Mechanical Characteristics Based on the Microstructure Analysis of Cementitious Composites Incorporating Polypropylene Powder

Abstract

Incorporating recycled plastics into construction materials offers environmental and economic benefits. This study examined the properties of cementitious composites incorporating recycled polypropylene (PP) powder to evaluate the feasibility of plastics as construction materials. Experimental parameters included PP content and a curing method. Ninety-six specimens were fabricated for compressive strength tests and 48 for flexural strength tests, with six specimens per parameter. The mechanical behavior of the PP cementitious composites was assessed through compressive and flexural strength tests alongside digital image correlation analysis. Field emission scanning electron microscopy (FE-SEM) and mercury intrusion porosimetry (MIP) were used to analyze the pore structure of cementitious composites. Additionally, X-ray diffraction and thermogravimetric analysis examined the thermal and chemical characteristics. Compared with the control specimens, cementitious composites containing 30% PP exhibited approximately 30% reduction in compressive strength but a 28% increase in flexural strength. FE-SEM and MIP results revealed defects that deteriorated the performance of the cementitious composites. However, the compressive strengths exceeded 30 MPa across all the tested parameters, which is satisfactory for construction applications. Furthermore, the addition of PP enhanced flexural strength, providing structural benefits that render it a viable option for sustainable construction materials.

1. Introduction

Plastic is widely used across various fields owing to its excellent productivity and durability, with global annual production exceeding 400 million tons. However, according to the Organization for Economic Co-operation and Development’s Global Plastics Outlook report, only approximately 9% of plastic used globally is recycled [1]. The low recycling rates of plastic pose considerable risks, including environmental pollution and challenges in waste management [2]. Most plastic waste (PW) is either discarded in terrestrial and marine environments or ends up in landfills. The resulting micro and nano plastics harm ecosystems and disrupt global carbon cycling [3,4,5]. Additionally, plastic production contributes substantially to accelerating global warming, accounting for 5% of total carbon emissions [6]. The COVID-19 pandemic exacerbated existing plastic issues by driving up plastic usage through increased demand for personal protective equipment, food packaging/delivery services, and online shopping. This surge led to a substantial rise in plastic packaging use [7,8]. These consumption habits and lifestyles still persist today, with projections indicating that PW could triple by 2060 [9,10,11]. Therefore, proper treatment and management of the rapidly rising volume of PW is becoming increasingly critical.

Currently, numerous policies and studies on plastic recycling are actively being developed [12,13,14,15,16,17]. Incorporating plastic into construction materials is a promising approach [18,19]. The incorporation of plastic into construction materials provides environmental and economic benefits [20,21]. This method not only aids in environmental protection by reducing greenhouse gas emissions from plastic production but also creates new value for plastic as a sustainable construction resource [22,23,24,25]. Awoyera and Adesina [26] found that PW could be applied in various forms, such as subbase, asphalt, filler, walls, and bricks for road construction. Incorporating PW into cementitious composites offers a highly efficient method for waste utilization [27,28,29,30]. Siddique et al. [31] revealed that incorporating plastic into concrete improves resistance to impact and shrinkage cracks. Numerous studies used various types of plastic in cementitious composites, typically incorporating plastic as either aggregate or fiber [32,33]. The effects of plastic on cementitious composites have been analyzed through experiments assessing mechanical properties, chemical properties, and durability [34,35,36,37,38]. These studies show that factors such as mix ratio, type of plastic, and particle shape and size result in significant variability in the outcomes [39,40,41,42].

Previous studies have revealed that small particles with rough and irregular shapes enhance the strength of concrete [43,44]. Juki et al. [45] found that smaller polyethylene (PET) particles (1–16 mm) enhanced concrete strength, whereas a higher plastic content reduced density and strength due to the low density of plastic. Ismail and Al-Hashmi [46] indicated that higher sand replacement rates decreased both compressive and flexural strength, with flexural strength decreasing by up to 30.5% compared to that of the control specimen. Mohammed et al. [47] showed that replacing up to 30% of aggregates with PVC (0.49–0.95 mm) caused less than an 8% strength loss while improving toughness, crack resistance, and wear resistance. Saxena et al. [48] reported that PET aggregates (0–20 mm) maintained compressive strength losses below 10% when the fine aggregate replacement was <10%, while significantly enhancing energy absorption (up to 421% and 378% increases at 20% fine and coarse replacements were observed, respectively). Similarly, Gesoglu et al. [49] found that PVC powder replacements of up to 25% of the cement weight maintained compressive strengths above 45 MPa. Overall, plastic with smaller particle sizes and optimized mixing ratios can improve the performance of cementitious composites.

Yin et al. [50] found that incorporating plastic fibers in concrete is effective in controlling crack formation resulting from the dry shrinkage of concrete, thereby preventing sudden failure [51]. In particular, polypropylene (PP) fibers exhibit high resistance to alkaline environments, contributing to long-term durability. Additionally, PP fibers offer both environmental and economic advantages over steel fibers while also significantly reducing work time.

