Investigation of Flexibility Enhancement Mechanisms and Microstructural Characteristics in Emulsified Asphalt and Latex-Modified Cement

Abstract

The inherent limitations of ordinary cement mortar—characterized by its high brittleness and low flexibility—result in a diminished load-bearing capacity, predisposing concrete pavements to cracking. A novel approach has been proposed to enhance material performance by incorporating emulsified asphalt and latex into ordinary cement mortar, aiming to improve the flexibility and durability of concrete pavements effectively. To further validate the feasibility of this proposed approach, a series of comprehensive experimental investigations were conducted, with corresponding conclusions detailed herein. As outlined below, the flexibility properties of the modified cement mortar were systematically evaluated at curing durations of 3, 7, and 28 days. The ratio of flexural to compressive strength can be increased by up to 38.9% at 8% emulsified asphalt content at the age of 28 days, and by up to 50% at 8% latex content. The mechanism of emulsified asphalt and latex-modified cement mortar was systematically investigated using a suite of analytical techniques: X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TG-DTG), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Through comprehensive analyses of microscopic morphology, hydration products, and elemental distribution, the enhancement in cement mortar toughness can be attributed to two primary mechanisms. First, Ca²⁺ ions combine with the carbonyl groups of emulsified asphalt to form a flexible film structure during cement hydration, thereby reducing the formation of brittle hydrates. Second, active functional groups in latex form a three-dimensional network, regulating internal expansion-contraction tension in the modified mortar and extending its service life.

1. Introduction

Cementitious materials exhibit notable advantages, particularly their high compressive strength and ease of construction [1]. It represents a key inorganic cementitious material widely utilized in diverse engineering applications, including roads, buildings, bridges, and other infrastructure projects [2]. Lyu et al. concluded that a Talbot grading index of 0.5 achieves the optimal Dynamic Particle Contact Stability (DPCS) for cement-stabilized aggregates, accompanied by minimal dissipated energy, the lowest fragmentation degree, the densest microstructure, and superior dynamic mechanical properties, with an enhanced anti-fragmentation mechanism [3]. The primary hydration product of cement hydration is C-S-H gel, which presents significant engineering challenges that limit its applicability, including high brittleness, low toughness, and a tendency to develop large cracks that are difficult to repair [4,5,6]. Indeed, cementitious materials can create an open structural system, thus making them widely favored in the construction industry [7,8,9]. Cao et al. proposed CO₂ Nano Bubble Water (CO₂NBW) as a novel additive for cemented backfill materials. This additive enhances the microstructure of cementitious systems, reduces micro-porosity and micro-cracking, and accelerates the hydration of cement [10]. Xiao and Cheng proposed a method for preparing rigid–flexible materials in the field of cementitious materials. This strategy significantly broadens the application scope of individual cement, asphalt, and latex cementing systems [11,12]. Thus, polymers with diverse properties and functionalities have been continuously integrated into cementitious materials to improve their performance.

Against the backdrop of rapid socioeconomic development, semi-rigid and semi-flexible materials have garnered increasing attention and been widely applied in diverse infrastructure sectors, including roadways, bridges, airports, and harbors. Incorporating emulsified asphalt and latex into ordinary cement mortar facilitates the preparation of modified composites, thereby enhancing their load-bearing capacity and seismic resistance [13,14]. Zhang demonstrated that semi-flexible composites (SFCMs) exhibit superior rutting resistance and moisture tolerance, while showing insensitivity to temperature fluctuations [15]. Li et al. demonstrated that the strength of semi-flexible pavement (SFP) increased by approximately sevenfold compared to the original open-graded asphalt mixture [16]. Song conducted innovative research on the preparation and performance evaluation of SFP materials. A comparative analysis revealed that SFP materials exhibit a significantly higher dynamic modulus, 1.4 to 1.7 times greater than that of conventional pavement materials, at 20 °C and 10 Hz [17]. Xiong et al. revealed that grout height significantly affects the reduction in maximum pavement buckling, while the vertical deformation of each structural layer decreases with increasing depth [18]. Cai et al. investigated the mechanism of SFP and found that grouted cement enhances its strength under high temperatures and low frequencies, while diminishing the loss modulus of SFP [19]. Both physical blending and chemical reactions are fundamental to enhancing the performance of cement mortars. Wang and Li showed that the improvement in polymer-modified cement mortar performance can be attributed to the dual role of polymers: affecting material structure, cement hydration, and porosity at the physical level, and mediating interactions between polymers and cement hydrates at the chemical level [20]. Sun established that the hydrophobic groups of styrene–butadiene latex (SBR) enhance the air-absorbing capabilities and fluidity of cement paste [21]. Ilango N et al. proposed that calcium atoms form coordination complexes with the carboxyl groups of poly methyl methacrylate (PMMA), thereby forming cross-links among polymer chains. Calcium ions in the adsorbed water phase enhance the physical bonding with the double-bonded oxygen of the ester functional group [22].

