Introduction
Recycled materials are receiving global attention thanks to the significant attainable environmental and economic benefits (Wang et al.
2018). However, large quantities of recycled materials (e.g., produced from construction and demolition waste (CDW), etc.) require significant management planning (Ritter et al.
2013). Given this, stringent legislative policies and regulations enforce the potential value. In Europe, for instance, one of the legal and action plan initiative for waste management is the Waste Framework Directive 2008/98/EC which present basic guidelines toward recycling and reuse (European Commission
2008). This initiative promotes resource efficiency by supporting recycle operations and the market value of recycled materials for construction activities (Haas et al.
2020).
Considering CDW, the European waste catalogue (EWC) specifies mineral waste that constitutes typical CDW in a table of sequence, where excavated soil, stones, and dredging spoil are listed. Hence, some countries consider excavated soil and land leveling materials as CDW (Ng and Engelsen
2018). The management of excavation materials particularly from construction activities has not received enough attention due to socio-economic and political reasons (Crawford et al.
2017; Dahlbo et al.
2015; Huang et al.
2018). This has made it almost difficult to trace and track global volume generated annually. Consequently, data on handling and use of excavation materials is under-reported (Magnusson et al.
2015). So far, readily accessible recycling technologies for CDW focuses more on other waste products such as wood, concrete, masonry, glass, etc. (Menegaki and Damigos
2018; Ng and Engelsen
2018; Tam and Tam
2006), than excavated soil. Nevertheless, a few countries such as France, Italy, Austria, and Switzerland have implemented national legislative and recycling guidelines to promote excavation materials, mainly from tunnel construction (Magnusson et al.
2015). Similarly, in Norway, national projects such as Kortreist stein (short-travelled aggregates) and RESGRAM (recycled aggregates from excavation masses) are designed to develop technological processing solutions for sustainable use of high-quality excavation materials. Norway has a long tradition of adopting national policies and regulations, economic incentives, and extended producer responsibility to promote recycle operations and to create a market for high-quality waste materials (Karstensen et al.
2020).
Currently in Western Norway, the production of recycled excavation materials (REM) using a modernized wet processing recycling technology is practiced. This technology may be regarded as one of the best processing methods when it comes to recycling large and complex stream of waste materials. It effectively produces quality products through its washing steps and separation efficiency. In addition, the technology balances processing, material quality and market performance. Given that recycled aggregates derived from REM vary from source to source in geology, and they constitute a significant amount of fine fraction with potential organic and clay contaminated particles, its management is complex. Hence, the technology operated in Norway is an ideal choice. The recurring challenge is that subsequent physical, mechanical, and chemical properties of processed REM are not consistent and stabilized. This is because the REM produced are occasionally constituted by phyllites in the stockpile (Norby
2020). Phyllites are characterized by layered silicates and have low strength properties (Dengg et al.
2018); hence in this context, this could contribute to the performance variation observed in processed REM. In the USA, New York City faces the challenge of enacting policies which may open for complete use of REM (Walsh et al.
2019). These developments increase the skepticisms about the service performance and overall use of REM. Nevertheless, some studies have demonstrated the feasible use of REM in other applications (Dengg et al.
2018; Lieb
2011; Voit and Kuschel
2020).
In general, the mechanical performance of recycled aggregates produced from excavation materials may be intrinsically linked to mineralogical constituents. In a recent study, the Los Angeles (LA) and micro-Deval (MD) performance of REM was studied (Norby
2020). The LA values were found to be in the region 25–28%, while the MD varied considerably from 7 to 20%. In another study, the LA of REM increased from 17–30% and 10–26% for MD (Barbieri et al.
2019). Both studies demonstrated in an X-ray diffraction (XRD) analysis that REM comprised of a significant amount of phyllosilicates (i.e., mica and chlorite minerals) (Barbieri et al.
2019; Norby
2020). Furthermore, a comprehensive review study of the influence of mineralogy and other geological parameters on the LA and MD performance of different rock aggregates has recently been published (Adomako et al.
2021). The study found that quartz and feldspar had a good correlation to LA and MD performance and that rock aggregates containing approximately 20% of mechanically weak minerals such as phyllosilicates show satisfactory mechanical properties. The study further identified some textural features such as spatial distribution, grain shape and size, morphology, etc. as influential factors (Adomako et al.
2021). In another study, recycled phosphate aggregates of sedimentary origin composed of limestone, marl, and flintstone from a single location showed a significant variation of 46–67% for LA and 50–70% for MD due to the presence of clay and flintstone (Amrani et al.
2019). At this point, conclusions may be reached that the influence of mineralogy on the performance of excavation materials is fundamental.