Orouji et al. [52] reported that incorporating 1.5% PP fibers maximized compressive and flexural strength, with specimens showing 7.3-times greater deformation than controls. Fraternali et al. [53] found that PET and PP fibers improved heat resistance, strength, and toughness, with PET fibers increasing compressive strength by 11.07 MPa. Ghernouti et al. [54] showed that waste vinyl fibers increased compressive strength by up to 13% and flexural strength by up to 14% depending on the fiber length, though shorter fibers had little effect. Overall, plastic fibers enhance the strength, ductility, and fracture resistance of concrete.

The hydrophobic nature of plastic interferes with the interaction between plastic and cement, which reduces the strength and performance of cementitious composites [55]. To address this issue, previous studies have investigated the addition of admixtures to plastic-containing concrete or treating the plastic before mixing [56]. Ali et al. [57] showed that adding silica fume mitigated the strength loss in plastic-containing concrete, increasing the compressive strength from 32.78 to 40.34 MPa by incorporating 20% plastic and 20% silica fume. Albano et al. [58] found that the flexural strength improved by 10% at 200 °C but declined at higher temperatures. Naik et al. [59] and Abbas et al. [60] reported that chemical and surface treatments involving high-density polyethylene (HDPE) enhanced matrix–plastic bonding, increasing the mechanical strength by 5–10%. This improvement was attributed to enhanced adhesion between the matrix and the plastic, which became hydrophilic owing to the sand coating.

Despite considerable advancements in incorporating various types of plastics into construction materials, most existing studies have primarily focused on PET, polyethylene (PE), and PVC. High-purity PP can be extracted from PW using solvent extraction methods [61]. PP is known for high toughness, flexibility, and substantial elongation capacity, which enables it to withstand considerable deformation. PP can absorb energy from cracking and inhibit crack propagation, thereby enhancing the overall durability and flexural strength of cementitious composites [62,63,64,65,66]. However, PP remains underexplored despite its high recycling potential [67]. Prior research primarily examined PP fibers and aggregates, focusing on their mechanical reinforcement capabilities in concrete. By contrast, limited research has addressed the use of PP powder derived from PW. PP in fiber form necessitates mechanical or thermal processing before being incorporated in cementitious composites. This processing can lead to changes in the physical and chemical structures of the material, potentially diminishing mechanical performance and altering surface characteristics. Additionally, the production of fibers increases process cost, which can reduce economic productivity in large-scale construction. By contrast, using PP powder enables direct incorporation without extensive processing. Reduced processing minimizes structural degradation of the polymer and lowers overall production costs. Utilizing PP powder derived from PW offers a practical alternative for enhancing both the economic and environmental performance of cementitious composites.

Understanding the influence of PP powder on the hydration process of cementitious composites is crucial for effective application. The effects of PP powder on cement hydration depend on curing conditions and incorporation rate, which influence the extent of hydration, pore structure development, and matrix development. Therefore, this study aimed to investigate the interaction between PP and hydration processes based on the curing method and the role of microstructural changes in increasing or decreasing the strength. Cementitious composites (hereafter referred to as PP cementitious composites) were fabricated with varying PP content and curing methods. Most previous studies have typically incorporated relatively small amounts of plastic into cementitious composites, usually less than 10% by weight of cement. However, in this study, PP powder was added at levels of up to 30% by weight of cement to explore the effects of high-volume incorporation. Admixtures were incorporated to enhance the strength of the PP cementitious composite [68]. Compressive and flexural strength tests evaluated the effects of PP powder on the mechanical characteristics of the cementitious composite. Digital image correlation (DIC) was used to analyze micro-deformation and cracking in the PP cementitious composite. Internal structure analysis was conducted to investigate the factors contributing to changes in the microstructure of the PP cementitious composite, as well as the effects of PP on the hydration reaction of the composite. Field emission scanning electron microscopy (FE-SEM), mercury intrusion porosimetry (MIP), X-ray diffraction (XRD) analysis, and thermogravimetric analysis (TGA) were conducted. Finally, the effects of experimental parameters and materials on the characteristics of the PP cementitious composite were analyzed using the Pearson correlation coefficient.

2. Experimental Programs

2.1. Fabrication of Specimens

The composition ratios of the PP cementitious composite used in this study are presented in Table 1. The materials in the specimens included Type 1 Portland cement, No. 6 silica sand (0.35–0.7 mm), and PP powder. In this study, PP powder with a particle size range of 50–300 μm was used. This range was selected considering that the mechanical and microstructural properties of cementitious composites are sensitive to the particle size of PP powder. This range is readily obtainable through conventional industrial milling and sieving processes, and the particle size is comparable to that of cement and sand. Therefore, PP particles in this range are less likely to interfere with cement hydration and dispersion within the composite matrix. Silica fume was used to compensate for the expected strength reduction in the PP cementitious composite due to the inclusion of plastic. Table 2 lists the chemical properties of silica fume. A superplasticizer was used to ensure flowability and improve the workability of the cementitious composite. Each admixture complied with ASTM C1240-20 and ASTM C494 regulations [69,70].