As discussed above, research on enhancing the mechanical properties of cementitious materials using organic additives has made significant progress, yet investigations into the underlying microscale mechanisms remain incomplete. Specifically, interactions within the material’s microstructure warrant urgent and in-depth clarification. Wang et al. revealed that the modification mechanism can be categorized into two distinct processes during cement modification: a physical mechanism occurring when polymer latex lacks active groups, and a chemical mechanism triggered by the presence of active groups [23]. Carboxylated styrene–butadiene latex and cationic emulsified asphalt with active functional groups offer notable advantages, including low pollution, strong adhesion, cost efficiency, excellent aging resistance, waterproof and anti-corrosion capabilities, and recyclable resource properties. Collectively, these attributes significantly augment the flexibility and durability of cement mortar, prolong its service life, and reduce pollution emissions [24,25,26]. Furthermore, waste asphalt pavement and latex-modified cement mortar can be crushed and reused as concrete aggregates, thereby facilitating resource recycling and minimizing construction waste [27,28].

In summary, this research has successfully developed a modified cement mortar that features excellent flexibility, superior safety performance, extended service life, and significant environmental benefits, achieved through a novel composite modification strategy. Microanalytical techniques, including XRD, FTIR, TG-DTG, XPS, and SEM, were utilized to investigate the effects of these two reagents on the degree of hydration and underlying mechanisms of cement mortar. This study aimed to propose a feasible solution for constructing rigid and semi-flexible pavements by modifying cement mortar through the incorporation of emulsified asphalt and latex, which effectively addresses the engineering challenges of brittleness and susceptibility to cracking in traditional cement mortar materials. It expands their application in complex stress environments, maximizes cement mortar utilization while prolonging service life, and reduces the generation of construction waste. For instance, the materials developed can serve as grout for porous pavement systems to mitigate rutting at traffic intersections, and as cushioning layer materials in railway infrastructure to improve track deformation resistance and leveling performance. The findings are expected to provide theoretical and technical support for developing high-performance, commercialized organic–inorganic composites as novel building materials.

2. Materials

2.1. Cement

The Xinjiang Tianshan ordinary silicate cement (P.O 42.5 R) was utilized in this study; its oxide compositions and physical properties are presented in

Table 1. Oxide fractions of cement (%).

Composition	SiO2	Al2O3	CaO	Fe2O3	MgO	K2O	SO3
Content	25.70	4.88	60.41	3.74	0.61	0.78	3.10

and

Table 2. Physical properties of cement.

Fluidity/mm	Flexural Strength/MPa		Compressive Strength/MPa		Setting Time/min		Soundness
	3 Days	28 Days	3 Days	28 Days	Initial	Final
199.5	5.3	9.0	28.5	50.2	158	201	Qualified

. The XRD pattern of silicate cement is shown in Figure 1. An analysis reveals that silicate cement contains four primary minerals—C₃S, C₂S, C₃A, and C₄AF—each with high hydration activity. Their interactions significantly influence key cement properties, including strength, setting time, and durability.

2.2. Aggregate and Water for Sample Preparation

Chinese ISO standard sand was utilized in this study, with its particle size distribution illustrated in Figure 2. This standard sand exhibits uniform particle grading, stable chemical composition, high hardness, and other advantageous properties. The silica (SiO₂) content should be greater than 96% according to the (Chinese national standard GB/T17671-2021) [29]. According to Equation (1), the water absorption rate of standard sand is 1.82%. The density of standard sand is 2100 kg/m³, which meets the requirements of (Chinese national standard GB/T 14684-2022) [30]. The water used in the experiment was sourced from the experimental water system of the Heavy Equipment Laboratory at Xinjiang University, which complies with the requirements of the Chinese national standard GB/T 17671-2021 [29].

$$\mathsf{\omega }={\displaystyle \frac{{\mathrm{m}}_2-m_1}{m_1}}\times 100\%$$

(1)

where $\mathsf{\omega }$ is the standard sand water absorption (%), $m_2$ is the saturated surface dry specimen mass (g), and $m_1$ is the drying specimen quality (g).

2.3. Emulsified Asphalt and Latex

Cationic emulsified asphalt from Xinjiang Tianchang Road Construction Co. was employed in this study. Its technical specifications are presented in

Table 3. Performance parameters of cationic emulsified asphalt.

