1. Introduction

It is well recognised that the overuse, or mismanagement, of a resource can lead to its depletion or exhaustion, which is unsustainable [1,2]^[1][2]; this situation is compounded by simultaneously sending wastes to landfill sites that could be recycled and reused in place of non-renewable resources, such as primary aggregate (PA). PA has been described as the most valuable non-fuel mineral commodity, and without it, life as we know it would be difficult to sustain ^[3]. The global consumption of PA combined with the extent of the landfilling of wastes that could act as aggregates is a case in point.

An estimated 50 billion tonnes of primary aggregate are currently produced annually worldwide, and this is predicted to increase 5% year after year ^[4]. The EU alone is estimated to consume 2 Gt of PA ^[5] whilst simultaneously sending over 1 Gt of waste (total weight, not just that which could be used as aggregates) to landfill sites annually ^[6].

In order to enhance the utilisation of recycled and/or secondary aggregates (RA and SA) as alternatives to PA, it is imperative that industry practices are modified significantly. This may require a substantial reduction in the demand for PA or a step change in how recycled and secondary aggregates are both viewed and produced within the industry (both in quantities and range of techniques used to maximise uptake of available wastes, i.e., not just crushing materials but using additional processes to valorise previously unsuitable materials, such as accelerated carbonation to produce lightweight aggregates from ashes: [12,13,14]^[7][8][9].

The term ‘aggregate’ is used herein to describe potential materials for geotechnical applications, as this term is commonly used within the existing literature, although it is not used without reservation. The term aggregate seems evocative of ‘coarse-grained’ materials (used in railway and highway applications, where engineering properties must be stringent), which are not necessarily required for many geotechnical applications. Perhaps a better term would be ‘particles’, as the materials under review provide the solids for the placed soils (many of which form three-phase materials), although as noted above aggregate is used.

2. The Ethos of the Circular Economy as Motivation for the Use of Alternative Aggregates

The adoption of alternative aggregates diverts waste from landfill sites and can lead to large financial savings, especially since in the UK where RA and SA are not subject to the aggregate levy and in the case of hazardous waste streams where landfill gate fees are typically higher [63]^[10]. Furthermore, many of these waste streams are generated in, or comparatively near, urban settings (i.e., near the centres of demand for aggregates); hence, there is the potential for reduced transport distances (when compared to quarries).

Treating previously unsuitable wastes to produce SA offers benefits; for example, chemical treatment of hazardous materials can remove the potential to contaminate, producing aggregates from wastes with particles that would otherwise be too small to be workable (i.e., silt and clay-sized particle sizes), although these processes can be energy-intensive (in the case of sintering, etc.), so some techniques are not without cost. Examples of SA include lightweight products such as Lytag (sintering of pulverised fuel ash, PFA), carbonated ashes and cemented or thermally treated wastes [12,13,14,64]^{[7][8][9][11]}.

The introduction of the revised Directive on Waste, Directive 2008/98/EC, intended to simplify the existing legislation on waste within Europe to promote the diversion of waste from landfills and to encourage lifecycle thinking, shifting the perception of waste from something to be disposed to a potentially valuable resource [31]^[12]. One of the important aspects of the revision was to clarify the definition of waste and the distinction between recovery and disposal [65^[13][14],66], and the introduction of ‘End-of-Waste’ (EoW) criteria aimed to achieve this. In order for a waste to achieve EoW status, the following criteria must be met: the substance or object is commonly used for specific purposes; there is an existing market or demand for the substance or object; the use is lawful (substance or object fulfils the technical requirements for the specific purposes and meets the existing legislation and standards applicable to products); the use will not lead to overall adverse environmental or human health impacts.

Another aim of the revised Directive is the promotion of reducing natural resource use [66,67]^[14][15]. This aspect is explicitly met by the diversion of wastes from landfills via their utilisation as aggregates (and has commercial benefits due to the simultaneous avoidance of the landfill and aggregate tax). However, there are more benefits to be gained during the production of the recycled/secondary aggregates, such as the sequestering of carbon dioxide during the accelerated carbonation of certain waste streams. In addition, research is currently being carried out into the extraction of valuable metals from waste streams prior to their treatment and subsequent production into aggregates [68^[16][17],69], further preserving natural resources and adding to the commercial viability.

3. Examples of the Utilisation of Wastes as Alternative Aggregates in Geotechnical Engineering and an Alternative that Potentially Could Be Used

Crushed concrete aggregate (CCA), produced by crushing material recovered during demolition of structures, has been considered inferior with qualities such as lower strength and stiffness when compared to other primary materials. It is the case that CCA could be considered as poorer quality due to the stiff core particles being surrounded by weak layers of mortar [124]^[18]. However, the extensive research that has been carried out into the properties of CCA reflects that AAs can exhibit strength and stiffness equivalent or better than higher grade materials when utilised in applications where it is well-compacted [124]^[18].

WGC has numerous possible applications, including drainage material, filter media or drainage blankets and load-bearing material in road pavements and asphalt aggregate projects [126,129,131,136]^{[19][20][21][22]}. Furthermore, waste glass is an inert material and non-biodegradable, thus staying in landfills for up to 1 million years [137]^[23], which is an obvious disadvantage in landfills but an advantage for AAs in geotechnical applications. Case studies of WCG being successfully used as pipe bedding material in Australia have been published, and many states in America and New Zealand have adopted the WGC-blends for use in road construction [138]^[24].

The varying chemical composition of waste glass, debris content and the ability to sort it by colour are limiting factors in the recycling rates of this material [128,132]^[25][26]. The recycling of flat glass (e.g., windows from buildings or vehicles) is substantially lower than cullet, which is largely due to the stricter quality requirements [138,139,140,141]^{[24][27][28][29]}. This is disappointing as it is possible to directly reuse windows (e.g., from buildings), but due to constraints that include: additional time and labour costs for glass removal, ease of damage during transportation and difficulties in matching supply with demand, much of this is landfilled [134,142,143]^[30][31][32].

There is currently very little geotechnical information available on these materials, and the authors argue that they have the potential to act as AAs in geotechnical engineering applications and thus merit research in this area.

4. Conclusions

Despite numerous studies highlighting the potential of RA/SA, the construction industry, and by extension geotechnical engineering, still relies heavily on PA for applications that could adopt non-virgin materials. With sources of PA under pressure and landfilling being politically undesirable, now is the time to change the way in which resources are consumed, especially if the stated desire to adopt a circular economy is to be realised.

AAs have been produced from waste streams (such as scrap tyres, ashes, waste glass and demolition rubble) and have successfully been utilised in geotechnical solutions. Despite this, there are several barriers preventing the widespread use of waste materials, including the lack of confidence and perceived risk with product quality, lack of suitable specifications, financial concerns and a lack of awareness. Case studies, such as the BDU project, offer inspiration to others to utilise materials that initially may not seem suitable. In addition, various laboratory studies have been undertaken to further understand the engineering properties of AAs and consider their use in (somewhat limited) geotechnical engineering applications.

It is clear, however, that there is still much to do with regards to AAs before the barriers (or a number of influencing financial constraints is probably beyond the remit of such studies) preventing a much greater utilisation in geotechnical applications are addressed. This includes furthering the understanding of engineering behaviour in more geotechnical contexts (other than road base, etc.), assessing new materials as they become available (such as AAs from APCr) and addressing the required criteria for use (standards) in these applications. If this can be achieved, then greater uptake of AAs may be facilitated, improving the use of non-renewable resources, limiting the amount of waste sent to landfills and moving towards the desired circular economy.