## 1 Introduction
Additionally, the concrete mixes are prepared with fillers such as lime powder to increase the particle packing of the mixing components, which minimizes the required amount of binder [15]. This potentially yields a binder with less environmental impact than OPCC. Figure 1: Green concrete strategies and types. Therefore, the starting point of studying a green concrete type is to create alternative concrete types that reduce the environmental impact of OPCC [52]. Life cycle assessment (LCA) is the most widely-accepted tool to assess and compare these acclaimed environmental benefits [53]. According to ISO 14040:2006, LCA is defined as \"the compilation and evaluation of the inputs, outputs and potential environmental impacts of a product system throughout its life cycle.\" An LCA study is divided into four main stages: (i) Scope and goal definition. (ii) Defining the inventory for the life cycle processes. (iii) Characterizing and measuring the life cycle impact. (iv) Interpretation of the results [54]. First, goal and scope definition involves outlining the system boundary, the functional unit (FU) selection and any assumptions and/or limitations that need to be considered. A system boundary of a concrete product could be cradle-to-gate, which means including all processes and emissions until the production of its different constituents, or cradle-to-grave which includes the \"Use\" and \"End-of-Life\" phases as per Figure 2[55]. A Cradle-to-Cradle LCA scope is that which assumes that all waste generated will be recycled in the future [23]. A FU is the basis for quantifying the inputs and outputs between alternatives. Hence, its selection needs to be reflective of the nature of the LCA subjects [56]. The second LCA stage includes collecting the data of energy and emissions associated with the aforementioned scope. The data needed for standard processes can mostly be obtained from primary sources or found in databases such as Eoinvent and European reference Life Cycle Database (ELCD) [57]. A further source of inventory data is the environmental product declarations (EPD) of the concrete's raw materials, which are produced by the local manufacturers according to a local binding legal framework [16]. At this stage, it is also important to decide on allocation, which is basically portioning the environmental burden of the original process to the product under study [17]. The third and final stage of an LCA is to calculate the environmental impact of the product being studied. This is performed by adding up the individual impacts of all the associated processes as per ISO 14040:2006 to calculate an environmental impact indicator, a number that makes the output of the impact assessment study more understandable to the user [58]. According to [PERSON] [59], there are two main types of indicators: mid-point indicators, which correlate with the estimated impact of a specific change in the environment, such as global warming potential, and end-point indicators, which correlate that same impact to damages via cause-effect changes, such as human health. Figure 2: Different system boundaries of green concrete life cycle assessment (LCA) [21]. As explained before, the LCA of green concrete cannot be easily assessed. In fact, there are a lot of uncertainties in the assessment process due to lack of a standardized methodology. To highlight this issue, an attempt was made by this study to compare the absolute values for the environmental impacts of the aforementioned green concrete types to that of OPCC, by considering around 300 different mixes from 39 journal papers [11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49].