Emulsified Asphalt	Evaporated Residue of Emulsified Asphalt
Viscosity/s	Residual content/%	Solubility/%	Needle penetration/DMM	Ductility/cm
8–20	50	97.5	45–150	40

. The ATR-FTIR spectrum of the emulsified asphalt is illustrated in Figure 3. As depicted in the figure, the absorption band at 3056.45 cm⁻¹ is attributed to C–H stretching vibrations in the benzene ring structure. The bands at 2921.65 cm⁻¹ and 2851.59 cm⁻¹ correspond to asymmetric and symmetric stretching vibrations of methylene groups, respectively. The wavenumber associated with the benzene ring is red-shifted, while the band at 1706.98 cm⁻¹ is assigned to C=O stretching vibrations in –RCOOH groups. This indicates that the emulsified asphalt contains both aromatic rings and carboxyl functional groups.

Carboxylated styrene–butadiene latex (XSBRL) produced by BASF Paper Chemicals (Huizhou) Co. was utilized, and its technical specifications are listed in

Table 4. XSBRL technical index.

Viscosity /MPa·s	Solid Content /%	pH	Density/ (g/cm3)	Mean Grain size/nm	Glass State Temperature/°C	Surface Tension/ (mN/m)
35–150	50–52	7.8–10	1.01	150	13	30–48

. The ATR-FTIR spectra of XSBRL are shown in Figure 4. An analysis indicated that the characteristic absorption peak at 1715.59 cm⁻¹ is attributed to the C=O stretching vibration of –RCOOH groups, confirming the presence of carboxyl functionalities in XSBRL.

3. Experimental Design and Methods

3.1. Design of Experiments and Specimen Preparation

This study investigated the effect of varying concentrations of cationic emulsified asphalt and XSBRL on the mechanical properties of cement composite mortar with a water-cement ratio of 0.5. Drawing from a comprehensive literature review and prioritizing experimental feasibility and repeatability, six modifier dosages were designed to analyze the impact of additive content on cement mortar using the controlled-variable method, with all other conditions held constant. The ingredient ratios are shown in

Table 5. Emulsified asphalt/XSBRL cement mortar ratios.

Cement Content (%)	Cement Quality (g)	Water (g)	Emulsified Asphalt or XSBRL Content (%)	Emulsified Asphalt or XSBRL Quality (g)	Standard Sand (g)
33	450	225	0	0	1350
			2	4.5
			4	9
			6	11.25
			8	13.5
			10	18

, and samples were prepared according to the Chinese national standard GB/T 17671-2021 [29]. The preparation of the composite material involved three sequential steps: First, ordinary silicate cement and a specified quantity of water were added to a planetary cement mixer. The mixture was thoroughly blended at the lowest rotational speed for 60 s under ambient temperature conditions. Second, the emulsified asphalt or XSBRL solution was incorporated, and mixing continued at the same low speed for 30 s. Finally, standard sand was added, and the mixture was rapidly agitated for 240 s before mixing was terminated. This experiment was performed under ambient temperature and pressure conditions, thereby enhancing the safety, cost efficiency, operational simplicity, and environmental sustainability.

The mortar was poured into a 40 mm × 40 mm × 160 mm mold, placed on a vibrating table, and vibrated for 60 s to eliminate air bubbles. The specimens were cured in a regulated environment (20 ± 2 °C, RH ≥ 95%) for 24 h within a standard curing chamber. The emulsified asphalt cement mortar (AC) and XSBRL cement mortar (LC) were cured in the standard curing room for 3, 7, and 28 days. The preparation method is depicted in Figure 5. The mortar was poured into a 40 mm × 40 mm × 160 mm mold, placed on a vibrating table, and vibrated for 60 s to remove air bubbles. Specimens were cured in a controlled environment (20 ± 2 °C, RH ≥ 95%) within a standard curing chamber for 24 h. The emulsified asphalt cement mortar (AC) and XSBRL cement mortar (LC) were cured in the same chamber for 3, 7, and 28 days. The preparation procedure is illustrated in Figure 5.

3.2. Cement Mortar Fluidity Test

This study evaluated the fluidity of cement mortar following the (Chinese national standard GB/T 2419-2005) [31]. The mortar was filled into the mold in two layers, each compacted in two steps. The first layer, which reached two-thirds of the mold height, was tamped 15 times to achieve initial compaction. The second layer was filled to the mold’s brim and continuously tamped until fully densified. Excess mortar beyond the mold’s truncated cone profile was trimmed by wiping from the center to the edge, after which the mold was gently lifted in a vertical direction. The cement mortar fluidity test was conducted at a rate of 25 jumps over 25 ± 1 s, with a frequency of one jump per second.

3.3. Flexibility Test for Modified Cement Mortar

Flexural and compressive tests of modified cement mortar specimens were conducted using a YAW-SERIES flexural–compressive integrated testing machine according to the Chinese national standard GB/T 17671-2021 [29], as illustrated in Figure 6.

Flexural Testing Procedure: The specimen was positioned on the machine’s support columns with its long axis perpendicular to the supports. Using the loading piston, a uniform load was applied to the lateral central position of the prism at a loading rate of 50 N/s, with a tolerance of ±10 N/s, until a fracture occurred.

