Research on Mechanical Properties of Cement Emulsified Asphalt Mortar Under the Influence of Water-to-Cement Ratios and Water-Reducing Agent

Abstract

To understand the mechanical behavior of CRTS (China Railway Track System) II cement emulsified asphalt mortar (CA mortar), this study tested the compressive strength and flexural strength of CA mortar at different ages under varying water-to-cement ratios and dosages of water-reducing agent. Based on X-ray diffraction (XRD) and scanning electron microscopy (SEM) results, the hydration products and microstructure of CA mortar at different ages were analyzed. The main conclusions are as follows. As the water-to-cement ratio increases, the compressive strength and flexural strength of CA mortar generally exhibit a decreasing trend. The strength increases rapidly in the early stages, with the 7-day compressive strength reaching over 80% of the 28-day compressive strength, and the 7-day flexural strength reaching over 93% of the 28-day flexural strength. As the dosage of water-reducing agent increases, both the compressive strength and flexural strength of CA mortar first increase and then decrease, with a reasonable range of water-reducing agent dosage being between 0.2% and 1.0%, and 0.5% is most appropriate. The hydration reaction of CA mortar is nearly complete at 3 days, with the increase in ages, the cement hydration slows down due to the coating action of asphalt, and the strength no longer changes greatly. Hydration products are mainly Ettringite, which is the main source of strength of CA mortar. After the emulsified asphalt breaks, it adsorbs onto the hydration products and sand surfaces, gradually forming a continuous phase, which enhances the structural toughness of the CA mortar.

1. Introduction

Cement emulsified asphalt mortar (CA mortar) is an inorganic–organic composite cementitious material made from cement, emulsified asphalt, sand, water, and additives [1,2,3]. It is used as the filling material in the ballastless track structure layer of high-speed railways in China, where it plays a significant role in bearing loads, transmitting forces, geometric adjustment, and providing damping [4,5,6]. CA mortar can be classified into CRTS I and CRTS II types. During construction, CRTS I type CA mortar is poured into the perfusion bag, and there is no bond between it and the track plate and the base plate, and only bears the longitudinal load from the track plate. The CRTS II type CA mortar is injected into the gap between the track plate and the base through the grout hole in the middle of the track plate and is directly bonded to the track plate and the base to bear and transfer the longitudinal and transverse loads from the track plate [7]. In addition, CRTS II type mortar exhibits higher mechanical strength, with a 28-day compressive strength typically exceeding 15 MPa, making it more commonly used in China [8]. Currently, CA mortar is being used as a grouting material, which has led to higher demands for its flowability [9]. This is typically achieved by increasing the free water content or adding water-reducing agents to improve flowability. However, the preparation of CA mortar with high flowability can introduce more air bubbles [10], and the increase in free water can further weaken the matrix strength. Therefore, it is necessary to study the mechanical properties of CA mortar.

Wang [11], using the 28-day compressive strength of CA mortar as an indicator, found that the compressive strength of high-strength CA mortar slightly decreases as the water-to-cement ratio increases, while it significantly declines as the asphalt-to-cement mass ratio or sand-to-cement mass ratio increases. The strength of CA mortar is primarily determined by the cement–asphalt binder and the interface properties between the binder and sand [12]. Tan [13] conducted experimental studies and found that the compressive strength of high-strength CA mortar at 1 day increases with the increase in the sand-to-cement ratio. However, when the sand-to-cement ratio exceeds 1.4, the 28-day compressive strength of CA mortar significantly decreases. Tian [12] pointed out that, with sufficient CA binder, sand can play a certain skeletal role, but its impact on the compressive strength of CA mortar is minimal. The results of Tan [13] and Tian [12] are not in conflict, as sand has a limited impact on the strength of CA mortar when the sand-to-cement ratio is low. Hu [14] compared the effects of three types of water-reducing agents—polycarboxylate, sodium lignosulfonate, and naphthalene-based agents—and found that polycarboxylate water-reducing agent had the least impact on reducing the compressive strength of CA mortar. Therefore, subsequent studies all used polycarboxylate water-reducing agent. Li [15] studied the optimal dosage of water reducing agent and found that 0.33% of water reducing agent was more appropriate. The content is quite different from that in this study, mainly because the CA mortar in this study has better fluidity, so the content of water reducing agent is higher. Additionally, Wang [16] used environmental scanning electron microscopy and calorimetry to observe that asphalt emulsion delayed the early hydration of cement, and the asphalt film negatively affected the further hydration of the cement. The framework formed by the hardened slurry matrix is the main structural skeleton of high-elastic modulus CA mortar, with the asphalt phase acting as a filler, thus increasing the weak phase of the structural system. Based on the above studies, it can be seen that compressive strength is an important evaluation index of CA mortar. Current studies mostly focus on the influencing factors of compressive strength and the formation mechanism of strength and lack comparative studies at different ages under different factors, failing to describe the strength growth process of CA mortar during the whole curing process. In practical engineering, the early strength of CA mortar is an important factor to determine the maintenance method and construction process, so it is necessary to test the different ages of CA mortar. In addition, although polycarboxylate water-reducing agent has the best effect, the optimal dosage and promotion mechanism of superplasticizer still need to be further explored.