Figure 1 illustrates the particle morphology of PP. The PP particles have thin, elongated structures with smooth surfaces [71] and exhibit a slightly porous structure. The material properties of the PP powder are presented in Table 3 [72,73,74,75,76,77].

The parameters considered for specimen production were PP incorporation rate, curing method, and curing period. Table 4 lists the corresponding parameter values pertaining to the mechanical experiments. Specimen names were assigned according to the test type, PP content, and curing method. “C” and “F” represent the test type, with “C” indicating a compressive strength test and “F” denoting a flexural strength test. “PP” signifies the incorporation of PP, while the numbers “0,” “10,” “20,” and “30” correspond to the ratio of the PP powder content relative to the cement weight. “R” and “H” indicate the curing methods, with “R” representing the room temperature (RT)-cured specimens and “H” indicating the HT-cured specimens. RT curing was conducted at 23 ± 2 °C, whereas high-temperature (HT) curing was first conducted in a chamber at 200 °C for 2 h and then followed by RT curing. To promote the hydration of cementitious composite and induce physical bonding between the cement matrix and PP powder, HT curing was conducted at 200 °C, exceeding the melting point of PP. A PP cementitious composite (containing 20% PP powder) used for compressive strength testing and cured under HT conditions is designated as “C-PP-20H.”

The water/cement ratio of the PP cementitious composite was 0.4, and the cement-to-sand ratio was 1:2. Control specimens used for comparison with the PP cementitious composite were made using only cement and sand. The PP powder incorporation ratios were 0%, 10%, 20%, and 30% relative to the cement weight. A total of 144 specimens were fabricated, with six specimens per parameter. Overall, 96 specimens were prepared for compressive strength tests and 48 specimens for flexural strength tests.

The curing periods of specimens for compressive strength were set at 7 and 28 d. The curing period of specimens for flexural strength was 28 d. The specimens for the compressive strength test were prepared with dimensions of $50 \mathrm{m}\mathrm{m}\times 50 \mathrm{m}\mathrm{m}\times 50 \mathrm{m}\mathrm{m}$, in accordance with ASTM C109 regulations [78]. For the flexural test, specimens were fabricated with dimensions of $40 \mathrm{m}\mathrm{m}\times 40 \mathrm{m}\mathrm{m}\times 160 \mathrm{m}\mathrm{m}$, in accordance with ASTM C348 regulations [79].

Figure 2 provides an overview of the specimen manufacturing process for strength tests. Figure 2a illustrates the materials required for specimen production. As illustrated in Figure 2b, cement, sand, PP powder, and silica fume were dry mixed in a mixer bowl for 5 min. Subsequently, water or a mixture of water and superplasticizer was added and mixed for 3 min, as presented in Figure 2c. As shown in Figure 2d, the mixture was compacted into appropriate molds. The compacted specimens were then cured in molds for 1 d, as illustrated in Figure 2e. RT or HT curing was then conducted according to the specified periods. These two curing methods were employed to examine the hardening characteristics of the cementitious composite based on the temperature.

2.2. Mechanical Characteristics of the Specimens

Compressive and flexural strength tests investigated the effects of the curing method and PP powder incorporation on the overall mechanical performance of cementitious composites. Figure 3 presents the longitudinal section of the cementitious composite subjected to the mechanical strength tests, while Figure 4 shows the setup for the mechanical testing.

The mechanical strength of the cementitious composite was tested using a universal testing machine with a 1000 kN capacity. In accordance with ASTM C109 [78], the compressive strength was measured using a displacement control method at a loading rate of 1 mm/min. The flexural strength of the cementitious composite was measured using a three-point bending test, with a loading rate of 50 N/s in accordance with ASTM C348 [79]. Rollers, placed at 100 mm intervals, were used to support the specimen. The compressive strength and flexural strength of the cementitious composite, measured through the three-point bending test, were calculated using the following equations.

(1)$f_c={\displaystyle \frac{P}{A}}$

(2)$f_r={\displaystyle \frac{3PL}{2b{d}^2}}$

where $f_c$ denotes compressive strength, P denotes the maximum load applied during failure and A represents the cross-sectional area of the material resisting the load.$f_r$ represents flexural strength, L denotes the span of the specimen, b represents the specimen width, and d represents the specimen height.

Figure 4b presents the setup for the flexural test and DIC analysis. DIC analysis was used to visualize crack behavior under the three-point bending load. The distribution of deformation in the specimen was precisely measured through real-time monitoring. For smooth DIC tracking, a speckled pattern was applied to the surface of the specimen using a spray. An LED light source was used to uniformly illuminate the specimen, enhancing the precision of DIC analysis. A high-speed camera recorded the deformation process of the specimen over time at a rate of 5 fps. Mercury RT software version.3.0.12 was employed to analyze the experimental data [80].

