphalt	Fine Aggregate	Coarse Aggregate	Water	Water Reducer
EA-0	480	--	858	858	192	6
EA-2.5	474	12	858	858	186	5
EA-5	468	24	858	858	180	4
EA-7.5	462	36	858	858	174	3
EA-10	456	48	858	858	168	2

.

2.3. Test Methods

To accurately simulate a negative-temperature environment at high altitudes, a constant-temperature and constant-humidity environment test chamber was used to maintain all the raw materials and specimens. The ambient temperature was set to −5 °C, and the relative humidity was set to 50%. The appearance of the test chamber is shown in Figure 3. All the raw materials were placed in a test box to maintain a warm temperature for 24 h, and all the spraying tests were completed within 30 min to eliminate the effects of temperature changes on the test results. The specimens to be tested indoors were demolded after 1 d of maintenance, after which the specimens were covered with cling film to continue maintenance. Finally, the specimens that were maintained under these conditions for this duration were removed for an indoor test.

2.3.1. Hydration Temperature Rise Test

In this test, 10 cm thick frozen soil with 10% moisture content was placed in a 50 cm × 50 cm × 50 cm open acrylic box. Then, a certain thickness of EASC was applied to the surface of the frozen soil. Afterward, temperature sensors were buried at the interface between EASC and frozen soil and at a frozen soil depth of 3 cm to monitor the temperature change at the interface and inside the frozen soil in real time, and the real-time temperature value was recorded once every 3 min. The test principle and device are shown in Figure 4.

2.3.2. Bond Strength Test

A 100 mm × 100 mm × 50 mm permafrost specimen with a moisture content of 20% was placed in a 100 mm × 100 mm × 100 mm test mold. Then, the top of the test mold was sprayed with freshly mixed EASC. Afterward, the negative-temperature top surface of the mold was cured. After one day, the film was cured for 28 days, and the permafrost and the EASC bonding surface of the splitting test were removed, as shown in Figure 5. The split tensile strength of the concrete was used as the bond strength value of the EASC, and the split tensile test was based on the provisions of GB/T 50081-2019 Standard for Test Methods of Physical and Mechanical Properties of Concrete [22]. The whole test process was completed within 5 min to eliminate the effect of temperature on the thermal disturbance of the frozen soil specimen.

The splitting tensile strength of the EASC was calculated using Formula (1):

$$f_{ts}=\frac{2P}{\pi A}=0.637\frac{P}{A}$$

(1)

where f_ts denotes EASC splitting tensile strength (MPa), P denotes breaking load (N), and A denotes specimen splitting area (mm²).

2.3.3. Rebound Rate Test

In the EASC spraying test, we adopted the large-plate method of molding; the sprayed concrete that was obtained through this method exhibited excellent compactness and uniformity, and its rebound rate could be tested with a small amount of concrete material, effectively preventing waste and ensuring the effective utilization of resources. The dimensions of the large plate were 50 cm × 50 cm × 20 cm, and a spraying pressure of 0.4 MPa was applied. The vertical distance from the nozzle to the spraying surface was 1.5 m, and the amount of concrete attached to the slab was approximately 125 kg per square meter. The test principle is shown in Figure 6, and the test process is shown in Figure 7. The formula for calculating the rebound rate is shown in Equation (2):

$$R=\frac{{\mathrm{W}}_2}{{\mathrm{W}}_1{+\mathrm{W}}_2}\times 100\%$$

(2)

where R is the rebound rate (%), W₂ is the mass of rebounded concrete (g), and W₁ is the mass of concrete stuck to the mold (g).

2.3.4. Mechanical Property Tests

The compressive and flexural strength specimens of EASC were fabricated with reference to GB50086-2015 Technical Specification for Rock Anchor and Shotcrete Support Engineering [23], and the test method was selected with reference to GB/T 50081-2019. The compressive and flexural strengths of the EASC specimens were tested after curing them at negative temperatures for 3 d, 7 d, and 28 d, after which the flexural compression ratio was calculated.

