## 2 Methodology
### System Boundary  
Having defined the objective of the project and the LCA case study, the scope of the study must be determined. This means proposing a preliminary outline of the boundaries of the desired study system. However, both the objectives and the scope can be redefined or modified during the course of the study as the LCA methodology is not a stationary/static process but a dynamic methodology. The production of natural aggregates falls within the construction sector. Therefore, the UNE-ISO 14040 [14], UNE-ISO 14044 [15], UNE-EN15643 [19] standards establish a special configuration when it comes to assessing sustainability in buildings and civil engineering works. The first stage of production (A1-A3) corresponds to the activities necessary to produce the materials that make up a building/work [1]. The construction and commissioning stage (A4-A5) characterizes the processes of implementation of the previously manufactured materials in the building. Subsequent stages such as use and maintenance (B1-B8) cover the processes that are necessary and arise in the process of the building's service life. Finally, the last stage, known as end of life (C1-C4), encompasses the activities of disposal, demolition and management of building waste. Considering the main objective of the research project, namely the production of natural aggregates that can be used in the future in the construction sector, the scope is limited only to the first stage, that is the raw material production stage such as natural aggregates. This first stage consists of the internal processes such as the extraction of raw materials (A1), their transport to the processing site (A2) and finally the manufacturing process until the desired material/product is produced (A3). It is therefore justified that in the specific case of the research project, the system boundaries encompassing the production of NCA in the gravel pit will be studied (see Figure 1). This system boundary is referred to in the cradle-to-gate LCA methodology and its use is in line with previous research in the field [1; 5; 20; 21]. The production process modelled in this research is discussed. First of all, once a site has been selected which has the potential to produce natural aggregates due to its soil geomorphology, all existing vegetation cover (trees, bushes, etc.) must be cleared in order to make the site uniform. Subsequently, depending on the location of the specific natural resource, i.e., the virgin bedrock from which the aggregates are transformed, basically two types of techniques are used. The first of these is the use of explosives to blast the ground to achieve the desired elevation and reduce the size of the virgin material [1]. On the other hand, and avoiding the use of explosives, layers of material can be extracted and removed with the use of heavy machinery such as excavators and backhoes for alluvial deposits (sediment deposits such as sand and gravel) [1]. This research will focus on this type of gravel pits where no explosives are used. Finally, once the necessary level of virgin material has been reached, it is extracted, also with heavy machinery. These machines excavate the beta and produce virgin material. This material cannot be used directly as aggregate as it contains unwanted impurities such as soil, heterogeneous rocks, etc. This virgin material is then loaded into dump trucks and transported to the so-called aggregate production station, which is fed by the arrival of these dump trucks. This aggregate production station is a mobile plant and is located in the quarry itself close to the production sites, so the transport process from the production face to the station is a few hundred meters. In contrast to a fixed station, which can be located kilometers away from the quarry, and consequently the process of transporting the virgin material is more environmentally burdensome [22]. The mobile aggregate production station is fed with the virgin material produced.