*4.1. Cement-Based Mortar Design*

Figure 2 gives the results of the setting times of cement-based mortar samples. There are seven samples given in the figure. They were designed through adding accelerator admixture in certain ratios ranging from 0‰ to 3‰ (0:CH00; 5‰:CH05; 10‰:CH10; 15‰:CH15; 20‰:CH; 25‰:CH25; 30‰:CH30). The results show that the setting time of cement-based mortars was shortened by increasing the ratio of the added accelerator admixture. Designing the optimum mixture for the 3D printed concrete paving stones was the aim. In fact, recent findings have some defects and design proportions that should be improved.

**Figure 2.** Cement-based mortar setting times.

The print time, setting time, and layer interval time in 3D printing should be considered as the loading increases on newly printed layers [17]. In general, the initial setting

time of cement-based mortars is usually longer than ten hours, and this is not acceptable for the 3D printing abilities of the materials [5]. The printable construction materials should exhibit good workability, an appropriate setting time, and high strength values. The setting time should be kept at 20–80 min, so that the material coagulates rapidly and exhibits high strength values and the desired styling ability within a short time [18]. According to Figure 2, the cement-based mortars designed were insufficient in terms of consistency and quality of printing. Therefore, in the context of the study, more experiments should be conducted to obtain the most suitable mortar design to produce 3D printed concrete paving stones.