2.3.5. Durability Performance Tests

The water penetration resistance and chloride penetration resistance of EASC were evaluated using the stepwise pressurization method and the electric flux method, respectively, in accordance with GB/T 50082-2009, Standard of Test Methods for Long-Term Performance and Durability of Ordinary Concrete [24].

Using the fast-freezing method in GB/T 50082-2009 as a reference, EASC specimens with a curing age of 28 days were subjected to 300 freezing and thawing cycles, and the mass loss rate and relative dynamic elastic modulus of the specimens were tested every 25 freezing and thawing cycles.

2.3.6. Scanning Electron Microscopy (SEM) and Energy-Dispersive Spectrometry (EDS)

A Nova Nano 230 scanning electron microscope (from FEI, Hillsboro, OR, USA) was used for microimaging the EASC test block samples with a conservation age of 28 days at a magnification of 1000×, and it was used in EDS face-scanning mode for trace element analysis.

2.3.7. Mercury Intrusion Porosimetry (MIP)

The shotcrete specimens cured at a negative temperature for 28 days were cracked and placed in an ethanol solution to terminate hydration. The specimens were removed and placed in a 60 °C oven to allow them to dry for 4 h before the test, after which the test was initiated. The pore structures of the EASC specimens were tested using a fully automatic mercuric pressure apparatus (modeled as an Autopore IV 9500; manufactured by Mack Corporation from the United States, Atlanta, GA, USA).

3. Results and Discussion

3.1. Heat Transfer Performance

In the process of excavating perennial permafrost tunnels, steel arches are usually assembled after 1 d of shotcrete construction to provide temporary support [25]. After the construction of shotcrete, its hydration exotherm creates a thermal disturbance affecting the rock surrounding the permafrost. However, to prevent thawing and collapse, shotcrete should not induce a large thermal disturbance affecting the rock surrounding the permafrost, as this can affect the quality of a project. The influences of thermal disturbance of the EASC on the permafrost within 1 day are shown in Figure 8. The test results show that 1 d after pouring EASC, the temperature at the frozen soil interface and 3 cm inside the frozen soil first increases and then decreases. This is because calcium aluminate cement can quickly hydrate and release a large amount of heat in a negative-temperature environment; with the passage of time, the rates of cement hydration and heat release slow down. Upon increasing the mixing of emulsified asphalt, the maximum temperature at the interface between the EASC and permafrost decreases from 4.52 °C to 0.84 °C, and the time until the maximum temperature is reached extends from 16.95 h to 18.75 h. Similarly, the maximum temperature at 3 cm from the surface layer of permafrost decreases from 0.35 °C to −3.92 °C, and the time until the maximum temperature is reached extends from 18.00 h to 22.25 h. These results arose because after mixing emulsified asphalt, the demulsification process of emulsified asphalt absorbs some of the heat generated by the hydration of cement, which slows the rate of exothermic cement hydration and extends the time required to reach the exothermic peak of hydration [26]. Therefore, when increasing the emulsified asphalt content, the thermal perturbation of permafrost by EASC gradually decreases.

3.2. Bond Strength

Increased bonding of shotcrete to a wall reduces its rebound rate and improves the densification of the concrete. Figure 9 shows the variation curve of the bond strength between the EASC and permafrost for different emulsified asphalt proportions. The bond strength test results reveal that the bond strength between the EASC and permafrost increases with an increasing emulsified asphalt content. When the content of emulsified asphalt increases to 10%, the bond strength of the EASC reaches 2.11 MPa, which is 75.8% greater than the bond strength of 1.20 MPa between ordinary shotcrete and permafrost. This increased strength arises because cement hydration accelerates the demulsification behavior of emulsified asphalt, and small asphalt particles gradually aggregate into large particles, restoring the high viscosity of the asphalt and increasing the adhesion between the interface of the EASC and permafrost. In addition, under the adverse influence of a negative-temperature maintenance environment and the adsorption effect of emulsified asphalt on cement particles, a few cement particles do not participate in the hydration reaction. The cement particles that have not undergone sufficient hydration reactions can activate mineral powder, further improving the adhesion between the two materials and the bond strength of EASC [27]. Therefore, the incorporation of emulsified asphalt in shotcrete can effectively improve the bonding between the aggregate and the permafrost interface.