Compressive Testing Procedure: After the flexural test, the two half-prisms were removed and placed on the testing surface with their cut faces down. The center of each half-prism was aligned within ±0.5 mm of the press plate’s center, leaving approximately 10 mm of the prism projecting above the plate. A constant loading rate of 2400 ± 200 N/s was maintained until failure, at which point the peak failure load was recorded. Flexural and compressive strengths were then calculated using Equations (2) and (3).

The folding and compression ratio of mortar specimens is defined as the ratio of flexural strength to compressive strength. Using a flexural–compressive automatic testing machine, the flexural and compressive strengths of the specimens were obtained, after which the ratio was calculated. The pliability of cement mortar specimens was then determined according to Equation (4).

$$R_f={\displaystyle \frac{1.5F_{f}L}{b^3}}$$

(2)

$$\mathit{Rc}={\displaystyle \frac{F_c}{A}}$$

(3)

$$S={\displaystyle \frac{R_f}{R_c}}$$

(4)

${“R}_f”$ represents the flexural strength (MPa). $“F_f”$ denotes the load applied to the prism’s central point during fracture (N). $“L”$ signifies the spacing between the supporting cylinders (mm), and $“b”$ denotes the side length of the square cross-section of the prism. ${”R}_c”$ denotes the compressive strength (MPa), with $“F_c”$ representing the peak load (N) at the moment of ring failure, and $“A”$ being the loading area (typically 1600 mm²). $“S”$ indicates the folding pressure ratio (%).

3.4. Materials Characterization

The XRD (Bruker D8 Advance, Billerica, MA, USA) was used to characterize the crystalline phases within a diffraction angle range of 10° to 80°, with a step size of 0.02° and a scan rate of 10°/min, as shown in Figure 7a. The SEM (Hitachi S4800, Tokyo, Japan) was used to examine the morphological features of cement mortar at magnifications of 5000× and 1000×, as depicted in Figure 7b. FTIR spectroscopy (Great 10, Tianjin, China) with KBr pellet technique was applied to analyze functional groups in hydration products, featuring a scanning range of 400–4000 cm⁻¹ and 32 scans, as shown in Figure 7c. TG-DSC measurements were conducted using a STA 7300 thermogravimetric analyzer (Selb, Germany) at a heating rate of 10 °C/min from 25 °C to 1000 °C, as illustrated in Figure 7d. XPS (Shimadzu Axis Supra+, Manchester, UK) was utilized to investigate the effect of emulsified asphalt/XSBRL on cement hydration mechanisms, as shown in Figure 7e.

4. Results and Analysis

4.1. Fluidity Analysis

Cement mortar fluidity refers to the flow performance of the mortar mixture under its own weight or external forces, serving as a key indicator of its workability. This parameter reflects how easily the mortar can be poured, vibrated, or paved, directly influencing construction quality. Specifically, mortar with high fluidity can readily form a uniform surface during paving, thereby minimizing manual troweling workload and reducing construction errors, as illustrated in Figure 8 and Figure 9. The flowability comparison of emulsified asphalt-modified (AC) and XSBRL-modified (LC) cement mortars is presented in Figure 10.

As evident in the figures, increasing the dosage of emulsified asphalt or XSBRL enhances the fluidity of cement mortar. Specifically, XSBRL-modified cement mortar exhibits a 15.4% increase in flowability compared to the ordinary cement mortar, whereas asphalt-modified mortar shows an 11.3% improvement. These results indicate that both XSBRL and emulsified asphalt significantly enhance the flowability of the ordinary cement mortar. The underlying mechanism involves organic polymers entrapping air bubbles, which reduce interparticle friction between cement grains, thereby facilitating smoother mortar flow.

4.2. Flexibility Analysis

The flexural-to-compressive strength ratio serves as a key parameter for evaluating material flexibility and resilience [32]; a higher ratio indicates greater flexibility and crack resistance. The fluctuations in flexural-to-compressive strength ratios and flexural strength values were analyzed, with results shown in Figure 11 and Figure 12. An analysis of variance (ANOVA) for the flexibility data of emulsified asphalt/XSBRL cement mortar is presented in

Table 6. Pliability data for ANOVA of emulsified asphalt cement mortar.

Source of Variation	SS	df	MS	F	p-Value	F Crit
Intergroup	0.026317	5	0.005263	21.53182	0.00	3.105875
Within a group	0.002933	12	0.000244
Total	0.02925	17

ANOVA was performed with a curing age of 3 days and emulsified asphalt content of 0%, 2%, 4%, 6%, 8%, and 10% at the 6-factor 3-level design, revealing a significant difference (p < 0.05).

and

Table 7. Pliability data for ANOVA of XSBRL cement mortar.

