## 1 Introduction  
For every living individual, around 4 tonnes of conventional concrete, comprised primarily of ordinary Portland cement (OPC) and naturally sourced aggregates (NA), were produced in 2015 [1]. Due to its inherent strength and durability properties, concrete is the second most used substance on Earth after water [2]. Unfortunately, the use of concrete is associated with immense negative environmental impacts. The current production rate of more than 4 billion tonnes of OPC annually is responsible for 7% of the global CO\({}_{2}\) emissions [3]. It also risks depleting natural resources, since more than 50 billion tonnes of aggregates are being extracted annually [4]. Concrete has an environmental impact of 320 kg eq CO\({}_{2}\)/m\({}^{3}\) on average as will be concluded in the next section of this paper, of which 90% is attributable to OPC [5]. Although this is less than that of steel and most polymers per unit mass [6], the intensive use of OPC concrete results in alarming environmental hazards. In China for example, the over-reliance on concrete alone resulted in approximately 1.5 billion tonnes of greenhouse gases (GHG) emissions in 2014 [7], which represents around 20% of the total produced in the same year [8]. Nevertheless, projections indicate that the growing global urbanization will double the demand on concrete by 2050 [9]. Hence, recent research has been directed to meet the \"2015 Paris climate conference\" guidelines of enhancing the sustainability of concrete [10]. As seen in Figure 1, there are five main families of concrete types found in the literature that are considered more eco-friendly [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]. Those are the types that were chosen to be studied in this paper as a sample for green concrete. First, there is the use of natural materials, such as bacteria, and agricultural waste, such as hemp [11], in order to create natural biological concrete (BioC) with reduced environmental impact. Second, there is recycling aggregate concrete (RAC), where construction and demolition wastes (CDWs) are used as aggregates in concrete. This reduces the landfill potential of concrete by 50-75% and its embodied carbon by 10-30% [12; 13]. Blended cement concrete (BCC), where OPC in the binder, is partially replaced with various pozzolanic materials called supplementary cementitious materials (SCM), and is considered the third family. Examples of these are secondary materials such as fly ash (FA), which is a by-product of coal combustion; ground granulated blastfurnace slag (GGBS) which is a by-product of steel manufacturing; and silica fume (SF), which is generated from glass manufacturing. Additionally, some primary materials that are manufactured with lower energy demands than OPC can be used to partially replace it, such as calcined clay (CC) and lime. The mechanical and durability properties of the resulting concretes vary significantly between the different types of materials and the percentages by which OPC is being replaced, and similarly, the environmental impact varies [50]. For example, the embodied emissions of concrete could decrease by up to 30% and 60% with the incorporation of 35% and 70% of FA and GGBS, respectively [51]. In order to totally replace OPC, alkali activated concretes (AAC), are made with precursors of 100% FA, (CC) or GGBS that are activated using an alkaline solution from usually sodium hydroxide or sodium silicate. This is the forth concrete type selected in this study. AAC causes 70-75% less GHG emissions compared to OPC concrete [52]. The fifth family is high performance concrete (HPA), which is recognized as a concrete type with enhanced mechanical and durability properties compared to OPC concrete (OPCC). This allows for a reduction in the required volume of concrete in certain applications.