3.3. Rebound Rate

To reduce the rebound rate of concrete, accelerators are usually added to ordinary shotcrete to accelerate the hydration and hardening of concrete to allow it to bond quickly to a wall [28,29,30,31]. In this study, to reduce the rebound rate of shotcrete, emulsified asphalt was added to increase viscosity. Figure 10 shows the change curves of the rebound rates of EASC specimens with different emulsified asphalt admixtures. As shown in Figure 10, the rebound rate of EASC in a negative-temperature environment continues to decrease upon increasing the emulsified asphalt content. The lowest rebound rate was only 13.8% when the content of emulsified asphalt was 10%. This low rate arose because cement hydration accelerates the consumption of water. Conversely, water dissolves Ca²⁺, Al³⁺, and other cations, destroying the double-electron layer structure of emulsified asphalt and thus accelerating asphalt emulsification under negative-temperature conditions [32,33]. After demulsification, the emulsified asphalt provides sufficient bonding force for EASC, which, in turn, improves bonding performance with the wall and reduces the rebound rate.

3.4. Mechanical Properties

The variation in the mechanical strength of EASC containing different proportions of emulsified asphalt is shown in Figure 11, and Figure 11a,b show that the content of emulsified asphalt reduces the compressive and flexural strength of EASC. At a curing age of 28 days, the compressive and flexural strength values of EASC with a 10% emulsified asphalt admixture are 59.4% and 74.6%, respectively, of those of ordinary shotcrete. This reduction in concrete strength induced by the emulsified asphalt occurs due to the lack of strength of the asphalt material itself. In addition, an increase in the emulsified asphalt proportion slows the hydration and hardening processes of cement, thus reducing the mechanical strength of EASC [34]. In the negative-temperature curing environment, the late strength growth of EASC was relatively slow. The 28 d compressive strength and flexural strength of ordinary shotcrete increased by 13.0% and 7.9%, respectively, compared with those at 7 days. This strength increase occurred because the incorporation of sodium pyrophosphate can significantly accelerate the hydration of cement; moreover, the incorporation of calcium aluminate cement is conducive to the production of high compressive strength in the EASC during the first 3 days of negative-temperature curing but is not conducive to strength increases in the later stage [21].

The concrete flexural/compressive ratio is the ratio of concrete’s flexural strength to its compressive strength, and it is a dimensionless parameter used to indicate the brittleness of concrete; the greater the flexural/compressive ratio, the better the flexibility and cracking resistance of the concrete [35]. Figure 11c shows that an increase in emulsified asphalt content increases the flexural/compressive ratio of EASC; that is, the material is compressed to a relatively great extent at the same pressure. This phenomenon occurs due to the high viscosity and flexibility of the emulsified bitumen, which increases the plastic deformation capacity of the material. Under pressure, the asphalt component of the EASC acts as a cushion, facilitating the plastic deformation of the concrete and increasing its flexural/compressive ratio and crack resistance.

3.5. Durability

3.5.1. Water Penetration Resistance

Water pressure was applied in a step-by-step manner to determine the water penetration resistance of the EASC, expressed in terms of impermeability grade. The results of the EASC water penetration test are shown in

Table 4. Permeability ratings of EASC specimens containing different emulsified asphalt proportions.

Grou**	EA-0	EA-2.5	EA-5	EA-7.5	EA-10
Impermeability rating	P12	P16	P20	P8	P6

. According to the table, with an increase in the emulsified asphalt content, the impermeability of EASC first tends to increase and then decrease. When the content of emulsified asphalt is 5%, the impermeability grade of EASC reaches P20, the highest level of resistance to water penetration. This phenomenon arises because when the content of emulsified asphalt is 5%, the hydration reactions that occur during the emulsion breaking of the cement and emulsified asphalt are synchronized in a negative-temperature maintenance environment, and the products formed by the two are independent yet intertwined with each other, infiltrating each other and then forming an effective spatial three-dimensional network structure [36]. This spatial structure can effectively impede the migration of water inside the EASC and improve the water infiltration resistance of the EASC. In addition, asphalt is a hydrophobic material, and the asphalt membrane structure formed after emulsified asphalt demulsification can effectively block the transmission of water in the EASC. When the content of emulsified asphalt is more than 5%, there is an obvious delay in the hydration of cement under negative-temperature maintenance, leading to the incomplete hydration of cement. This process does not generate enough hydration products to fill the internal pores of EASC; thus, the water penetration resistance of EASC decreases.