It is also essential to emphasize that some authors have generally reported satisfactory performance values for excavation materials. In one particular case involving recycled andesite and marble aggregates from the same source, the LA of both materials reached permissible strengths of 25% and 27%, respectively, and therefore the authors implied that both materials could be applied in asphalt pavements characterized by light to medium traffic (Akbulut and Gürer
2007). The LA performance of recycled basalt aggregates was reported to be 13% and was incorporated into stone mastic asphalt (Karakuş
2011). The LA of recycled crushed basaltic aggregates was reported to be satisfactory at a value of 21% in the study by Arulrajah et al. (
2012). In specific cases where the performance is compromised, it has been suggested by some authors that these materials (e.g., recycled basaltic aggregates) may be blended with other materials to achieve higher workability and strengths (Ali et al.
2011; Arulrajah et al.
2013). Speaking of blending recycled materials to achieve optimum performance, a function-based investigation by repeated load triaxial test was performed on REM which had been partially replaced by phyllite materials in different quantities (Adomako et al.
2022). The result first showed considerable stiffness variation between REM and phyllites in unblended condition, and phyllites substituted at 25% and 50% in REM confirmed a decrease in stiffness with increased content of RPM. Regarding the deformation behavior, both materials performed similarly. Conclusions reached by the authors were that the performance of the materials typically compares to other recycled materials despite the stiffness variation.
From the above review, it is clear that a few studies have extensively researched on the implications of mechanically weak rocks constituted in excavation materials. Questions related to which production level of REM may be expedient considering the presence of weak materials, and how the masses may be mixed to achieve satisfactory mechanical performance has not been studied in detail. Given this, the purpose of the present study is to investigate the LA and MD performance of REM mixed with mechanically weak materials in different quantities to establish the limit thresholds while maintaining acceptable performance. The study aims at establishing performance relationships based on the content of recycled phyllite materials (RPM) in REM and potential applications in unbound layers of road pavement. This approach may promote the use of REM in quantities significantly higher than current production levels and may serve as quality control guide in matters of REM and other mixtures. The last part of the study was to examine changing mineral assembly in the fine fractions extracted from blended mixtures in order to identify and understand which minerals abrade in both tests as indication of the effect of weak minerals. The study also compares the performance of other rock aggregates derived from different production sites across Norway.
Conclusions
This study has presented the results of the mechanical performance of recycled aggregates derived from excavation materials (REM) which is mixed with recycled phyllite materials (RPM). The Los Angeles (LA) and micro-Deval (MD) tests were used in the investigation. In addition, X-ray diffraction (XRD) and acid solubility tests were performed to identify mechanically weak minerals accumulated in the fine fraction, after the tests.
The results have shown that RPM and REM had similar and satisfactory LA of 28% and 26%, respectively, but a significant difference in MD performance of 26% for RPM and 6% for REM was found. At the intermix level, it was found that REM could tolerate up to 40% of RPM before it exceeded the MD limit of 15–20% defined in N200 by the Norwegian Road Public Administration. Blending REM with the hard rock PGr (Porphyritic granite) indicated that a higher content of PGr in this combination showed increased resistance to LA. Furthermore, the maximum intermix level of RPM in PGr was only 20%. Regarding the use of limestone (Lim) in REM blends, Lim’s maximum blending ratio should be less than 40% to satisfy the MD limit. The LA was the critical parameter of mixtures generated from local crushed rock material (CrR) and REM; hence, approximately 75% of CrR was the maximum tolerable content to reach the base course and subbase criteria.
Consistency was found when the mineralogy of the fine fractions (< 1.6 mm) from the LA and MD tests were assessed. Limestone minerals mainly seem to disintegrate when mixed with amphibolite—a product of pyroxene which also is characterized by soluble components when exposed to acidic environment. The XRD analyses of pure and blended mixes of RPM in REM and RPM in PGr showed a good relationship between increased intensities of mica and chlorite with increased RPM. Regarding the relationship encompassing LA and MD, and mineralogy, it was observed that the wearing of weak minerals (mica and chlorite) was high in the MD test compared to the disintegration effect by LA. This may be attributed to the conditions of MD test, namely (a) the wear nature of the test method, and (b) the moisturized condition of the test. The less effect to mineral wear in LA may be due to reasons such as the dry method of the test and expected cushioning effect by pulverized fraction which prevents large particles to further disintegrate during the test.
Given the study’s overall findings, it may be concluded that the REM used in this study present excellent mechanical properties due to the low presence of weak minerals and may be used for construction purposes. Furthermore, foreign rock materials (e.g., phyllite) in REM can be tolerable in quantities significantly higher than the average levels found in today’s production. The authors are of the opinion that the findings of the study may be applicable in other scenarios where excavation is characterized by satisfactory LA and MD performance and a low content of mechanically weak minerals.