To address these issues, this study focuses on the high-flow CRTS II type CA mortar. Under the influence of water–cement ratio and water-reducing agent, the strength of CA mortar at different ages of 3 days, 7 days, 14 days, and 28 days was compared and its growth rule was analyzed and the strength formation mechanism of CA mortar with the result of XRD and SEM analyses was explored.

2. Materials and Experimental Methods

2.1. Raw Materials

The primary materials for CA mortar include cement, emulsified asphalt, sand, and water. The specific material parameters are listed in

Table 1. Parameters of CA mortar raw materials.

Material Type	Cement	Emulsified Asphalt	Manufactured Sand	Water	Water-Reducing Agent
Parameters	P.I 42.5	Cationic 60% solid content	40–80 mesh	potable water meeting testing standards	Polycarboxylate superplasticizer

.

2.2. Experimental Methods

In accordance with the “Cement and Cement Concrete Testing Procedures for Highway Engineering” (JTG 3420-2020) [17], the specimens were prepared using a mortar mixer, and the size of the specimens were 40 mm × 40 mm × 160 mm. After 24 h of curing, the specimens were demolded and then placed in a curing chamber maintained at a temperature of 20 °C ± 1 °C (including the strength testing room), with a relative humidity greater than 90%.

2.2.1. Compressive Strength Test Method

The compressive strength test was conducted using a compressive fixture, aligning the fixture with the center of the platen of the testing machine. The loading surface of the specimen was the two sides of the specimen formed during casting, with an area of 40 mm × 40 mm. The loading rate for the compressive strength test was 0.5 kN/s. The schematic diagram of the test setup is shown in Figure 1. The formula for calculating the compressive strength is given by Equation (1).

$$R_c={\displaystyle \frac{F_c}{A}}$$

(1)

where $R_c$ is the compressive strength (MPa); $F_c$ is the failure load (N); $A$ is the loaded area (mm²).

For each group of tests, six parallel tests were conducted, and the final compressive strength result was taken as the arithmetic average of the six measured compressive strengths, accurate to 0.1 MPa. If any of the six strength values deviates by more than ±10% from the average, it should be excluded, and the final result should be calculated from the remaining five values. If, within these five values, any one exceeds ±10% of the average, the entire group of specimens is considered invalid.

2.2.2. Flexural Strength Test Method

In this study, the flexural strength was determined using the center loading method. The loading rate for the flexural strength test was 0.02 kN/s, continued until failure occurred. The schematic diagram of the test setup is shown in Figure 2. The formula for calculating the flexural strength is given by Equation (2).

$$R_f={\displaystyle \frac{{1.5F}_f·L}{b^3}}$$

(2)

where $R_f$ is the flexural strength (MPa); $F_f$ is the failure load (N); $L$ is the center-to-center distance between the support cylinders (mm); $b$ is the side length of the square cross-section of the specimen, which is 40 mm.

For each group of tests, three parallel tests were conducted, and the final flexural strength result was taken as the arithmetic average of the three measured flexural strengths, accurate to 0.1 MPa. If any of the three strength values deviates by more than ±10% from the average, it should be excluded, and the final result should be calculated from the remaining values.