3.5.2. Resistance to Chloride Penetration

The electrical flux of concrete can be used to characterize its resistance to chloride ion permeation. The resistance of concrete to chloride ion permeation can be assessed by applying a specific voltage to the ends of a concrete specimen and measuring the cumulative electrical flux through the concrete over a certain period. Depending on the value of the electric flux, concrete can be categorized into different classes of resistance to chloride ion permeation. When the electric flux is greater than 4000 coulombs, the concrete has high chloride permeability. When the electric flux is 2000–4000 coulombs, the concrete has medium chloride permeability. When the electric flux is 1000–2000 coulombs, the concrete has low chloride permeability. When the electric flux is 100–1000 coulombs, the concrete has very low chloride permeability. Finally, when the electric flux is less than 100 coulombs, the concrete has negligible chloride permeability [37].

The results of the EASC flux test on the specimens with different emulsified asphalt proportions are shown in Figure 12. As shown in Figure 12, when increasing the emulsified asphalt content, the anti-chlorine penetration performance of EASC tends to first increase and then decrease. When the content of emulsified asphalt is 2.5% or 5%, the EASC has very low chlorine penetration. When the content of emulsified asphalt is 5%, the flux of the EASC is only 863C, and its anti-chlorine penetration performance is the best. When the chloride permeability performance is the best, continuing to increase the content of emulsified asphalt will have a detrimental effect on the EASC’s anti-chlorine permeability. The reason for this degradation is that the appropriate mixing of emulsified asphalt reduces the number of harmful pores inside the EASC, weakening the chloride ion penetration channel; additionally, emulsified asphalt is a hydrophobic material that can resist the penetration of water in the concrete, thus hindering the transmission of chloride ions. If the content of emulsified asphalt continues to increase, an overly high amount of emulsified asphalt greatly impedes the cement hydration process, leading to incomplete hydration and ultimately affecting the formation of a dense microstructure in the EASC [38].

3.5.3. Frost Resistance

Figure 13a shows the variation curve of the EASC mass loss rate under the action of freeze–thaw cycles. With an increase in the number of freeze–thaw cycles, the mass loss rate of EASC continually increases. Under the same number of freezing and thawing cycles, the EASC composite containing 5% emulsified asphalt has the lowest mass loss rate and the best frost resistance. Its mass loss rate after 300 freeze–thaw cycles is only 1%, and the mass loss rate of EASC containing more than 5% emulsified asphalt is greater than that of ordinary shotcrete. On the one hand, when the emulsified asphalt content is less than 5%, the emulsified asphalt can enhance the adhesion between aggregates after emulsification, thus improving the integrity of the concrete. On the other hand, because the asphalt composition of the water barrier reduces the water content of the concrete, the probability of concrete micropore freezing damage occurring is also reduced under freezing and thawing cycles. When the content of emulsified asphalt is greater than 5%, too many asphalt particles form on the surface of the cement emulsion. In addition, too many asphalt particles adsorb onto the surface of the cement particles, affecting the normal hydration of the cement. As a result, the hydration products are not abundant, and the structure is not dense. At this time, the water migration channel inside the concrete widens, and it very easily expands during freeze–thaw cycling, which leads to the destruction of the internal structure of the concrete and a decrease in frost resistance. The mass loss rates of the EASC specimens with different emulsified asphalt proportions show similar patterns of change. Mass loss is the first to increase rapidly, and the mass loss rate reaches its first peak after 50 freeze–thaw cycles. This phenomenon occurs due to the freezing stress generated during the freeze–thaw cycle, leading to the gradual peeling of the surface layer of the test specimen. In addition, the mass loss rate increases. After 50~150 cycles, the mass loss rates of the EASC specimens with different emulsified asphalt proportions decrease because freezing and thawing damage occurs only on the surfaces. After 150 freeze–thaw cycles, the mass loss rate of shotcrete with various emulsified asphalt proportions accelerates again due to the continuous action of freeze–thaw cycles; these cycles result in the intensification of matrix damage, and the mass loss rate increases significantly [39].

