**3. Results and Discussion**

The compressive strength was determined before the creep tests. Four specimens were used to determine compressive strength values for printed and cast specimens. The specimen's age at the time of testing was 28 days. The compressive strength values are shown in Figure 3.

**Figure 3.** The 3D-printed and cast cement composite compressive strength values with measurement errors.

As visible from the compressive strength diagram, printed specimens loaded longitudinally to their layer placement exhibit more than two times lower compressive strength than cast specimens; furthermore, their standard measurement error is 17.9% larger than cast specimens.

Afterward, 28 day-long early creep and shrinkage tests were run, and the resulting readings are compiled in Figure 4.

**Figure 4.** The 3D-printed and cast cement composite total recorded, creep, and shrinkage strains.

As it is clear from the curves in Figure 4, the shrinkage strains for printed and cast specimens are very close, or even identical. However, the creep strains have significant differences. Printed specimens show, at the peak values, 28.3% less creep strains. Additionally, the creep curve of printed specimens shows strain-decreasing relations starting from day 12 until day 28, while creep strains in cast specimens rise until day 18 and then exhibit a slight decrease. This implies that there must be some layer adhesion issue or that the load impact to the specimen resulted in the degradation of the structure. As the printed specimens have significantly lower compressive strength than cast specimens, it is necessary to calculate specific creep to see the creep strains without applied stress impact.

Specific creep values are calculated according to the equation, and the results are in Figure 5:

$$\chi\_{cr}(t, t\_0) = \frac{\varepsilon\_{cr}(t, t\_0)}{\sigma} = \frac{\varepsilon\_{kop}(t) - \varepsilon\_{sl}(t) - \varepsilon\_{cl}(t, t\_0)}{\sigma} = \frac{1}{E\_{cr}(t, t\_0)}\tag{1}$$

where:

*χcr*(*t*, *t*0) is the specific creep, *εcr*(*t*, *t*0) is the creep strain, *εkop*(*t*) is the total strain, *εsh*(*t*) is the shrinkage strain, *εel*(*t*, *t*0) is the elastic strain, *σ* is the compressive stress, and *Ecr*(*t*, *t*0) is the modulus of creep.

In Figure 5, it is clear that 3D-printed cement composites exhibit significantly higher specific creep; in other words, they are more willing to creep. On average, they have 32.8% higher specific creep than cast specimens. Furthermore, their specific creep appears within a couple of days, while it develops during the first 21 days of testing in cast specimens.

**Figure 5.** The 3D-printed and cast cement composite specific creep.

To further elaborate the assumption that printed specimens have some issues in the printed layers, the strain readings were divided into those that were measured to the top surface layer and those in which the surface consists of layer-side surfaces (placement and description shown in Figure 6).

**Figure 6.** The 3D-printed cement composite strain gauge placement on layer sides and surface for creep (**a**) and shrinkage (**b**) specimens.

The long-term shrinkage and creep strain curves according to strain gauge fitment are shown in Figure 7.

Here, it becomes clear that while long-term strain curves in the relation are similar, the creep strains and shrinkage strains are very different. While creep and shrinkage curves rise steadily to the layer top surfaces, the sides seem to have deterioration due to shrinkage. As the specimens were tested at the age of 28 days, the main shrinkage effect came from drying shrinkage. The shrinkage strain curves from the layer side surfaces lead to the conclusion that layers have been partially separated. To further elaborate, printed specimens after long-term tests were saturated in epoxy resin and used to make polished section specimens that then had their microstructure examined. It was determined that for all specimens, one side of the layer was more porous (see Figure 8) than the rest of the polished section surface.

**Figure 7.** The 3D-printed cement composite total recorded, creep, and shrinkage strains according to strain gauge placement.

**Figure 8.** Quantitative image analysis images with matrix and filler part (red) and air void parts (blue) to the left side (**a**), middle part (**b**), and right side (**c**) of the specimen cross-section.

The specimens were printed using a plastic nozzle that had been printed on the plastic 3D printer. It had a stitched part that, as it turns out, frothed up part of the cement composite that interacted with this part of the nozzle. The model of the nozzle is shown in Figure 9. The white part in the model is the part where plastic layers are connected, and the stitch is developed.

**Figure 9.** The 3D printer nozzle printed and modeled part.
