## 2 Materials and Methods  
A customized 3D printer was utilized for the dry powder supply and liquid binder supply, as shown in Figure 2. A positive displacement metering valve MV 250 (Poly Dispensing Systems, Orgeval, France) was integrated into the printer to ensure accurate binder deposition using a 700 ETC+ controller (Poly Dispensing Systems, Orgeval, France), thus enhancing the precision of the specimen's fabrication. The hopper was used for the dry powder supply while the binder was precisely sprayed in proper places to be effectively activated using a nozzle operated using a positive displacement valve. The printed samples had dimensions of 160 mm \(\times\) 40 mm \(\times\) 40 mm and were produced with a layer thickness of 4 mm and a hatch distance of 4 mm. The velocity of the powder spread was 700 mm/min and the liquid binder was also operated at the same velocity, while maintaining the pressure at 2-4 bar using a 700 ETC+ valve controller (Poly Dispensing Systems, Orgeval, France). The dry mix included limestone (BETOCARB P2-MX from (Omya SAS, Orgon, France) containing 98.4% calcite)--density 2700 kg/m\({}^{3}\)--and standardized sand--density 2640 kg/m\({}^{3}\)--(per EN 196-1 [21]) at a ratio of 1:3. Figure 2: (**a**,**b**) Customized 3D printer setup; (**c**) printed green specimen; (**d**) printed specimen; (**e**) 700 ETC+ valve controller. The particle size distribution of limestone and standardized sand was analyzed using the laser scattering technique (Partica LA-960V2--Horiba Scientific, 91120 Palaiseau, France) (Figure 3), and the distributions agreed with the technical data provided by the suppliers. Three different concentrations of NaOH in water solutions (12.9 mol/L, 15.6 mol/L, and 19 mol/L) were considered in the present work. It is interesting to note that traditional casting techniques have generally utilized around 13 mol/L; however, by utilizing a selective limestone activation process, we were able to safely investigate higher concentrations [20]. The solutions were prepared in an airtight polypropylene Erlenmeyer flask to minimize CO\({}_{2}\) exposure and prevent carbonate formation, and were stored under a controlled hood, along with the other materials, at 20 \({}^{\circ}\)C for at least 24 h. The properties of the liquid binders were analyzed using the [PERSON] rheometer MCR 102 in accordance with DIN 53019 [22]; the relationship between shear stress and shear strain is shown in Figure 4.  
Printing was carried out by depositing successive 4 mm thick layers (ten) of a mixture of dry limestone and normalized sand, each activated using a NaOH water solution over  
Figure 4: Rheological behavior of the NaOH water solutions. Figure 3: Particle size distribution for limestone and sand. a defined area of 160 mm \(\times\) 40 mm. The printed samples were first cured in situ on the powder bed for 24 h to enable the binder to initiate the hydration process and allow for setting without interference. This was followed by additional curing for 48 h at room temperature to enable continued hydration and strength development under air exposure. After curing, loose powder was removed (de-powdering). After printing, the specimens were manually abraded using standardized grit paper in order to achieve reproducible edge profiles that are required for mechanical testing. To ensure a reproducible flat surface, smoothing was conducted on a precision-ground, level steel plate as an invariant reference by maintaining a combination of calibrated hand pressure techniques and continuous visual inspection--and the specimens were intermittently rotated to minimize localized surface deviations. The printed specimens were finally dried at 45 \({}^{\circ}\)C for 96 h and 600 h before mechanical testing, which was equivalent to 7 days and 28 days of curing time, respectively. The choice of this drying temperature was based on previous literature [23]. The weight loss of the printed specimens was measured after post-processing to the end of curing time (28 days).