2.2.3. Microscopic Test

The X-ray diffraction (XRD) test device is X’Pert PRO MPD with a scanning range of 5°~90° and a scanning speed of 3.0°/min. When testing CA mortar of different ages, the sample was mashed and soaked in anhydrous ethanol for 24 h at the specified age to terminate hydration. Then, the sample was taken out and dried at 40 °C to remove isopropyl alcohol, ground, and passed through a 200-mesh screen. The Scanning Electron Microscope (SEM) test equipment is Hitachi Regulus8100 (Hitachi High-Tech, Japan), the acceleration voltage is 15 kv, and the magnification is 500,000 and 2,000,000.

3. Experimental Results and Analysis

The water-to-cement ratio is a key factor influencing the mechanical strength of CRTS II type CA mortar [18,19]. Under high-flow conditions (with flowability less than 100 s), CA mortar, due to its higher water-to-cement ratio, experiences a complex interaction between cement hydration and the de-emulsification of emulsified asphalt at different curing stages. This leads to significant variations in the mechanical properties of CA mortar at different ages [20,21]. Therefore, in this study, compressive strength and flexural strength were tested under different water-to-cement ratios (including the water in the emulsified asphalt) and varying dosages of water-reducing agents, following the test methods in Section 2.2.1 and Section 2.2.2. The strength mechanism of CA mortar was explained in conjunction with XRD and SEM analysis results. The experimental process employed a controlled variable method, with the material mix proportions shown in

Table 2. Mix ratio of CA mortar.

Material	Cement	Emulsified Asphalt	Manufactured Sand	Water (Total)	Water-Reducing Agent
Mass Ratio	1	0.5	1.2	0.46~0.49	0.0001~0.02

.

3.1. Strength Trends of CA Mortar

3.1.1. Effect of Water-to-Cement Ratio on the Compressive Strength of CA Mortar

The compressive strength of CA mortar with water-to-cement ratios of 0.46, 0.47, 0.48, and 0.49 at 3, 7, 14, and 28 days is shown in Figure 3.

From Figure 3, it can be observed that the compressive strength decreases at all ages with an increase in the water-to-cement ratio. However, the differences at early stages are small, and the gap gradually widens with increasing curing time. As the water-to-cement ratio increases, the amount of water in the CA mortar also increases. While water is necessary for the hydration process, excess water after hydration leaves behind additional voids. These voids weaken the internal structure of the mortar, making it more susceptible to stress concentration under external forces, ultimately leading to material failure. Therefore, a higher water-to-cement ratio typically results in a decrease in compressive strength. In contrast, a lower water-to-cement ratio allows the cement particles to pack more tightly with less water, and the hydration products can better fill the voids, creating a denser structure that effectively resists external forces, leading to higher compressive strength.

At the same time, de-emulsification requires some water to escape, and if the water-to-cement ratio is high, excessive water will hinder the escape of moisture, slowing down the de-emulsification of the asphalt. In the early curing stages (such as 3 and 7 days), this delayed de-emulsification prevents the asphalt membrane from forming fully, hindering its toughening effect, and leading to lower compressive strength in the early stages.

The compressive strength of CA mortar increases rapidly in the early stage. The compressive strength at 3 days can reach over 65% of the 28-day strength, and at 7 days, it can reach over 80% of the 28-day strength. After this, the strength growth becomes slower. Cement hydration consumes the free water in the CA mortar system, promoting the de-emulsification of the asphalt. Once the asphalt de-emulsifies, the asphalt particles adsorb onto the surface of the cement particles. As de-emulsification progresses, the surface of the cement particles becomes gradually enveloped by asphalt, hindering further hydration of the cement. However, as the asphalt particles coat the cement, they form a continuous phase that forms the initial strength framework of the CA mortar. The cement particles gradually break through the asphalt coating, continuing the hydration process upon contact with water molecules. The resulting cement hydration products fill the asphalt framework, forming a denser structure, and the formation of a continuous phase of hydration products leads to the development of the strength framework. This explains the rapid early growth of compressive strength.

CA mortar is essentially a multiphase structure. Once the asphalt framework is formed, it becomes the weak phase of the entire structure. Under compressive stress, cracks are more likely to form within the asphalt phase, gradually propagating and causing fracture. Therefore, the effect of later hydration on the compressive strength of CA mortar is limited.