2.3. Internal Structure Analysis Methods

Figure 5 shows the equipment used for internal structure analysis of PP cementitious composites. The internal structure was examined across different PP contents and curing methods, focusing on specimens that demonstrated the highest results in the 28 d strength tests. FE-SEM and MIP experiments examined the pore structure of the PP cementitious composites. Additionally, the chemical and thermal properties of the PP cementitious composite were analyzed through XRD and TGA experiments.

2.3.1. FE-SEM Test Method

FE-SEM SU8600 observed the structure of PP, while FE-SEM S-4800 observed the microstructure, bonding conditions, and other characteristics of the specimens (Figure 5a). Before testing, dust was removed from the specimens, and specimens were then coated with platinum to improve conductivity. The specimens were viewed at high magnifications in accordance with ISO 19749 [81]. The high-resolution images obtained from FE-SEM provide detailed insights into microdefects and phase distribution within the specimens.

2.3.2. MIP Test Method

AutoPore V 9620 was used to analyze the influences of curing methods and PP on the pore structure of the cementitious composite (Figure 5b). The porosity and pore size distribution of the PP cementitious composite were evaluated by measuring the amount of mercury that intruded into the pores under various pressures. The pressure and pore size ranges were 3.45 kPa–413.69 MPa and 0.003–500 μm, respectively. The contact angle was in the range of 130–140°. The pore diameter was calculated using the Washburn equation [82,83]:

(3)$d=-{\displaystyle \frac{4\gamma \mathit{cos}\theta }{P}}$

where d denotes pore diameter, $\gamma$ represents the surface tension of mercury, $\theta$ denotes the contact angle, and P represents mercury penetration pressure.

2.3.3. XRD Test Method

XRD experiments were performed using the X’Pert PRO system to analyze the effects of PP incorporation and curing methods on hydrate formation in the cementitious composite. X-ray radiation was applied at 40 kV and 40 mA, in accordance with ASTM E915-96, and 2θ scanning was performed in the range of 10–80° (Figure 5c) [84].

2.3.4. TGA Test Method

TGA experiments were conducted in accordance with ASTM C1872 by heating specimens at a rate of 10 °C/min over a temperature range of 25–1000 °C [85]. Mass changes were measured as the specimens thermally decomposed with increasing temperature. The TGA 2050 analyzed the thermal characteristics of the PP cementitious composite by measuring the mass changes in the specimens (Figure 5d).

2.4. Pearson Correlation Analysis

The relationship between the compressive and flexural strength of cementitious composites and the various influencing variables was analyzed by quantifying correlation. The range of the Pearson correlation coefficient was from −1 to 1. The closer the value is to 0, the weaker the correlation, while −1 and 1 indicate strong negative correlation and positive correlation, respectively. The dataset comprised five input variables and two output variables. The input variables affecting the strength of the cementitious composite were curing days, PP content, silica fume, sand, and superplasticizer. The output variables were compressive strength and flexural strength.

3. Experimental Results and Discussion

3.1. Mechanical Characteristics of PP Cementitious Composites

3.1.1. Compressive Strength and Trend Analysis

Figure 6 illustrates how the compressive strength of PP cementitious composites varies with changes in PP content and curing method. The error bars represent the standard deviations calculated from measurements of six specimens tested using each compressive strength test. Generally, irrespective of curing methods, the compressive strength of the cementitious composite tends to decrease as the PP content increases. The trendlines in Figure 6 confirm a consistent decrease in compressive strength with increasing PP content, regardless of the curing method or period. The 28 d compressive strength of C-Control-R and C-Control-H, without PP incorporation, was 45.84 and 48.22 MPa, respectively. The 28 d compressive strength of C-PP-10R and C-PP-10H, with 10% PP incorporation, was 42.47 and 41.58 MPa, respectively, which is 85% of that of the Control specimen. The compressive strength of C-PP-30R and C-PP-30H, with the maximum PP incorporation, was 33.12 and 32.38 MPa, respectively. The decrease in compressive strength in these cementitious composites is attributed to the adhesion problem between plastic and cement paste caused by the hydrophobicity of the plastic. However, the compressive strength of all specimens remained above 30 MPa when appropriate admixtures were used. A compressive strength exceeding 30 MPa is considered sufficient for use in construction applications such as road pavement, indicating that a cementitious composite containing 30% PP can be suitably utilized.

The compressive strength of the RT-cured PP cementitious material increased by up to 20% as the curing period extended. By contrast, the compressive strength of the HT-cured PP cementitious composite showed minimal change with an extended curing period. Compared to the PP cementitious composite cured at room temperature, the composite cured at HT exhibited higher compressive strength at 7 d but lower compressive strength at 28 d. The HT of 200 °C promotes early crystallization, enhancing early strength but reducing the long-term strength of the PP cementitious composite [86]. HT curing enhances early strength but induces microcracking via thermal expansion. These microstructural defects compromise the integrity of the matrix over time, leading to lower compressive strength at later ages compared to RT curing.