Figure 13b shows the change curve of the relative dynamic elastic modulus of the EASC under freeze–thaw cycling. With the increase in the number of freeze–thaw cycles, the relative dynamic elastic modulus of the EASC decreased continuously, and the relative dynamic elastic moduli of the EASC specimens mixed with 7.5% and 10% emulsified asphalt decreased to below 60% after 125 and 175 freeze–thaw cycles, respectively. Then, the freeze–thaw test was stopped. At 300 freeze–thaw cycles, the relative kinetic modulus of elasticity of ordinary shotcrete decreased by 20.1%, and the relative kinetic moduli of elasticity of the EASC specimens with 2.5% and 5% emulsified asphalt decreased by 14.3% and 12.9%, respectively. Compared with the EASC with an emulsified asphalt content greater than 5%, the type with an emulsified asphalt content less than 5% has better frost resistance because the emulsifier is a surfactant. This component is similar to an air-entraining agent, which introduces stable and tiny air bubbles in concrete to isolate the water transport channel, thus relieving the water crystallization pressure [17,40]. Conversely, because the low content of emulsified asphalt does not negatively affect cement hydration, the emulsion-breaking process and hydration process occur at the same time, generating a flocculent colloid intertwined with the asphalt film and cement hydration products. This flocculent colloid fills some of the large pores inside the concrete; thus, the EASC specimens containing 2.5% and 5% emulsified asphalt have great frost resistance levels. When the amount of emulsified asphalt admixture is further increased, the excess emulsifier and asphalt components affect the hydration of the cement and reduce the compactness of the concrete, thus weakening the frost resistance of the concrete. In summary, the frost resistance of EASC first increases and then decreases when increasing the emulsified asphalt content, and the frost resistance is greatest when the content of emulsified asphalt is 5%.

3.6. Hole Structure Analysis

The pore structure of EASC can be observed to analyze the modification mechanism of emulsified asphalt at a fine scale. In this study, the pore structure and pore size distribution of EASC were systematically observed and calculated by using scanning electron microscopy and mercury intrusion porosimetry.

3.6.1. SEM Images

A 1000-times-magnified SEM image of the EASC sample after 28 days of negative-temperature curing is shown in Figure 14. Figure 14a shows that the ordinary shotcrete was fully hydrated in the negative-temperature environment, the hydration products were well developed and concentrated, and the structure of the concrete is dense. Figure 14b shows that when the amount of emulsified asphalt is 2.5%, the aggregate surface is flat and covered with a layer of asphalt. Figure 14c shows that when the content of emulsified asphalt is 5%, the aggregate surface is covered with fibrous hydrates generated by the cementation of cement and emulsified asphalt. Figure 14d shows that when the content of emulsified asphalt is 7.5%, the internal EASC is affected by asphalt emulsification, which generates many connected pores. Figure 14e shows that when the content of emulsified asphalt is 7.5%, the internal EASC is covered with fibrous hydrates. Moreover, when the content of emulsified asphalt is 10%, the coating behavior of the emulsified asphalt on the cement particles seriously affects the hydration process of the cement. In addition, there are insufficient hydration products inside the EASC, which, in turn, results in the formation of additional connected large pores. The EDS elemental analysis of EASC specimens with different emulsified asphalt proportions shows that with an increasing content, the concentrations of C and O gradually increase, and the concentrations of Si, Ca, and Al gradually decrease. This finding indicates that the emulsified asphalt adsorbs onto the surfaces of the cement particles and hinders the generation of cement hydration products, a finding that is consistent with the results of Li et al. [41].

