## 3 Sources of Discrepancies in LCA Stages
### Stage 2: Inventory Data
In all cases, the discrepancy in the impact of cement attributed has a great impact on the final environmental impact calculated through an LCA for a concrete alternative. (iii) The variability in inventory data is not only in the OPC, but as seen in Table 4, it is the same with the rest of the concrete mix components. The most variable components found were the supplementary cementitious materials, such as FA, GGBS and SF whether in BCC or AAC mixes. The reason behind  
Figure 10: A review of the global warming potential (GWP) of a tonne of ordinary Portland cement (OPC) reported in 25 papers. Figure 9: The meta data for the primary versus secondary sources for LCA studies reviewed and the databases used. this could be attributed to the case of environmental impact allocation. Impact allocation is the process of portioning the environmental burden of the original process to the waste material being recycled in the product under study [17]. According to the EU directive 2008, waste can be considered to be a by-product when its further use is certain, it is produced as an integral part of a production process, it can be used without any further processing other than normal industrial practice and its further use is lawful [77]. All four points apply to FA, GGBS and SF; hence, they ought to be considered by-products, not waste. This means that they ought to be allocated a percentage of the environmental burden of their original production processes, which are coal combustion, steel production and glass manufacturing, respectively [19]. The first impact allocation scenario is \"mass allocation\" where the percentage allocation is based on the relative mass between the waste material as a by-product and the total mass (the effective mass of electricity + the mass of FA) as shown in Equation (1). The second scenario is \"economic allocation\" in which the percentage allocated is based on the relative market value between the final product, which is FA, and electricity, as per Equation (2) [77]. \[\text{Mass Allocation}\;=\;\frac{(\text{m})_{\text{by-product}}}{(\text{m})_{ \text{main product}}+(\text{m})_{\text{by-product}}} \tag{1}\]  
applied. However, the fact that the fluctuation in market prices of the raw materials should now be a part of the environmental impact assessment of concrete through the economic allocation, creates room for further discrepancies in the results, as seen in Table 5. It is recommended to keep the LCA results of concrete limited to the time frame for which the prices of the raw materials still stand unchanged. (iv) Another source of discrepancy in LCAs of concrete with regard to inventory data is the impact attributed to the transportation of raw materials to the concrete manufacturing plant. It could vary between 5% and 20% depending on the location of the raw materials relative to the concrete batch plant [94]. While examining a sample of the papers being reviewed, it was apparent, as seen in Table 6, that the transportation distances vary widely between the different studies. A study by [PERSON] et al. [95] concluded that the critical distance for importing FA that would still yield a BCC mix with a positive overall environmental impact profile compared to OPCC is around 900 km as opposed to the 3000 km proposed by [PERSON] et al. [160] and 5700 km by [PERSON] et al. [96]. Additionally, [PERSON] et al. [13] argue that if the recycled aggregates are sourced from a landfill that is more than 230 km from the concrete batch plant, the RAC produced would have a higher environmental impact compared to OPCC, which is a larger figure than the 145 km concluded by [PERSON] et al. [53]. In all cases, it is recommended to perform a scenario analysis in each LCA study for concrete in case the sources for the raw materials are not exact, to determine the sensitivity of the output relative to the change in distances.