3.1.2. Flexural Strength and DIC Analysis

Figure 7 illustrates the flexural strength of PP cementitious composites cured for 28 d. The error bars represent the standard deviations calculated from measurements of six specimens tested using each flexural strength test. Under RT curing, the incorporation of PP increased the flexural strength of the cementitious composite. The flexural strength of F-PP-10R was 11.81 MPa, which is approximately 10% higher than that of the Control (10.74 MPa), while F-PP-30R exhibited a flexural strength of 13.75 MPa, showing an improvement of approximately 28% compared to the Control. Generally, the flexural strength of cementitious composites tends to be proportional to compressive strength [87,88,89,90]. However, the flexural strength of RT-cured PP cementitious composite was inversely proportional to compressive strength. The increase in flexural strength of PP cementitious composites under RT curing can be attributed to the high elongation rate of polypropylene, which allows the plastic to enhance the resistance to bending.

By contrast, the flexural strength of HT-cured PP cementitious composites decreased as the PP content increased. The flexural strengths of F-PP-10H, F-PP-20H, and F-PP-30H were 10.02, 9.83, and 9.78 MPa, respectively, which were up to 40% lower than that of the flexural strength of specimens cured at room temperature. This reduction may be attributed to the thermal degradation of PP at elevated temperatures (200 °C), which diminishes the reinforcing capability of PP within the cementitious composite. During HT curing, thermal expansion and contraction of PP can lead to deformation and non-uniform pore distribution within the matrix. Furthermore, the increase in temperature accelerates thermal degradation, resulting in a decline in the mechanical properties and durability of PP. Although the high elongation rate of PP enhances flexural strength under RT curing, this reinforcing function cannot be maintained effectively under HT curing owing to a combination of the aforementioned reasons.

The trend observed in the flexural strength results is supported by the DIC analysis. DIC analysis was used to monitor the flexural failure deformation of PP cementitious composites. Figure 8 presents the full-field strain distribution images of the composites, showing variations based on the curing method and incorporation rate. During the three-point bending test, compressive forces were applied to the upper part of the neutral axis of the specimen, while tensile forces acted on the lower part. Deformation in all specimens was concentrated around the middle of the span. The greatest deformation was observed close to the crack, which appeared at the lower central part of the specimen, aligning with the point of flexural failure. As shown in Figure 8a, cracks in specimens cured at room temperature propagated vertically in a straight line. Conversely, crack patterns in specimens cured at HTs were characterized by irregular curves. Figure 8b shows that the degree of bending increased with higher PP content. Under HT curing, PP exhibits thermal degradation and disperses irregularly within the cementitious composite. The non-uniformly distributed PP undergoes thermal shrinkage and expansion, resulting in the formation of uneven pores throughout the composite. These uneven pores disrupt the homogeneity of the matrix and contribute to the development of irregular crack patterns, as illustrated in Figure 8b. These internal defects weaken the stress transfer efficiency within the composite and reduce the ability of PP to resist flexural loads. Consequently, these factors are closely associated with the decrease in flexural strength observed in HT-cured specimens.

3.2. Internal Structure Analysis

3.2.1. FE-SEM

Using 5000× magnification FE-SEM images, the internal pores and hydrates of PP cementitious composites were examined in relation to PP incorporation and curing temperature. Specimens cured for 28 d were analyzed, and Figure 9 shows the FE-SEM image. Control-R and Control-H, without PP incorporation, exhibited abundant calcium silicate hydrate (C-S-H) and dense internal structures. By contrast, as observed in previous studies, porous structures were present in all specimens containing PP. Belmokaddem et al. [91] analyzed pores resulting from poor adhesion between plastic and the cement matrix using FE-SEM. The interfacial transition zone and pores within PP cementitious composites, caused by the hydrophobic nature of plastic, were observed in several studies [92,93]. FE-SEM observations revealed a weak interfacial transition zone (ITZ) between the PP particles and the cement hydration products, characterized by localized debonding and micropore formation. This poor interface is attributed to the hydrophobic nature of PP, which inhibits effective chemical bonding.

Figure 9a illustrates that the frequency of micropores increases with higher PP content. In PP-30R and PP-30H, pores caused by poor bonding between plastic and cement paste become more prominent as the PP ratio increases. The internal structure of specimens cured at an HT exhibited distinct patterns compared to those cured at room temperature. While Figure 9a depicts small and uniformly sized pores, Figure 9b reveals a range of pore sizes and a relatively smooth surface. Plastic does not physically or chemically interact with cement paste owing to its hydrophobic nature. However, HT (200 °C), exceeding the melting point of PP (130–171 °C), enables physical bonding between plastic and cement paste by imparting liquid-like properties to the plastic. Consequently, PP-10H, PP-20H, and PP-30H specimens exhibited smoother surfaces. Moreover, at HT curing, PP undergoes shrinkage and expansion during the melting and hardening processes, which facilitates physical bonding with the cement paste. The resulting variation in pore sizes (small and large) within PP cementitious composites is responsible for the diverse-sized pores depicted in Figure 9b, as compared to the more uniform pore sizes illustrated in Figure 9a. While physical bonding between PP and the cement paste can enhance the strength and internal structure of the composite, the detrimental effects of micropores outweigh these benefits, significantly impacting the overall strength of the PP cementitious composite.