3.1.2. Effect of Water-to-Cement Ratio on the Flexural Strength of CA Mortar

The flexural strength of CA mortar with water-to-cement ratios of 0.46, 0.47, 0.48, and 0.49 at 3, 7, 14, and 28 days is shown in Figure 4.

From Figure 4, it can be seen that with the increase in the water-to-cement ratio, the 28-day flexural strength of the CA mortar shows a downward trend, and this pattern generally holds for other curing stages as well. Overall, the 3-day flexural strength of CA mortar exceeds 5 MPa, and the 28-day flexural strength exceeds 5.5 MPa, indicating good flexural strength. The early flexural strength of CA mortar increases rapidly, with the 3-day flexural strength reaching about 90% of the 28-day flexural strength, and the 7-day flexural strength reaching more than 93% of the 28-day flexural strength. The formation of flexural strength in CA mortar is essentially completed within the first 7 days. When the water-to-cement ratio changes slightly, significant changes in flexural strength are observed, suggesting that the flexural strength of CA mortar is quite sensitive to the water-to-cement ratio. Furthermore, the higher the water-to-cement ratio, the faster the early increase in flexural strength and the slower the later increase.

With the increase in water-to-cement ratio, the moisture content in CA mortar increases. Excess water during the hydration process leads to the formation of more capillary pores, weakening the overall density of the material. This makes the mortar more susceptible to stress concentration when subjected to bending stress, resulting in a decrease in flexural strength. The increase in water-to-cement ratio also leads to a reduction in the quality of the interfacial transition zone (ITZ) between the CA paste and the aggregate. Under high water-to-cement ratio conditions, excessive moisture in the cement paste causes more micro-pores and micro-cracks to form in the ITZ, making this area more prone to failure, thus reducing the overall flexural strength of the material.

During the early ages, the hydration reaction of cement progresses rapidly, generating a large amount of hydration products, such as ettringite and calcium silicate hydrate (C-S-H). These products quickly fill the pores inside the material, forming a dense structure that provides high early flexural strength. However, the role of the asphalt also cannot be overlooked. After preparation, the emulsified asphalt particles gradually break and form a continuous asphalt membrane. These membrane structures act to toughen the mortar, buffering the stress concentration within the cement matrix, and further enhancing the material’s flexural strength. Particularly within the first 3 days, although the hydration products of the cement dominate the formation of strength, the presence of asphalt significantly improves the material’s crack resistance and toughness, which is why the flexural strength at 3 days approaches 90% of the final strength.

3.1.3. Effect of Water-Reducing Agent on the Compressive Strength of CA Mortar

Water-reducing agent is an important additive in CA mortar. Its primary function is to reduce the mixing water content, thereby improving the density of the cement paste and influencing the compressive strength of CA mortar [22,23]. This study tested the compressive strength of CA mortar with different water-reducing agent contents (0.1%, 0.2%, 0.5%, 1.0%, 2.0%) at 3, 7, 14, and 28 days, as shown in Figure 5.

From Figure 5, it can be seen that with the increase in the water-reducing agent content, the compressive strength of CA mortar initially increases and then decreases, following a pattern of first increasing and then decreasing. When the content of water reducing agent is 0.001, the compressive strength of different stages is basically below 8MPa, and the water reducing agent has basically no effect. When the content of superplasticizer is 0.002~0.01, the 28-day compressive strength is above 14 MPa, and the effect of superplasticizer is obvious. The water-reducing agent effectively lowers the water-to-cement ratio, reduces the porosity inside the mortar, and makes the CA mortar denser, thereby significantly improving its compressive strength. Additionally, the water-reducing agent can disperse the cement particles, promoting their even distribution in the mortar and facilitating the full hydration of the cement, which forms a dense network of hydration products.