Source of Variation	SS	df	MS	F	p-Value	F Crit
Intergroup	0.026983	5	0.005396	51.126315	0.00	3.105875
Within a group	0.001266	12	0.000105
Total	0.02825	17

ANOVA was performed using a curing age of 3 days and XSBRL content levels of 0%, 2%, 4%, 6%, 8%, and 10% at a 6-factor 3-level design, revealing a significant difference (p < 0.05).

.

As shown in Figure 11, the flexural-to-compressive strength ratio of mortars with additives was significantly higher than that of ordinary cement mortar under 28-day curing conditions, and XSBRL modification outperformed emulsified asphalt in this regard. Both emulsified asphalt and XSBRL yield a non-monotonic trend in the flexural-to-compressive strength ratio of cement mortar, where the ratio initially increases and then decreases with increasing dosage of emulsified asphalt or XSBRL. This is because emulsified asphalt and XSBRL exhibit inherent flexibility, causing the flexural-to-compressive strength ratio to grow with their dosage. Li et al. reached a similar conclusion: when the XSBRL concentration gradually reaches a critical level, numerous surfactant molecules instantaneously aggregate into large micelles, thereby disrupting the system’s internal structure [33]. However, the flexibility ratio decreases with curing age for all test groups. This is because as the cement mortar continues to hydrate, voids within the material are filled, and the microstructure densifies, leading to reduced flexibility.

As shown in Figure 12, the XSBRL-modified mortar with 8% additive exhibits flexural-to-compressive strength ratios of 0.32, 0.30, and 0.27 at 28, 7, and 3 days of curing, respectively—the highest among all groups. Emulsified asphalt-modified mortar with 8% content achieves peaks of 0.32, 0.27, and 0.25 under the exact curing durations. The higher flexural-to-compressive strength ratio of XSBRL-modified cement mortar compared to emulsified asphalt-modified mortar is attributed to the polymer latex film formed during hydration, which coats cement particles and enhances adhesive contact surfaces. These findings are consistent with the research by Xie and Kong [34,35], who systematically elucidated the micromechanical mechanism whereby the hydrated polymer latex film bridges internal voids and enhances matrix structural stability.

As shown in Table 6 and Table 7, the ANOVA results for the flexibility data of emulsified asphalt/XSBRL cement mortar indicate a p-value < 0.001 for the response variable, which is below the significance threshold of 0.05. This confirms a statistically significant difference in flexibility across different additive dosages.

4.3. Physical Composition and Morphology Analysis

4.3.1. XRD Analysis

XRD analysis was performed to investigate the effect of emulsified asphalt and XSBRL on the hydration products of cement, with the results shown in Figure 13, Figure 14 and Figure 15. As shown in Figure 13, the hydration products of cement mortar primarily include calcium silicate hydrate (C-S-H), ettringite (AFt), and Ca(OH)₂. These findings align with results reported in the studies by Kong and Gastaldi [36,37]. The peak intensities of C-(A)-S-H and Ca(OH)₂ from cement hydration increased with curing age. This can be attributed to the hydration reaction in the modified mortar system, where the C₂S, C₃S, C₃A, and C₄AF in the cement further generate C-(A)-S-H and Ca(OH)₂ as the emulsification reaction progresses, as shown in Chemical Reactions (1)–(3).

$${2(3\mathrm{CaO}·\mathrm{SiO}}_2)+6{\mathrm{H}}_2\mathrm{O}\to {3\mathrm{CaO}·2\mathrm{SiO}}_2·3{\mathrm{H}}_2\mathrm{O}+{3\mathrm{Ca}\left(\mathrm{OH}\right)}_2$$

(R1)

$${2(2\mathrm{CaO}·\mathrm{SiO}}_2)+4{\mathrm{H}}_2\mathrm{O}\to {3\mathrm{CaO}·2\mathrm{SiO}}_2·3{\mathrm{H}}_2\mathrm{O}+{\mathrm{Ca}\left(\mathrm{OH}\right)}_2$$

(R2)

$${3\mathrm{CaO}·\mathrm{A}{\mathrm{l}}_2\mathrm{O}}_3+6{\mathrm{H}}_2\mathrm{O}\to {3\mathrm{CaO}·\mathrm{A}{\mathrm{l}}_2\mathrm{O}}_3·6{\mathrm{H}}_2\mathrm{O}$$

(R3)

As shown in Figure 14, the peak intensities of C-(A)-S-H and Ca(OH)₂ decreased progressively with increasing dosages of emulsified asphalt and XSBRL, indicating the inhibition of Ca(OH)₂ formation. This phenomenon arises because the XSBRL cationic emulsion impedes the further dissolution of cement particles, thereby reducing the concentrations of Ca²⁺ and OH⁻ ions in the paste. Furthermore, specific surfactant molecules rapidly aggregate into micelles, inhibiting the formation of brittle hydration products. Comparative microscopic analyses indicate that both emulsified asphalt and XSBRL suppress the progression of brittle cement hydration reactions.