3.2.2. MIP

The pore structure of PP cementitious composites observed through FE-SEM was further verified through MIP experiments, which measured specimen porosity under various parameters. Figure 10 illustrates the relationship between the 28 d compressive strength and porosity of the PP cementitious composite. Consistent with the FE-SEM images, the results indicated that the porosity of the PP cementitious composite increased with higher PP content. The porosity of PP-30 increased by approximately 60% compared to that of the Control specimen. This increase in porosity is attributed to poor contact and interference with hydrate formation owing to the hydrophobicity of the plastic. Furthermore, specimens cured at HT exhibited 2–4% higher porosity compared to those cured at room temperature. The growth of these interconnected pores reduces the load-bearing continuity of the cementitious matrix, directly explaining the observed decrease in compressive strength.

Generally, the porosity of cementitious composite is inversely proportional to compressive strength, as observed in previous studies [94,95]. The porosity of PP cementitious composites in this study followed the same trend. In addition to porosity, the pore size distribution also plays a substantial role in determining the properties of cementitious composites [96]. Figure 11 illustrates the pore size distribution within the total pore volume. In this study, pores were categorized based on size into four types: gel pores (<10 nm), medium capillary pores (10–50 nm), large capillary pores (50–1000 nm), and macro pores (>1000 nm). Among these, large capillary pores and macro pores exceeding 50 nm are considered detrimental to the performance of cementitious composites and are classified as harmful pores [97,98].

The pore structure of the Control specimen was dense, with a relatively uniform distribution of pore sizes. Control-R and Control-H, which did not incorporate PP, contained 24–28% gel pores, attributed to the high content of C-S-H gel. By contrast, the pore structure of specimens incorporating PP was predominantly characterized by macro pores. For instance, PP-10R exhibited 15.7% gel pores and 48.1% macro pores, with the gel pore ratio being nearly halved and a significant increase in the macro pore ratio compared to Control-R. While the overall pore structure of cementitious composite was not substantially influenced by the curing method, specimens cured at HT exhibited a 3–4% higher macro pore ratio.

These results clearly demonstrate the effects of PP incorporation on the pore structure of cementitious composites, corroborating the FE-SEM findings. As PP content increased, the porosity of the cementitious composite increased, along with the proportion of harmful pores. This increase in both porosity and the harmful pore ratio contributed to the reduction in compressive strength. In particular, macro pores accounted for the highest proportion of the pore structure of the PP cementitious composite. As a result of analyzing the pore structures of the cementitious composites via the mechanical strength test, macro pores were determined to be a major factor affecting the reduction in the strength of the PP cementitious composites.

3.2.3. XRD Analysis

The XRD analysis verified the amount of hydrate and investigated the effect of hydration on the mechanical properties of PP cementitious composites. To investigate the causes of strength variations in cementitious composites owing to PP incorporation and curing temperature, specimens without PP (Control-R and Control-H) and those with maximum PP content (PP-30R and PP-30H) were analyzed. Figure 12 presents the XRD patterns of specimens. The 2θ angles corresponding to the observed peaks were similar across all specimens. Silica peaks were consistently present in all patterns, with prominent peaks around 27°. Additionally, calcium hydroxide (C-H) and C-S-H peaks appeared at uniform angles, with no evidence of new crystalline phases caused by the addition of PP. The absence of chemical interaction between PP and the cement paste confirms that changes in the mechanical properties of the PP cementitious composite are primarily attributed to physical and microstructural alterations rather than chemical reactions.

While the XRD patterns of all specimens were generally similar, differences in peak intensity and composition were observed depending on the presence of plastic and curing temperature. The XRD peaks of specimens incorporating PP were reduced, which can be attributed to the hydrophobic nature of the plastic. Weak calcite peaks were identified around 27° in Control-R and Control-H but were absent in PP-30R and PP-30H, indicating that the plastic interfered with the hydration reaction. The HT promoted hydration and increased the amount of hydration products. Tobermorite was observed only in HT-cured specimens (Control-H and PP-30H). In general, an increase in hydration products tends to enhance the strength of cementitious composites; however, factors such as pore structure, cracking, and non-uniform dispersion negatively affect the mechanical strength of the composite.

3.2.4. TGA

TGA was performed to examine the thermal characteristics of PP cementitious composites. Figure 13 presents the weight loss graph as a function of temperature. Each specimen exhibited mass reduction in three distinct stages. The first stage, occurring between 80 and 150 °C, resulted in approximately 3% weight loss due to water evaporation and C-S-H dehydration. The second stage commenced at 400 °C, where rapid mass reduction was noted owing to the decomposition of Portlandite. During this phase, the decomposition of the PP powder between 400 and 480 °C further contributed to weight loss. The final stage began at 750 °C, where the release of CO₂ from the thermal decomposition of CaCO₃ caused additional mass reduction, which continued up to 1000 °C. The highest weight loss observed was in PP-30R, which reached 17%.

