Waste Polypropylene in Asphalt Pavements: A State-of-the-Art Review Toward Circular Economy

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study provides intensive review for one of the waste materials widely used in pavement engineering. The topic is interesting. However, the current version of the manuscript has a lot of issues that need to be addressed before acceptance

• The abstract should give brief about the advancements achieved by this waste material and challenges still need to be addressed
• The review must provide some analysis about data collection strategy and bibliometrics
• The introduction should report the review studies published on the waste plastic and identify the new contribution of this work
• Size scale is needed on figure 1
• Figure 3 is not critically important for the scope of this study
• Subtitles of section 2.2 are not recycling methods. It is a pre-recycling preparations and methods
• Section 3.1 has some similarity to the subsections of section 2. Both foster the preparation of the plastic before reusing
• Figure 6 does not target the main scope of the related section. The key elements of the section should be prioritized for figure illustration
• Figure 7 is poor presentation for the plastic reactions. More explanations and notes need to be added to make the figure more informative
• The study does not review the impact of plastic on the rheological performance of asphalt binder
• Softening point, penetration, and viscosity are basic physical properties not rheological characteristics. The related discussion and figures must be separated from the title of high-temperature performance and be in a separate section
• Also, BBR results are low-temperature performance and its related discussions and results should not be part of the section of compatibility
• Section 4.1 is not a mix design. It is mostly mix methods
• The study should show combined figures to report the results from different references not just copying some figures from specific studies
• The manuscript lacks any discussion for the methods and findings of previous studies

The study does not illuminate the gap in the literature and future perspectives

Author Response

See attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The review by Yang N. et al. surveys the current state of knowledge on the use of waste polypropylene (WPP) in asphalt pavement engineering. The authors begin by outlining the physicochemical properties of polypropylene and its main recycling routes – mechanical, chemical, and energy recovery – emphasizing the environmental drawbacks of incineration and landfilling. Then the authors focus on WPP as an asphalt modifier: pretreatment strategies (pyrolysis, grafting, thermo-mechanochemical degradation) are discussed as means to improve dispersion and lower mixing temperatures. Key performance aspects of WPP-modified asphalt binders are reviewed, including enhanced high-temperature stability and rutting resistance, and improved moisture damage resistance, but reduced low-temperature ductility. Compatibility and storage stability are shown to benefit from compatibilizers or nanofillers. At the mixture level, both dry and wet blending processes are covered with performance outcomes and highlighting trade-offs, e.g., improved stiffness and hydrophobicity vs. potential brittleness at low temperatures. The review concludes that WPP holds strong potential in sustainable pavement applications, especially when combined with functionalization or elastomeric co-modifiers, and calls for further work on life-cycle assessment, standardization, and multi-scale modeling. In general, the review is timely, well-structured, and technically sound. With appropriate revisions, it will serve as a valuable reference for researchers and practitioners in sustainable pavement materials.

The specific comments are as follows.

Title. “Recycling of Waste Polypropylene in Sustainable Asphalt Materials towards Circular Economy. State-of-the-Art Review”. The title is overly long and slightly redundant (“Sustainable Asphalt Materials” + “towards Circular Economy”). The authors should consider shortening: “Waste Polypropylene in Asphalt Pavements: A State-of-the-Art Review toward Circular Economy”.

Line 15. “WPP has become one of the major components of municipal solid waste…” No quantitative evidence is provided (e.g., % by mass or volume). The authors should add a reference or statistic (e.g., “accounting for ~X% of global plastic waste [refs.]”.

Line 21. “modification mechanisms, preparation methods…” The abstract lists many topics but does not indicate the novelty or gap this review fills relative to existing surveys. The authors should explicitly state the contribution: e.g., “This review uniquely integrates recent advances in thermo-mechanochemical upcycling with mixture-level performance, bridging molecular design and field application.”