As shown in Figure 15, the peak intensity of Ca(OH)₂ in emulsified asphalt-modified mortar is lower than that in XSBRL-modified mortar, indicating that emulsified asphalt inhibits the formation of brittle phases during cement hydration more significantly than XSBRL. This occurs because asphalt emulsion does not chemically bond with cement or its hydration products; instead, it forms flexible polymer films that coat these products, thereby leading to the more pronounced inhibition of brittle phases generated during hydration.

4.3.2. SEM Analysis

A SEM analysis was conducted to characterize the morphological features of cement mortar hydration products, with results shown in Figure 16 and Figure 17. The mortar’s components were observed at varying magnifications, as detailed below. The flaky Ca(OH)₂ crystals were visualized in the cement mortar microstructure at 1000× magnification, as depicted in Figure 16a. AFt contributes to chemical stability within the cement matrix, yet excessive formation leads to a weak interfacial transition zone, thereby compromising the matrix’s overall strength. Needle-shaped and rod-shaped AFt crystals were observed at 5000× magnification, as shown in Figure 16b. AFt, formed via the reaction between C₃A and gypsum, manifests as needle-like or rod-shaped crystals during the early hydration stage. These crystals initially enhance matrix toughness through a bridging effect; however, excessive AFt leads to the degradation of toughness due to expansion-induced microcracking. Honeycomb-like C-S-H structures are visible at 5000× magnification in Figure 16c. As an amorphous gel with a layered network structure, C-S-H retains water through physical adsorption and chemical bonding. As the primary phase contributing to the strength of cement paste, its densification is positively correlated with the mechanical strength of the paste. This is because cement hydration produces various compounds that accumulate, fill paste pores, and interlock with one another.

Microstructural comparisons are crucial for analyzing differences among ordinary, emulsified asphalt-modified, and XSBRL-modified cement mortars, as detailed below. Figure 17a shows the morphologies of C-S-H, Ca(OH)₂, and AFt hydration products in ordinary cement mortar. Upon mixing, cement particles and emulsified asphalt droplets adsorb to each other, forming agglomerates of cement and emulsified asphalt polymers, as seen in Figure 17b. In Figure 17c, XSBRL emulsion is observed coating the surfaces of acicular and rod-like crystals. This occurs because the XSBRL emulsion film fills cracks and pores in the cement matrix.

A distinct asphalt film is visible in Figure 18, which, as a multi-component organic mixture, exhibits a complex chemical composition. A molecular structure analysis confirms its major components as hydrocarbons, predominantly alkanes and aromatic hydrocarbons. Corresponding EDS diagrams illustrate the precise elemental proportions, which align with the elemental mapping results for C, O, Al, Si, S, and Ca. As indicated by the elemental distribution in the maps, the asphalt film is primarily composed of carbon and oxygen, consistent with its chemical constitution.

Figure 19 shows the sample surface covered by a latex film, an organic polymer composed primarily of alkanes, aromatic hydrocarbons, and esters. Specific elemental proportions are revealed in corresponding EDS spectra and elemental mapping images for C, O, Al, Si, S, and Ca. As indicated by the elemental distribution in these maps, the latex film is predominantly composed of carbon and oxygen, consistent with its chemical constitution.

4.4. Mechanistic Study of Modified Cement Mortar

4.4.1. TG-DTG Analysis

The effect of varying emulsified asphalt and XSBRL dosages on cement hydration products was investigated using a thermogravimetric analysis, with the results shown in Figure 20 and Figure 21. Figure 18 illustrates the three phases of thermal decomposition in cement hydration products, clearly depicting the process. The endothermic peaks correspond to the evaporation of free water, dehydration of C-S-H gel, and decomposition of AFt, aligning with the findings of Li et al. [38]. The three-stage thermal decomposition occurs within distinct temperature ranges. The first stage involves the formation of C-S-H hydrate at temperatures ranging from 50 to 200 °C. The second stage is an endothermic decomposition of Ca(OH)₂ at temperatures ranging from 350 to 550 °C, as described by Chemical Reaction (4). The third stage involves the carbonation of Ca(OH)₂ to form CaCO₃, which decomposes into CO₂ at temperatures ranging from 600 to 750 °C, as shown in Chemical Reactions (4)–(6).

$$\mathrm{Ca}(\mathrm{OH}{)}_2\to \mathrm{CaO}+{\mathrm{H}}_2\mathrm{O}$$

(R4)

$$\mathrm{CaO}+\mathrm{C}{\mathrm{O}}_2\to \mathrm{CaC}{\mathrm{O}}_3$$

(R5)

$$\mathrm{CaC}{\mathrm{O}}_3\to \mathrm{C}{\mathrm{O}}_2+\mathrm{CaO}$$

(R6)