TGA results indicated that the partial decomposition of PP increased the thermal weight loss of PP cementitious composites. HT curing accelerated dehydration and partial decomposition of hydration products, such as C-S-H and CH, during the curing process. This change reduced the amount of water remaining in the cement matrix. As a result, HT-cured specimens exhibited lower thermal weight loss compared to RT-cured specimens during the TGA test.

3.3. Pearson Correlation Analysis of the PP Cementitious Composites

Figure 14 presents the Pearson correlation coefficients between input variables and the strength properties of cementitious composites. The analysis was conducted without normalization to preserve the original relationships among variables. The correlation between curing duration and flexural strength was excluded because the flexural strength test was performed at a fixed curing age of 28 d.

The compressive strength shows a moderately strong negative correlation with PP content (r = −0.62) in Figure 14a, indicating that an increase in PP content reduces compressive performance. The microstructural analysis demonstrated that the presence of PP interferes with cement hydration and induces pore formation, which explains the observed decrease in compressive strength. The correlation between PP content and flexural strength is positive (r = 0.17). While the incorporation of PP impairs hydration and lowers flexural performance, the presence of PP contributes to localized deformation control and supports load transfer during bending. Silica fume shows a weakly positive correlation with compressive strength (r = 0.05), attributed to the pozzolanic reaction that facilitates secondary gel formation and matrix densification. Superplasticizer exhibits a positive correlation with compressive strength (r = 0.06). Increased workability resulting from superplasticizer usage promotes hydration and matrix development. The curing duration demonstrates a positive correlation with the compressive strength (r = 0.44), reflecting continuous hydration under RT curing.

In Figure 14b, the negative correlation between PP content and compressive strength is less pronounced (r = −0.59). Although thermal degradation due to HT curing typically reduces compressive strength, the strength loss appears partially offset by the accelerated hydration and enhanced pozzolanic reaction under elevated temperatures. By comparison, the positive correlation between curing duration and compressive strength is less pronounced (r = 0.44). Exposure to 200 °C causes significant moisture loss in the cementitious composite, which limits hydration owing to insufficient water during the curing period. PP content and flexural strength show a weak negative correlation owing to the deterioration of PP (r = −0.08). HT curing leads to microstructural degradation of the cementitious composite, including increased internal porosity and shrinkage-induced cracking. Silica fume and sand show relatively strong positive correlations with compressive strength. The $\mathrm{S}\mathrm{i}{\mathrm{O}}_2$ in silica fume and sand enhances the pozzolanic reaction, which is further activated by elevated temperature, resulting in improved strength of the cementitious composite [99]. Superplasticizer also shows a positive correlation with compressive strength (r = 0.31), which is consistent with enhanced fluidity and reduced void formation in the matrix.

The PP content exhibits a strong negative correlation with compressive strength (r = −0.62), indicating that the compressive strength can be preserved by limiting the amount of incorporated PP. By contrast, under RT curing, the PP content shows a weak positive correlation with flexural strength (r = 0.17), suggesting that the flexural performance can be enhanced by increasing PP incorporation to an appropriate level. Therefore, the PP content in cementitious composites should be strategically adjusted to balance compressive and flexural strengths. Moreover, the correlation analysis reveals that curing temperature has a significant impact on the strength development of PP cementitious composites. Elevated temperatures can enhance strength through accelerated hydration and improved pozzolanic reactions; however, they may also impair mechanical properties owing to matrix degradation and microstructural defects. Therefore, selecting an appropriate curing method is essential to ensure the reliable performance of PP cementitious composites.

4. Conclusions

In this study, compressive and flexural strength tests were conducted to evaluate the mechanical characteristics of PP cementitious composites. Additionally, crack patterns were examined using DIC analysis. FE-SEM, MIP, XRD, and TGA were used to analyze the effects of different parameters on the internal structure of the PP cementitious composites. Finally, overall trends regarding the effects of PP powder on the cementitious composite were analyzed using the Pearson correlation coefficient and machine learning approaches. The main conclusions are as follows:

Increased PP content reduced compressive strength under all curing conditions owing to poor bonding and interruption of hydration. HT curing accelerated early strength but decreased long-term strength because of microcracks and porosity.

At RT curing, PP enhanced the flexural strength of a cementitious composite by up to 28%. Under HT curing, degradation of the PP and cement matrix reduced flexural strength by up to 40%. DIC analysis revealed irregular cracks resulting from uneven pore distribution and thermal deformation.

FE-SEM and MIP confirmed increased porosity and altered pore structure with PP addition. HT curing resulted in the formation of multi-scale pores and defects due to PP shrinkage and expansion, lowering structural integrity. As a result, the compressive strength of the PP cementitious composite decreased by up to 30%.

XRD results indicated that PP did not chemically react with the components of the cementitious composite and interfered with hydrate formation. The HT promoted hydration, but the resulting products did not improve mechanical strength owing to microstructural defects in the cementitious composite. TGA revealed greater weight loss with higher PP content owing to thermal decomposition and reduced hydration.

