## 1 Introduction
Metakaolin (MK) or high grades of calcined kaolinite clay with high purity and price is generally used by other industries, for instance, paper, ceramics and refractory [16]. In the concrete industry, it appears to be an economical choice to use lower grades of calcined clay, which are widely available compared to MK. However, the calcined clay sourced from different suppliers may contain various percentages of MK. Different calcined clay may have different compositions, secondary phases, fineness, and specific surfaces [22] which will influence the rheological behaviors of fresh cementitious materials. To our knowledge, at this moment few researchers are focusing on developing limestone and calcined clay-based cementitious materials for 3 DCP. The effects of different grades of calcined clay (different MK content) on fresh behavior and early-age strength development is to be further explored. Extrudability and early-age strength should be emphasized as two major constraints of developing printable cementitious materials. As cement is a time-dependent material, the workability of a given printable mix design is not constant and shows loss of consistency with time. Printability window/open time was used to describe the valid time for extruding the fresh material with acceptable quality [8; 23]. Thus, the printability window/open time of a printable mix design could be understood as the timespan of extrudability. As explained by [PERSON] [24], extrusion behavior of fresh cementitious materials are dependent on several rheological parameters, such as yield stress, plastic viscosity and shear strength. Many experimental methods have been proposed from the literature to quantify how these rheological parameters change over time. [PERSON] et al. [7] used a shear vane apparatus to measure the change of shear strength of fresh mortar with time. More recently, rheological behavior of fresh cementitious materials were precisely determined by using a coaxial-cylinder rheometer (equipped with a unit cell and vane rotor) [4; 25]. Besides, the ram extruder was also one method to characterize the extrudability of cement-based material [2]. The ram extruder could be regarded as a model to simulate the process of material extrusion and deposition in the 3 DCP process. The extrusion technology as a cost-efficientand environmental-friendly fabrication method has been used in the cementitious materials industry for a long time [26] before applied in 3 DCP. According to [PERSON] et al. [27], it is feasible to use a ram extruder as a rheo-tripometer to calculate the extrusion factors of the high yield fluids, such as bulk and shear yield stresses, wall friction yield stress, and others. The extrusion pressure could reflect the combined impacts of rheological and tribological properties of cement-based materials. Since a cement-based material is heterogeneous and time-dependent, the extrusion behavior should be changing with time. In this study, a new extrusion test based on the ram extruder is proposed for characterizing the extrudability of the developed limestone and calcined clay-based cementitious materials at different ages. On the other hand, elimination of formwork in 3 DCP introduces many challenges for developing printable cementitious materials. Without the supports from formwork, the extruded fresh mortar layer should emerge with high yield stress immediately to maintain the shape and sustain the load from subsequent layers once they are deposited [28]. That is the main reason why it is essential to monitor the mechanical performance of the printable mix design at an early age. In this study, the early age strength contains two sections: the green strength, as well as the compressive strength at 1, 7, and 28 days. The phase of green strength which was defined by [PERSON] et al. [29] is used to describe the uniaxial compressive strength of mixtures at the fresh state. According to the simulated building rates (1.1-6.2 m/h) from [PERSON] et al. [28], it will take at most 3h to print a large concrete component with a height of 3m.