As shown in Figure 21, the formation of Ca(OH)₂ in modified cement mortar decreases with increasing dosages of emulsified asphalt and XSBRL, indicating that these admixtures inhibit the generation of brittle hydration products. Emulsified asphalt and XSBRL coalesce into films that adsorb onto cement particle surfaces, hindering the dissolution of cement particle ions. This suppression reduces Ca²⁺ and OH⁻ concentrations in the mortar, thereby preventing the formation of hydration products. The polymer films formed by emulsified asphalt and XSBRL coat cement particles, blocking the hydration activity of inorganic ions and thus suppressing Ca(OH)₂ formation.

4.4.2. FTIR Modification Mechanism Analysis

To characterize functional group changes in the emulsified asphalt/XSBRL-modified cement mortar system, FTIR spectroscopy was employed, as shown in Figure 22 and Figure 23. As evident in Figure 22, the FTIR spectra of emulsified asphalt and XSBRL differ significantly from those of ordinary cement mortar. The results reveal distinct carbonyl (C=O) absorption peaks at 1700 cm⁻¹ for emulsified asphalt and XSBRL, which are absent in their modified cement mortar counterparts. These findings align with the results reported by Liu and Fang [39,40], indicating that the functional groups chemically react with the C=O moieties in the XSBRL-modified system. The presence of these functional groups enhances molecular cross-linking within the cement mortar matrix, thereby improving its resistance to tensile and compressive external forces.

As shown in Figure 23, a vibrational peak for water -OH stretching appears at 3429 cm⁻¹. The bending vibration peak of bound water -OH at 1609 cm⁻¹ is attributed to H₂O molecules in the C-(A)-S-H gel matrix. The wavenumber of 960 cm⁻¹ corresponds to Si-O stretching vibrations in the C-S-H gel, while the -OH stretching peak of Ca(OH)₂ at 3647 cm⁻¹ serves as an indicator of the degree of hydration in the cement. With curing progression, both the Si-O stretching peak (960 cm⁻¹ in C-S-H) and the Ca(OH)₂ -OH stretching peak (3647 cm⁻¹) exhibit increased intensities and broader peak profiles.

Figure 24 shows that the -OH absorption peak of Ca(OH)₂ appears at 3647 cm⁻¹, with its intensity exhibiting a post-peak decrease as the dosages of emulsified asphalt and XSBRL increase. This trend suggests that the cement hydration process is retarded after the peak as the additive dosage increases. The underlying mechanism involves interactions between the -COOH groups of emulsified asphalt/XSBRL and Ca(OH)₂, leading to the formation of a polymer film. This film restricts cement–water contact when adsorbed onto particle surfaces, thereby inhibiting the growth of hydrates and suppressing the generation of brittle hydration products. As shown in Figure 24, the polymer film formed by emulsified asphalt and XSBRL impedes the formation of C-S-H gel, thereby arresting the progression of brittle cement hydration. This finding aligns with the research by Li and Singh [41,42], who proposed that the film formed during emulsified asphalt demulsification inhibits the dissolution of H₂SiO₄²⁻ and retards cement hydration.

4.4.3. XPS Microscopic Interface Analysis

To investigate the microcompositional and interfacial characteristics of modified cement mortar, an XPS analysis was conducted, with the results presented in Figure 25 and Figure 26. As shown in Figure 25, the XPS spectrum of emulsified asphalt/XSBRL-modified cement mortar is dominated by peaks from C, Si, O, and Ca. The Ca 2p peaks in emulsified asphalt/XSBRL cement mortar exhibit variations in shape and intensity with increasing admixture content, indicating that calcium is involved in chemical reactions within the modified cement system. Figure 26 presents the high-resolution XPS spectra of plain, emulsified asphalt-modified, and XSBRL-modified cement mortars. The high-resolution C 1s spectra of emulsified asphalt/XSBRL cement mortar exhibit distinct peaks at 284.8 eV (C–C), 286.1 eV (C–O), 288.6 eV (C=O), and 289.6 eV (O–C=O). In the O 1s high-resolution analysis, binding energies at 531.2 eV, 532.1 eV, and 532.8 eV are assigned to C=O, C–O, and Si=O=Si moieties, respectively. The Si 2p spectra display binding energies at 101.2 eV, 102.1 eV, and 103.2 eV, corresponding to Si–O bonds and Si=O=Si configurations. For Ca 2p, high-resolution spectra reveal binding energies at 346.8 eV, 347.4 eV (attributed to Ca(RCOO)₂), 348.2 eV, 350.4 eV, and 351.2 eV. Collectively, the binding energies of C 1s, Si 2p, O 1s, and Ca 2p indicate the presence of CaCO₃, Ca(OH)₂, C–S–H, and Ca(RCOO)₂ in the system.