However, when the content of water reducing agent is 0.02, the compressive strength decreases obviously, it has two negative effects. First, excessive water-reducing agent may overcoat the cement particles, affecting the contact between the cement particles and inhibiting the hydration reaction, which reduces the amount of hydration products. Second, an excessive amount of water-reducing agent can introduce too many air bubbles during the hardening process or cause water to evaporate too quickly, creating more voids. In addition, excessive water reducing agent will lead to excessive dispersion of cement particles, but reduce its effective hydration surface area, resulting in inadequate hydration reaction between cement particles, hydration products (such as C-S-H, Ettringite) incomplete generation, affecting the densification and strength improvement of mortar. Therefore, the amount of water-reducing agent should be controlled to below 1%, the content of water reducing agent 0.002~0.01 is a reasonable range.

Within the reasonable range, the 28-day compressive strength increases with the water-reducing agent content, while the 3-day strength shows the opposite trend. This is primarily due to the effect of the water-reducing agent on the early hydration reaction of the cement. While the water-reducing agent lowers the water-to-cement ratio, it coats the surface of the cement particles. Although this improves the dispersibility of the cement, it may also suppress the early hydration reaction of the cement, leading to a reduced amount of hydration products in the short term. This delay in the early hydration reaction directly affects the strength development at 3 days, resulting in a relatively lower early compressive strength of the mortar.

3.1.4. Effect of Water-Reducing Agent on the Flexural Strength of CA Mortar

This study tested the flexural strength of CA mortar with 0.1%, 0.2%, 0.5%, 1.0%, and 2.0% water-reducing agent content at 3, 7, 14, and 28 days, as shown in Figure 6.

From Figure 6, it can be seen that with the increase in water-reducing agent content, the flexural strength of CA mortar initially increases and then decreases. The water-reducing agent has a significant impact on the flexural strength of CA mortar, with the maximum 28-day flexural strength reaching over 6.5 MPa, while the minimum 28-day flexural strength is only around 3.0 MPa. When the water-reducing agent content exceeds 1.0%, the early flexural strength is relatively low, but it gradually increases in the later ages.

In the lower range of water-reducing agent content, the addition of the water-reducing agent effectively reduces the water–cement ratio, making the CA paste more compact, reducing its porosity, and improving the flexural strength of the CA mortar. Therefore, with the increase in water-reducing agent content, the flexural strength of the mortar gradually increases in the initial age. However, when an excessive amount of water-reducing agent is added, the trend of flexural strength starts to reverse, showing a weakening phenomenon. The main reason is that an excessive amount of water-reducing agent causes the cement particles to disperse excessively, increasing the distance between the particles, which affects the effective bonding between them. At the same time, an excessively high water-reducing agent content significantly increases the fluidity of the mortar, which can lead to segregation and bleeding during preparation. These phenomena can result in uneven pore distribution during the hardening process, introducing excess air bubbles, reducing the density of the material, and thus weakening the flexural strength. This effect is most significant in the early stages, but as the hydration reaction continues, the “water-reducing” effect of the water-reducing agent gradually weakens, and the flexural strength gradually improves. In summary, the water-reducing agent content should be controlled to below 1%.

3.2. Strength Mechanism of CA Mortar

3.2.1. Hydration Products Analysis

The XRD patterns of CA mortar at different ages are shown in Figure 7. In the XRD patterns, asphalt materials generally do not show obvious diffraction peaks because asphalt is mainly composed of hydrocarbons, most of which are amorphous or non-crystalline materials. Therefore, the XRD pattern of asphalt usually presents a broad background rather than sharp diffraction peaks. Thus, during XRD analysis, the focus is mainly on the cement and its hydration products.

From Figure 7, it can be seen that the XRD pattern at 3 days is already close to that at 28 days. The diffraction peaks include most of the phase composition of the CA mortar after setting, but with slightly different peak intensities. This indicates that the cement hydration reaction in CA mortar occurs quickly, contributing to a higher early strength.

At the 3-day age, the generation of ettringite is particularly significant, as shown by a strong characteristic peak in the XRD pattern. Ettringite is a hydration product formed by the reaction of aluminates in cement with gypsum, and it is produced in large quantities in the early stage. This contributes significantly to the volumetric stability and early strength of CA mortar, which is why the material exhibits high early strength. Meanwhile, the formation of calcium hydroxide (CH) also becomes evident, further indicating that rapid hydration occurs in the early stage.

