## 4 Conclusions  
This paper was a fundamental study to develop the limestone and calcined clay based sustainable cementitious materials for the extrusion-based 3 DCP. Four mix designs with different binder compositions were proposed in this study. Mixtures MIX-L, MIX-M, and MIX-H contained 40% of Portland cement, 20% of limestone and 40% of calcined clay in the binder. While the different grades of calcined clay were used, respectively. Mixture MIX-R as the reference mix design had 40% of Portland cement and 60% of limestone as the binder. All mix designs were investigated through penetration resistance test and compressive strength test at 1, 7, and 28 days. The most important goal of this paper was to investigate the effects of different grades (MK content) of calcined clay on extrudability and the early-age strength development. Thus, the mixtures were also evaluated by the extrusion test, green strength test and ultrasonic pulse velocity test. The main findings of this study are summarized as follow. * A lab-scale extrusion test method based on the ram extruder was proposed in this study to efficiently observe the extruded filaments of the fresh mixture and quantify the required extrusion pressures at different ages. Mixtures MIX-L, MIX-M, and MIX-H showed good shape stability at the ages before their initial setting time. The extrusion pressures and the growth rate of pressure with time were significantly increased by increasing the MK content in calcined clay. Thus, using a higher grade of calcined clay may increase the extrusion shear strength of fresh mixtures which may bring difficulties for extruding and reduce the printability window/open time. * According to all results found during this study, increasing the MK dosage in calcined clay could significantly accelerate the initial cement hydration. Consequently, higher mechanical performance in the early stages (green strength) were achieved. Moreover, the buildability and structural build-up behavior of mix designs for 3 DCP could be enhanced by using higher grades of calcined clay. Figure 12: Measuring results of ultrasonic pulse velocity for mixtures MIX-L, MIX-M, and MIX-H. The linear fit based on measuring results from 100 min to 240 min. * Compared with mixture MIX-R, mix designs with calcined clay showed shorter initial setting time and higher compressive strength at 1, 7, and 28 days. * Increasing the content of MK in calcined clay could significantly reduce the initial setting time and increase the compressive strength at 1, 7, and 28 days. * Within the first 4h, the filler effect of the calcined clay may play a dominant role to promote the phase change of flocculation, nucleation and structuration of cement particles from the microstructure view. Increasing the percentage of MK in calcined clay could increase the content of very small particles in the binder system, which may enhance nucleation and facilitate the microstructure development. * Overall, it is essential to find the balance between extrudability and early-age strength to apply the discussed mix designs in 3 DCP. Results of this research showed that the MK content of calcined clay plays a key role in the performance of printable cementitious mortars. Conceptualization, Y.C. ; Formal analysis, Y.C. and Z.L. ; Investigation, Y.C. and Z.L. ; Methodology, Y.C. and S.C.F. ; Software, Y.C. ; Supervision, O.C., F.V. and E.S. ; Visualization, Y.C. ; Writing - original draft, Y.C. ; Writing - review & editing, Z.L., S.C.F., O.C., F.V. and E.S. [PERSON] and [PERSON] would like to acknowledge the funding supported by the China Scholarship Council (CSC) under grant No. 201807720005 and No. 201506120072 respectively. In addition, [PERSON] would like to acknowledge the funding from Science Without Borders from the National Council for Scientific and Technological Development of Brazil (201620/2014-6). Burgess Pigment Company is thanked for the donation of the Optipopzz@Burgess metakaolin. The authors appreciate Dr. [PERSON] and Mr. [PERSON] for their supports in tests.