Pearson correlation analysis was conducted to investigate the overall effect of parameters on cementitious composites. Curing temperature significantly affected hydration, pore structure, and overall strength development. Ensuring the desired performance of cementitious composites in specific structural or environmental applications requires careful selection of the curing method.

With the use of suitable admixtures, the compressive strength of PP cementitious composites exceeded 30 MPa across all tested parameters. These findings highlight the importance of optimizing the mix design to balance mechanical performance and sustainability, ensuring that the incorporation of PP provides environmental benefits while minimizing impact on structural integrity.

Overall, these results suggest that cementitious composites incorporating plastic may be viable construction materials for applications such as road pavements, bridge slabs, and piers. However, the durability of PP cementitious composites, including their resistance to carbonation, freeze–thaw cycles, shrinkage, chloride ion penetration, and life cycle performance, was not evaluated in this study. Without such assessments, the long-term performance and reliability of PP cementitious composites in real-world structural applications remain uncertain. Therefore, future studies will focus on comprehensive durability evaluations to determine the applicability of these composites in practical construction environments.

Author Contributions

J.M.: Conceptualization, Writing and Editing; D.K.: Investigation, Formal analysis, Writing—Original draft preparation; S.K.: Investigation, Formal analysis, Writing—Original draft preparation; H.L.: Writing and Editing, Project administration. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data will be made available on request.

Conflicts of Interest

The authors declare that they have no competing interests.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1 PP particle: (a) PP powder used in the test and (b) FE-SEM image of the PP powder.

Figure 2 Fabrication process of the PP cementitious composites: (a) preparation of materials, (b) dry mixing, (c) wet mixing, (d) compacting, (e) curing in a mold, and (f) curing in a chamber.

Figure 3 Longitudinal view of mechanical strength testing of cementitious composites: (a) compressive strength and (b) flexural strength.

Figure 4 Set up for mechanical characteristics testing: (a) compressive strength and (b) flexural strength and DIC system.

Figure 5 Equipment used for internal structure analysis: (a) FE-SEM, (b) MIP, (c) XRD, and (d) TGA.

Figure 6 Compressive strengths of the PP cementitious composites under different curing methods: (a) RT curing and (b) HT curing.

Figure 7 Flexural strengths of the PP cementitious composites.

Figure 8 DIC analysis of cracks in the PP cementitious composites: (a) RT curing and (b) HT curing.

Figure 9 FE-SEM images of the PP cementitious composites: (a) RT curing and (b) HT curing.

Figure 10 Relationship between the porosity and compressive strength of the PP cementitious composites.

Figure 11 Pore characteristics of the PP cementitious composites.

Figure 12 XRD analysis of the PP cementitious composites.

Figure 13 TGA curves of the PP cementitious composites.

Figure 14 Pearson correlation heatmap: (a) RT curing and (b) HT curing.

Table 1

Composition ratios of PP cementitious composites.

W/C	Cement	Sand	Silica Fume	Superplasticizer
0.4	1.0	2.0	0.125	0.005

Table 2

Chemical properties of silica fume.

	Chemical Composition (%)
	$\mathbf{S}\mathbf{i}{\mathbf{O}}_2$	$\mathbf{F}{\mathbf{e}}_2{\mathbf{O}}_3$	CaO	${\mathbf{A}\mathbf{l}}_2{\mathbf{O}}_3$	MgO
Silica fume	90.0	3.0	2.0	1.5	0.3

Table 3

Material properties of PP.

Category	Property	Test Method	Units	Value
Physical properties	Melt index (230 °C, 2.16 kg)	ASTM D1238 [72]	g/10 min	27
	Density	ASTM D792 [73]	$\mathrm{kg}/{\mathrm{m}}^3$	900
Mechanical properties	Yield stress	ASTM D638 [74]	MPa	22.6
	Elongation	ASTM D638 [74]	%	<100
	Flexural modulus	ASTM D790 [75]	MPa	1226
Impact properties	IZOD impact strength (23 °C)	ASTM D256 [76]	J/m	98
	IZOD impact strength (−10 °C)	ASTM D256 [76]	J/m	49
Thermal properties	Heat deflection temperature $(0.45 \mathrm{MPa})$	ASTM D648 [77]	°C	105

Table 4

Test parameters.

Specimen Name	Test Type	PP Content (%)	Curing Method	Curing Period
C-Control-R	Compressive strength test	0	Room-temperature curing (23 ± 2 °C)	7, 28
C-PP-10R		10
C-PP-20R		20
C-PP-30R		30
C-Control-H		0	High-temperature curing(200 °C)
C-PP-10H		10
C-PP-20H		20
C-PP-30H		30
F-Control-R	Flexural strength test	0	Room-temperature curing (23 ± 2 °C)	28
F-PP-10R		10
F-PP-20R		20
F-PP-30R		30
F-Control-H		0	High-temperature curing (200 °C)
F-PP-10H		10
F-PP-20H		20
F-PP-30H		30

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