# Development of Cementitious Mortars for Aerial Additive Manufacturing
## 3 Results
### Scanning Electron Microscopy  
As shown in Figure 6a-f, all cross-sectional fracture surfaces of each mix design show a structure typical of the C-S-H phases formed in CEM1-based cements. SEM images of mixes \(A\)-\(E\) illustrated were taken at \(\times\)4000 magnification unless noted otherwise. The smooth, spherical particle observed in mix \(B\) (Figure 6b) can be attributed to the PFA, while the spherical particles in Mix \(D\) (Figure 6d) and Mix \(E\) (Figure 6f) illustrate the presence of larger and smaller PFA particles, respectively; silica fume possesses smaller and more sub-angular to angular particle microstructure [38] which is distinct from the spherical PFA. The image shown in Mix \(E\) shows a polypropylene fibre with a diameter of approximately 40 \(\upmu\)m. Micro-cracking is visible in mixes \(A\) (Figure 6a), \(B\) (Figure 6b) and \(E\) (Figure 6f). Mix \(C\) (Figure 6c) shows a polyol resin modified cementitious matrix.  
Figure 5: Mechanical test results. (**a**) 28-day compressive strengths for mixes \(A\)–\(E\). (error bars denote the standard deviation). (**b**) The 28-day flexural strengths for mixes \(A\)–\(E\) (error bars denote the standard deviation). (**c**) Examples of flexural failure. Specimens for mixes \(A\), \(B\), \(C\) and \(E\) broke in two. Mix \(D\) specimens were held together by the polypropylene fibres to prevent complete breakage. (**d**) Creep results over a 50-day period, showing strain. (**e**) Temperature and humidity variations in the creep results.  
The PFA spherical particle visible in mix \(D\) (Figure 6d) is observed to have a closer, superior bond to the cementitious matrix than that observed in \(B\) (Figure 6b), which has a gap evident. Mix \(D\) contained silica fume and was stronger, denser and possessed more buildability, with the denser CSH structure a result of adding the silica fume to the mix. The close up of the polypropylene fibre visible in mix \(D\) (Figure 6a), imaged at \(\times\)2000 magnification, shows an uneven surface which would promote anchorage in the cementitious matrix and assist pull-out resistance, whereas a smoother fibre would pull-out more easily when tensile forces were applied. Possible traces of cement phases adhering to the fibre are visible in the image. Mix \(D\) was the strongest in the compression strength results and had the highest \(G^{*}\), but possessed low workability; in summary it was the stiffest mix. Mix \(E\) (Figure 6f), to which the limeX70 was added, shows a distinct diamond-shaped calcite crystal [39] towards the right of the image due to the lime-rich mix and excess of hydroxide, along with what are reasoned to be needle-shaped silicate crystals on the far-right of the image.  
Figure 6: SEM images of cross-sectional morphology of mixes \(A\)–\(E\) taken at \(\times\)4000 magnification unless otherwise stated. (**a**) Mix \(A\) with cracks in the matrix and Calcium Silicate Hydrate (CSH) phases visible. (**b**) Mix \(B\) with CSH phases and a distinct spherical fly ash particle visible. (**c**) Mix \(C\) which shows a denser resin-modified cementitious structure. (**d**) Mix \(D\) with the PFA particle showing a closer bond to the cementitious matrices than observed in mix Mix \(B\). (**e**) Mix \(D\) with an image of a polypropylene fibre taken at \(\times\)2000 magnification. (**f**) Mix \(E\) with a smaller spherical PFA particle visible along with a diamond shaped calcite crystal and needle-shaped silicate crystals on the right of the image.