The XPS analysis revealed that calcium in the emulsified asphalt/XSBRL-modified cement matrix undergoes a chemical reaction, forming Ca(RCOO)₂, as shown in Chemical Reaction (7). Further characterization indicated that the amount of Ca(RCOO)₂ increases with an increasing emulsified asphalt/XSBRL dosage, findings that align with the FTIR analysis results. A comparative analysis showed that emulsified asphalt/XSBRL-modified materials contain higher concentrations of Ca(RCOO)₂ than ordinary cement mortar, collectively serving as key contributors to the mortar’s flexibility.

$$2\mathrm{RCOOH}+\mathrm{Ca}(\mathrm{OH}{)}_2\to \mathrm{Ca}(\mathrm{RCOO}{)}_2+\mathrm{n}{\mathrm{H}}_2\mathrm{O}$$

(R7)

4.5. Discussion

The incorporation of emulsified asphalt and latex forms numerous organic–inorganic composite interfacial transition zones within the mortar matrix. As evidenced by the XRD patterns of emulsified asphalt/latex-modified cement mortar in Figure 14a,b, and the asphalt and latex films visualized in Figure 18 and Figure 19, the polymer films inhibit cement hydration. This leads to a distribution of abundant pore spaces and unhydrated cement particles within the interfacial transition zones [43]. The hydrophobicity of pore walls restricts sulfate transport into the matrix, thereby inhibiting the formation of hydration and expansion products. This mechanism reduces microstructural damage in emulsified asphalt/latex-modified cement mortar [7]. Consequently, the compressive strength change in emulsified asphalt/latex-modified cement mortar was less pronounced than that in ordinary cement mortar. This suggests that the inclusion of emulsified asphalt and latex mitigates the impact of sulfate on cement mortar, reducing both sulfate-induced erosion and microstructural damage, and extending the service life of mortar. Additionally, it reduces construction waste generation and contributes to environmental sustainability.

5. Conclusions

To tackle the performance limitations of cement mortar—including high brittleness, low toughness, and elevated crack susceptibility—two modifiers, emulsified asphalt and XSBRL, are incorporated into ordinary cement mortar. This study explores the correlation between the cement hydration mechanism and the flexural properties of the composite material. Key conclusions are summarized as follows.

(1)

The emulsified asphalt and XSBRL significantly enhance the flexibility of cement mortar through a novel modification strategy. The optimal flexibility was achieved at an 8% dosage of emulsified asphalt and XSBRL, exhibiting 38.9% and 50% improvements in performance compared to the ordinary cement mortar, respectively. The inclusion of emulsified asphalt and XSBRL in ordinary cement mortar significantly extends service life by enhancing crack resistance. Consequently, this minimizes the need for frequent repairs and replacements, thereby reducing overall lifecycle pollution, which encompasses energy consumption in production and waste generation during disposal.

(2)

Multi-technique analyses (including XRD, SEM, TG-DTG, FTIR, and XPS) revealed that excessive dosages of emulsified asphalt and XSBRL form polymer films on the surface of cement particles. These three-dimensional reticulated structures effectively mitigate the effects of external compressive and tensile stresses, thereby enhancing the flexibility of the modified mortar.

(3)

Both emulsified asphalt and XSBRL form asphalt film and latex film that weaken the inorganic ion stacking effect. The carbonyl groups in emulsified asphalt coordinate with Ca²⁺ in the cement hydration system, while the carboxyl groups in XSBRL chemically form flexible linkages with Ca(OH)₂. This mechanism prevents the formation of brittle hydrates and weak chemical bonds.

This study systematically analyzed the mechanical properties and microstructural mechanisms of emulsified asphalt and latex-modified cement mortars. The materials we researched can be utilized as grouting in porous material pavements to eliminate rutting at traffic crossings, or as a cushioning bedding material in rail-road traffic to improve track deformation. The emulsified asphalt/XSBRL-modified mortar exhibits excellent resistance to environmental factors (e.g., freeze-thaw cycles, moisture intrusion, and UV irradiation). The three-dimensional reticulated membrane structure mitigates environmental deterioration of cement mortar by ameliorating damage from such factors, thereby significantly enhancing its durability and extending service life. This study has not addressed the influence of mixing procedures and ambient conditions on the formation mechanisms of cement mortar and concrete properties, thus serving as a potential avenue for future investigations. In the future, the cement industry will continue to face a complex market environment. The insufficient demand in the cement market remains a prominent challenge, attributed to multiple interconnected factors. Notably, optimizing cement properties and service life through the incorporation of modifiers makes a significant contribution to sustainable economic development.