The SEM images and EDS elemental analyses of EASC specimens with different emulsified asphalt proportions show that when the emulsified asphalt content is less than 5%, the emulsified asphalt has no unnecessary effect on the hydration of the cement particles. In addition, the fibrous hydrate generated by the emulsified asphalt’s mutual cementation with cement fills the pores inside the concrete. However, when the emulsified asphalt content is greater than 5%, the emulsified asphalt breaks and negatively impacts the hydration process of the cement, impeding the formation of cement microstructures. This impediment leads to an increase in the number of large pores inside the EASC. The formation of a cement microstructure leads to an increase in the number of large pores and an increase in the porosity of EASC.

3.6.2. Pore size Distribution

According to the results obtained by Odler et al. [42], there are four pore sizes in cement concrete: gel pores (pore less than 10 nm), transition pores (pores ranging from 10 to 100 nm), capillary pores (pores ranging from 100 to 1000 nm), and macropores (pores greater than 1000 nm). An MIP test was carried out on the pore structures of EASCs with different emulsified asphalt proportions, and the results are shown in Figure 15. Figure 15 shows that the proportions of capillary pores and macropores in EASC first decrease and then increase upon increasing the emulsified asphalt content. When the content of emulsified asphalt is 5%, the proportions of capillary pores and macropores in EASC are the smallest, amounting to 19.47% and 4.03%, respectively. When the content of emulsified asphalt was increased to 10%, the volumes of capillary pores and macropores reach 30.58% and 19.02% of the total pore volume, respectively. Hover et al. [43] concluded that capillary pores and macropores mainly affect the durability of concrete. Therefore, the lower the capillary pore content and macroporous content of an EASC specimen, the better its durability performance.

The above analysis reveals that the cement and emulsified asphalt were cemented with each other, generated fibrous hydration, and further expanded into the surrounding space, allowing them to effectively fill large pores. However, too much emulsified asphalt hinders the hydration process of cement and affects the formation of the cement microstructure. Therefore, using an appropriate amount of emulsified asphalt in shotcrete can improve its durability by effectively remedying its high porosity and poor densification.

4. Conclusions

In this study, the effects of emulsified asphalt on the heat transfer performance, working performance, mechanical strength, and durability of shotcrete were comprehensively investigated in a negative-temperature environment. In addition, the pore structure characteristics were evaluated via SEM image analysis and MIP. These analyses were conducted to clarify the modification mechanism of emulsified asphalt for EASC. The main conclusions obtained are as follows:

(1)

In a negative-temperature environment, the incorporation of emulsified asphalt increased the viscosity of the concrete, which improved the bonding performance between EASC and permafrost and reduced the rebound rate. When the content of emulsified asphalt was 10%, the rebound rate of EASC was reduced to 13.8%, and the bond strength between it and permafrost was 2.11 MPa.

(2)

In a negative-temperature environment, the addition of sodium pyrophosphate to calcium aluminate cement accelerated the early mechanical strength development of EASC but was detrimental to its later strength development. Upon increasing the emulsified asphalt content, the mechanical strength of EASC decreased, and crack resistance gradually increased.

(3)

A content of emulsified asphalt less than 5% could improve the internal pore structure of EASC and reduce porosity, thus improving durability. When the content of emulsified asphalt was 5%, the durability of EASC was the best. The impermeability grade was P20, the electrical flux was 863C, the mass loss rate after 300 freeze–thaw cycles was 1%, and the relative dynamic elastic modulus decreased by 14.3%. Increasing the content of emulsified asphalt further affected the cement hydration process, thus reducing the impermeability and frost resistance of EASC.

(4)

When the content of emulsified asphalt exceeded 5%, the ratio of capillary pores to macropores in the EASC increased gradually when increasing the emulsified asphalt content, reducing the concrete’s durability. When the content of emulsified asphalt was 10%, the volumes of capillary pores and macropores reached 30.58% and 19.02%, respectively, of the total pore volume.

(5)

The results of this study show that we not only successfully reduced the rebound rate of sprayed concrete, avoided wasting raw materials, and improved the durability of sprayed concrete but also provided a theoretical basis for the use of wet sprayed concrete in permafrost areas in China’s plateau regions.