## 1 Introduction  
3D concrete printing (3 DCP), also referred to as digital fabrication or additive manufacturing of concrete, has been under development by both the academic research and construction industries in the past decade [1; 2; 3]. The unique advantages of 3 DCP over conventional concrete construction may include saving labors and costs, eliminating formwork, increasing flexibility in architecture design, as well as reducing the construction time [3; 4; 5; 6]. Until now, there are two main approaches in 3 DCP: the extrusion-based method and the powder-based method [2; 7]. This study only focuses on the extrusion-based 3 DCP, which generally employs an additive, layer-based, formwork-free extrusion and deposition process [7]. Considering conventional concrete technology principles, the extrusion-based 3 DCP cannot be regarded as a sustainable construction method. As reported by [PERSON] et al. [8], the amount of ordinary Portland cement (OPC) forms more than 70% of the binder in most of the available cementitious materials for 3 DCP. To satisfy the required fresh properties of 3D printable concrete, such as pumpability and extrudability, the content of aggregate is significantly reduced,which leads to a further increased quantity of OPC in the mix design [9]. Producing OPC consumes massive energy and emits large amounts of greenhouse gases [10]. According to [PERSON] et al. [11], the cement industry is responsible for nearly 7% of global total CO\({}_{2}\) emission. Partially replacing OPC by supplementary cementitious materials (SCMs) might be a proper way to start making the printable concrete sustainable. Recently, many researchers started to look for suitable SCMs for decreasing the use of OPC in 3D printable concrete. The mix designs for 3 DCP proposed by [PERSON] et al. [2] contain 55% of OPC (391 kg/m\({}^{3}\)), 30% of fly ash and 15% of microsilica in the binder. It is a good example to show the feasibility of using comparable content of fly ash and microsilica/silica fume to substitute OPC. The attempt of using a high volume of fly ash or slag to replace OPC can be found in the recent works by [PERSON] et al. [12] and [PERSON] and [PERSON] [13], respectively. A large amount of limestone is also utilized by [PERSON] et al. [14] for developing a sustainable cementitious binder. However, many limitations may appear by using the common SCMs like fly ash, silica fume, slag, and limestone as the main ingredients of 3D printable cementitious materials for a long-term application. The main problem for silica fume and slag is the limited quantity of their production worldwide [15; 16]. The amount of fly ash is relatively higher, whereas more than 66% of available fly ash is not suitable for blending with cement due to quality reasons [17]. Using the above 10% of limestone alone as the substitution of OPC in the binder would result in higher porosity and weaker strength [16; 18]. [PERSON] et al. [19] pointed out that it is worthwhile to use limestone and calcined clay as the OPC substitutions for developing sustainable and printable cementitious materials. Reasons include the following: (1) kaolinite clay and limestone exist in abundance; (2) the production of calcined clay requires much lower heating temperature (700-850 \({}^{\circ}\)C). Manufacturing 1 ton of calcined clay may emit only 0.3 ton of CO\({}_{2}\)[20] which is much less than producing the same mass of OPC (emitting 0.8-0.9 ton of CO\({}_{2}\)) [21]; (3) a mixture using limestone and calcined clay with 1:2 mass ratio not only could replace higher contents of OPC but generate carbo-aluminate hydrates which can fill the capillary pores [16]. The porosity refinement could contribute to increasing the compressive strength at an older age; (4) the lower grades of calcined clay (metakaolin content: 40-50%) also shows the comparable pozzolanic reactivity which has been demonstrated by [PERSON] et al. [22].