Pyrolysis of Reclaimed Asphalt Aggregates in Mortar

Asphalt pavement consists of aggregates resulting in a waste material at end of its life. The aggregates can be reused as basic material for asphalt or cementitious binding agents. In both scenarios, the recycled aggregates should provide a good bond with the binder to achieve strength. This study focuses on reusing recycled asphalt aggregates (RAA) in mortar. The major weakness of RAA is the thin oily film originating from the asphalt residue, weakening the bond with cement. The pyrolysis method is accessed in an attempt to overcome this weakness. Three scenarios were investigated; the use of virgin aggregates (VA), RAA, and pyrolysis recycled asphalt aggregate (PRAA) as  constituent in mortar. All variables were set a constant except for the aggregate type, the VA mortar function as controlling element. This research is methodologically based on experimental data conducted in the laboratory, while aggregate samples were taken from the field. To analyze the influence of pyrolysis to the aggregate-to-cement bond behaviour, qualitative and quantitative data were collected. The quantitative data were the mechanical properties, the mortar tensile, and compression strength. The qualitative data were obtained from scanning electron microscope readings to visually observe the aggregate surface roughness and voids, including the aggregates cross-section and pre-existing micro-cracks in the aggregate-to-cement interface. Supporting data were the aggregates‘ abrasion rate and absorption. The RAA resulted in a significant mortar strength decrease. This conclusion was supported by the findings of pre-existing cracks in the interfacial transition zone. The pyrolysis method improved the compression strength but negligibly affected the tensile behavior. The compression and tensile strength increased as a function of time for both RAA and PRAA, and a strength convergence was reached at 28 days. The PRAA is considered an option for reuse in mortar, supporting nature conservation.

Mechanical properties; Mortar constituent; Pyrolysis recycled asphalt aggregates

    The aggregates in cementitious composites originate from quarries or through stone crushing, which is basically recyclable (Ashadi et al., 2015; Turu’allo, 2015; Purnomo et al., 2021). Two most commonly used recycle aggregate, i.e., recycled concrete or paving-originated aggregates, supports the sustainability of the aggregate’s lifecycle. The main issue when dealing with recycled aggregates is the quality: the physical and mechanical properties that are highly influenced by the original binding agent residue. The quality and origin of cement highly influence the alteration of the aggregate properties when dealing with concrete and mortar. For asphalt pavements, the asphalt is the main affecting material. The untraceable origin and the wide range of cement and asphalts make the aggregates a very unreliable material for recycling purposes.    
    The use of recycled asphalt aggregate (RAA) in mortar has been widely studied (Abraham & Ransinchung, 2018; Sola & Ozyazgan, 2019; Debbarma et al., 2020; Shi et al., 2020). In general, RAA from road scarifying is used directly as a constituent in mortar. The studies concluded that the use of RAA in mortar decreases the mechanical properties as a function of asphalt content in the RAA. The main source of this depreciation is the poor bond. Debbarma et al. (2020) recommend applying surface treatments (mechanically or chemically) to remove the residue from the surface to stabilize the aggregate. The following is an overview of methods nowadays used.
    The use of microwaves for concrete aggregate recycling started as early as 2011 and is still being investigated to date (Akbarnezhad et al., 2011; Choi et al., 2014; Mousa et al., 2020; Wei et al., 2021a; 2021b; 2021c). Microwave heating is also used in asphalt road maintenance work, such as hot in-place recycling. This method does not completely remove asphalt from the aggregate surface but only softens the asphalt to facilitate mixing of the recycled asphalt pavement with additional asphalt or new aggregate (Benedetto & Calvi, 2013; Sun & Sheng, 2020; Gulisano & Gallego, 2021). Mechanically removing the thin layer of the bonding agent using acid and lime immersion and mechanical rubbing (Kazmi et al., 2019) was also attempted. Improving the contaminated surface was conducted by accelerated carbonation techniques and slurry wrapping (Wang et al., 2020). The two basic approaches differed in their method: the first was by film removal, and the second was by treatment of the coated surface. Most recent is the introduction of nano-silica technology through a combination of pre-spraying, air-drying, and particle size evaluation (Gao et al., 2020; Li et al., 2021), and the self-healing concept. The latter evolves around re-hydrating, bacterial self-healing, and micro-encapsulation (Li et al., 2021). An overview of methods, their advantages, and disadvantages, are outlined in the following works (Wang et al., 2020; Tam et al., 2021).
    This work is focused on the reuse of RAA in mortar. The focus is directed on mortar, since the behavior of mortar represents a wide range of cement-made products such as paving blocks, concrete, and masonry. Three types of aggregate were used: virgin aggregate (VA), recycled asphalt aggregate (RAA), and RAA after removing the thin asphalt film using the pyrolysis method (designated as pyrolysis recycled asphalt aggregate (PRAA)). The Pyrolysis method is a unique new surface treatment method to remove and stabilize the residual asphalt film by heating and is expected to contribute to increasing the mechanical properties of the mortar due to an aggregate-to-mortar bond improvement. 

Research on pyrolysis of recycled asphalt aggregates is limited. Most of the work on improving recycled aggregates does not focus on pyrolysis to stabilize the residual film surrounding the aggregates. Comparing the impact of methods in general, recycled aggregates result in a decrease in compressive and tensile strengths due to the residue of previous binding agents and the non-standard quality of the original material, as stated in the majority of previous research works. The attempt to remove the residual film using a broad range of methods showed that the mechanical properties of the new composite using mortar or asphalt as the binder improved, but could never reach the strength of the original virgin aggregates (VA).

The findings in this study showed that, in compression, the decrease was independent of mortar age; the strength reduction was 55% and 30% for the RAA and PRAA mortars. The strength loss of the RAA predominantly originated from the surface conditions: the thickness of asphalt film, smoother surface, pre-existing micro-cracks, which promote crack initiation and propagation under loading, and the decrease in abrasion resistance, in combination with the presence of loose, unstable particles. The PRAA had a better compression strength, but, despite the removal of the asphalt film, a thin layer of residue remained. The PRAA had a relatively high abrasion rate, weakening the bond between the aggregate and the cement. While no preliminary micro-cracks were present in the aggregate surface and the absorption rate was better compared to RAA, substantial pre-existing cracks were detected in the aggregate-to-mortar interface.

In tension, both the RAA and PRAA exhibited a 40% strength decrease, which is slightly fluctuates with age and stabilises at 28 days. The pyrolysis did not substantially affect the tensile behavior due to the presence of pre-existing cracks. The relationship between the tensile and compression strength can be represented by the coefficient C ranging from 1.3 and 1.2 for the VA and RAA, and 1.0 for the PRAA, using the equation at any age from 14 to 180 days.

The RAA had a 1.49% absorption, but the PRAA had a substantially lower absorption of 0.59%. The pyrolysis stabilized the surface and prevented water intrusion into the aggregate's inner pores.
    The pyrolysis method applied on RAA had a number of advantages: the compression strength increased, and the absorption decreased, reducing the formation of a water film surrounding the aggregate during the cement hydration process. The pyrolysis procedure did not have any impact on the tensile strength for both the RAA and PRAA, however, the overall tensile strength decrease was relatively low, with a minimum reduction of 42% in the VA mortar. The pyrolysis procedure offers a solution to rejuvenate the RAA for mortar usage. The reuse of RAA supports the conservation of natural aggregates.

    This research was financially supported by the Faculty of Engineering, Diponegoro University, Indonesia through the Excellent Research Grant 2021 [grant number 3179/U/1/UN7.5.3.2/PP/2021].

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