At 7 and 14 days, the XRD pattern did not change significantly compared to the 3-day pattern. The diffraction peak of ettringite increased slightly, suggesting that the hydration reaction was still ongoing. At this point, the characteristic peak of ettringite remained strong, indicating that it continued to be an important hydration product in the system even at 14 days. However, compared to the early stage, the rate of ettringite formation began to slow down, suggesting that the internal structure of the material was gradually stabilizing.

At 28 days, the hydration reaction was essentially complete. The XRD pattern showed a slight reduction in the peak of C₃S, but it was still relatively prominent, indicating that some unhydrated C₃S remained in the CA mortar system. At this stage, the peak intensities of CH and ettringite reached their maximum, but the difference compared to the 3-day peak was limited, suggesting that hydration had been slowly continuing from 3 days to 28 days. In addition, the diffraction peak of C₂S did not change significantly throughout the entire aging process. On the one hand, C₂S typically undergoes a slower hydration process in the early stages. On the other hand, the presence of asphalt inhibits the cement hydration reaction, making the hydration conditions more challenging, so the variation in C₂S was minimal throughout the process.

In summary, the hydration reaction of CA mortar is nearly complete at 3 days, which is an important reason why the 3-day compressive strength of CA mortar can reach more than 65% and the flexural strength can reach about 90%, and while slow hydration continues afterward, the change is minimal; therefore, the increasing amplitude of mechanical strength in the later period is small. The main hydration product is ettringite, which is the primary contributor to the strength of CA mortar. Hao [24] also compared the change in compressive strength of CA mortar under different emulsified asphalt content and found that hydration of cementing material was the main source of strength of cement emulsified asphalt grouting material, which was confirmed from the side. Furthermore, at 28 days, the material still contains considerable C₃S and C₂S, indicating that the presence of asphalt has a certain inhibitory effect on the cement hydration reaction, and the material still has potential for further hydration. In addition, the higher the water-to-cement ratios, the more C₃S and C₂S participated in the hydration reaction in the early stage, which was one of the important factors for the growth rate of strength in the early stage.

3.2.2. Temporal Variation in Microstructure in CA Mortar

The SEM images of CA mortar at different ages are shown in Figure 8. From Figure 8, it can be seen that with increasing age, the microstructure of CA mortar gradually becomes more compact, with the generated hydration products and asphalt particles progressively filling the pores in the structure. At 3 days, a significant amount of C-H-S and AFt has already formed in the microstructure. These products are the main contributors to the strength of CA mortar. Observing the image with a 500 mm scale, it can be seen that most of the cement and hydration products are coated with a layer of asphalt membrane, and some of these asphalt membranes are interconnected to form a continuous phase. The presence of asphalt effectively binds the hydration products together, enhancing the compactness of the structure. The generation of a large number of hydration products improves the compressive strength of CA mortar, and the existence of asphalt film increases the structural toughness, which is an important reason why the 3-day flexural strength of CA mortar can reach more than 5.0 MPa.

At 7 days, there is no significant change compared to the 3-day microstructure, the main difference being the generation of more hydration products. At 14 days, more C-H-S products are visibly formed, and the structure becomes denser, with large pore structures almost disappearing. Additionally, as a large number of hydration products are generated, they begin to break through the asphalt membrane, and the binding effect of the asphalt gradually weakens. At 28 days, the SEM image is nearly identical to that at 14 days; therefore, from 14 days to 28 days, the compressive strength and flexural strength of CA mortar increase by no more than 10%. The overall structure is quite compact, but there are still many unhydrated cement particles in the image. The early encapsulation by asphalt delayed the hydration process, leading to insufficient hydration in the later stages.

In summary, the early hydration products in CA mortar are the main source of its strength. After the emulsified asphalt breaks, it adsorbs onto the surface of the hydration products and sand particles to form a continuous phase, playing a binding role and also improving the structural toughness of CA mortar. With increasing age, the encapsulating effect of asphalt slows down the cement hydration, leading to a lower amount of hydration product formation and limited strength increase. In the process of preparing CA mortar, excessive emulsified asphalt may lead to thick coverage of asphalt film, hinder the intensification of hydration of cementified materials, and thus hinder the development and connection of hydration products [24]. Therefore, the content of emulsified asphalt should be carefully considered. At 28 days, there is still a significant amount of unhydrated cement particles in the CA mortar system, indicating that the cement still has potential for further hydration. Additionally, although asphalt acts as a binder, it has relatively low strength and is the weakest phase in the entire CA mortar system. During compressive failure, it is prone to damage at the asphalt phase or its interface. As cement hydration progresses, the asphalt membrane is gradually penetrated by hydration products, weakening the asphalt’s role, and improving strength. When CA mortar undergoes shear failure, the elastoplastic constitutive behavior of asphalt can effectively absorb interface stress, and the presence of the asphalt membrane enhances the bending strength of CA mortar.