Lines 39–41, 257–260, 392–400: While the review emphasizes PP’s susceptibility to ultraviolet aging and highlights that few studies address long-term aging resistance of WPP-modified asphalt, there are experimental results that provide direct evidence that asphaltenes, which are ubiquitous in asphalt, act as effective UV stabilizers for PP (e.g., 20 wt % asphaltenes suppress UV-induced viscosity drop by ~6 times and strength loss by ~2 times, see 10.3390/polym15214313). This is highly relevant: in asphalt, PP is embedded in a matrix rich in asphaltenes, which may mitigate UV- and thermo-oxidative degradation. The authors should discuss the protective role of asphaltenes to polypropylene chains using this reference in Section 3.2.3 or 4.2.2.

Line 49. “PP is one of the most widely used materials due to its excellent comprehensive properties…” The authors should be specific, e.g., “high specific strength, chemical resistance, and processability” (as later in lines 97–100).

Line 61. “WPP is mainly treated by incineration, landfilling, and recycling…” Missing quantitative context: e.g., according to OECD (2022), globally ~9% of plastics are recycled, ~19% incinerated, ~50% landfilled. The authors should add such data to ground claims.

Lines 73–74, 233–240, 380–382: The authors discuss PP’s compatibility with asphalt and the use of compatibilizers (e.g., PP-g-MAH). However, compatibility between PP and asphaltenes as a major aromatic polar fraction of asphalt has already been demonstrated experimentally. Melekhina et al. (see the abovementioned reference) showed that asphaltenes disperse well in PP up to 30 wt %, forming stable composites with good interfacial adhesion and no coarse aggregates (according to optical microscopy, XRD, rheology). This supports the feasibility of PP–asphaltene interaction in binder systems and suggests intrinsic (though not perfect) thermodynamic affinity, likely due to dispersion forces and partial solubilization. The authors should use this work data when addressing compatibility.

Table 1, caption. “Comparison in the physical properties of PP, PE, PET, and PLA”. The word “in” is superfluous. The authors should write “Comparison of the physical properties…”.

Table 1. “WPP [33–35]” as header—confusing, since WPP = waste, but properties listed (density, melting point, etc.) are for virgin PP. The authors should rename column to “PP (virgin or typical recycled)” and clarify in footnote that WPP properties depend on prior use/history.

Lines 142–145. Discussion of Lamtai et al. and Main et al. implies WPP behavior mirrors virgin PP under recycling – yet no distinction is made between post-industrial vs. post-consumer WPP, which differ significantly in contamination and degradation. The authors should add a sentence acknowledging this variability and its impact on asphalt performance.

Line 242. “mixing conditions that are typically more than 20 °C higher than the conventional asphalt blending temperature [61]”. But Ref [61] (Ma et al. 2021) is a review – not a primary study reporting a specific temperature. The authors should cite an experimental source.

Line 267. “highly effective… Zhou et al. [69] used WPP… prepared a low-cost warm-mix asphalt modifier through pyrolysis”. But pyrolysis consumes energy and requires controlled atmosphere – “low-cost” is debatable. The authors should qualify: “potentially lower-cost, depending on pyrolysis scale and energy source”.

Line 399. “WPP-modified asphalt exhibited a reduced ductility of 8–12 cm… indicating… decreased low-temperature toughness”. No reference to specification limits. Without context, 8–12 cm may or may not be acceptable. The authors should add: “though still above minimum thresholds for certain pavement classes” or cite relevant standards.

Line 412. “as the WPP content increased from 0 to 12 wt%…” 12 wt% is exceptionally high (most studies use ≤6 wt%); outlier data may skew conclusions. The authors should note that >8 wt% is rarely considered practical due to workability and storage stability issues.

Line 477. Dry vs. wet process: the dry process is described as “easily implementable at conventional asphalt plants”. This is not universally true – many batch plants are not designed for polymer/fiber dosing, risking clogging or segregation. The authors should add “provided dosing and mixing equipment are adapted for fibrous or granular additives”.

Author Response

See attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

can be accepted