4. Conclusions

This study tested the strength development patterns of CA mortar with different water-to-cement ratios and water-reducing agent dosages, and based on the XRD and SEM analysis results, the strength mechanisms of CA mortar were explained. The following conclusions can be drawn from this work:

(1)

Compressive strength and flexural strength of CA mortar shows a decreasing trend as water-to-cement ratio increases. The compressive strength and flexural strength of CA mortar increases rapidly in the early stage, with the 3-day compressive strength reaching more than 65% of the 28-day compressive strength, and the 7-day compressive strength reaching more than 80% of the 28-day compressive strength, with the 3-day flexural strength reaching about 90% of the 28-day flexural strength, and the 7-day flexural strength reaching more than 93% of the 28-day flexural strength. The formation of flexural strength is basically completed within the first 7 days.

(2)

Both the compressive strength and flexural strength of CA mortar show a pattern of first increasing and then decreasing as the water-reducing agent dosage increases. The water-reducing agent dosage range of 0.2% to 1.0% is a more reasonable range.

(3)

The hydration reaction of CA mortar is nearly completed at 3 days. As the age increases, the encapsulating effect of asphalt slows down the cement hydration, resulting in a lower rate of hydration product formation and limited strength improvement. The main hydration product is Ettringite, which is the primary source of strength in CA mortar. In addition, at 28 days, there are still significant amounts of unhydrated C₃S and C₂S inside the material, indicating that the presence of asphalt has a certain inhibitory effect on the hydration reaction of cement, and the material still has potential for further hydration.

This study provides a reference for the optimal design of ballastless track materials for high-speed railway. First of all, control the water-to-cement ratio to a reasonable range to prevent strength decline, especially in the parts with high bearing requirements, it is necessary to choose a mortar with a lower water-to-cement ratio to ensure compressive and flexural strength. Secondly, the content of water-reducing agent should be controlled between 0.2% and 1.0%, which can not only improve the strength, but also maintain good constructability. Finally, emulsified asphalt is effective in improving the toughness of mortar, but its property of inhibiting cement hydration requires reasonable regulation of its dosage to avoid excessive inhibition of hydration reaction. During the construction of the project, the material ratios should be adjusted according to the specific requirements of different construction stages and parts to ensure the comprehensive performance and long-term durability of mortar.

5. Future Work

In this study, only the water-to-cement ratios and water-reducing agent content were changed, but the content of emulsified asphalt did not change. Emulsified asphalt is very important in CA mortar system, it can improve the structural toughness of the material and has a great influence on the mechanical strength. Therefore, in future research, we will focus on the strength growth law of CA mortar under different emulsified asphalt content and explore the elastic–plastic constitutive of CA mortar and the interaction effect of cement and emulsified asphalt based on the test results.

(1)

The influence of other factors on the mechanical properties of CA mortar requires further study.

(2)

The coupled effect of cement hydration and the demulsification of emulsified asphalt on the mechanical properties of CA mortar should be further refined.

(3)

The influence of different curing conditions on the mechanical properties of cement emulsified asphalt mortar.

(4)

Establishment and optimization of stress–strain constitutive model of cement emulsified asphalt mortar.

Cement production is a significant source of carbon emissions globally, and its environmental impact is a major concern. To mitigate this, future studies should consider the use of supplementary cementitious materials (SCMs) such as fly ash, slag, or recycled aggregates, which can reduce the carbon footprint of cement-based materials. Additionally, while the incorporation of emulsified asphalt improves the mechanical properties of the mortar, it also involves petroleum-based products, which may contribute to environmental pollution during both production and disposal stages. Therefore, it is essential to select low-impact alternatives and optimize their dosages to balance material performance with environmental sustainability.