## 4 Discussion  
Within the presented research, we compared the rheological properties of different spherical filler materials with similar particle size distribution at three filler loadings (30, 45, and 60 vol.%). The composition of the three-component binder was the same for each filler loading level. The results of the influence of filler loading on rheological properties showed that the increasing filler loading increased storage, G\({}^{\prime}\) and loss, G\({}^{\prime\prime}\) modulus of all materials. The increase of both moduli was observed in the whole frequency range measured (frequency sweep test, Figure 7), as well as at all tested shear strains (amplitude sweep tests, Figure 3). This observation is in line with the literature [21; 34; 37; 38]. As opposed to the general behaviour, the extent of moduli change depends on the material. Increasing filler loading also changed the rheological character of the material. As the filler loading was increased to 60 vol.%, all four composites exhibited solid-like behaviour at a constant frequency and small shear strains (Figure 4). Moreover, filler loading affected the solid-like to liquid-like transition in the broader frequency domain (Figures 7 and 8). In general, it was observed that in the range of lower frequencies, where particle-particle interactions are dominant, all composites exhibited solid-like behaviour. As the frequency increased, the material behaviour changed to liquid-like. Furthermore, when the concentration of filler particles increased, the transition from solid-like to liquid-like behaviour  
Figure 13: ATR-FTIR spectra of GB materials, reference binder system, and GB filler material. appeared at higher frequencies since the particle network was denser. The value of the frequency, where the transition occurred, depended on the filler material. Comparison of the effect of filler materials on rheological properties showed that the composites with Al fillers and 316L filler exhibited remarkably similar rheological behaviour. This can be seen through the amplitude sweep tests (Figure 5) and frequency tests (Figure 6), where the composites with both materials exhibited similar values of storage, G\({}^{\prime}\) and loss, G\({}^{\prime\prime}\) modulus regardless of the filler loading. On the other hand, it was shown that the composite materials with Ti fillers exhibited the lowest dynamic moduli in amplitude and frequency tests; again, regardless of the filler loading. The rheological behaviour of the composites with GB filler was similar to the behaviour of Ti composites at concentrations 30 and 45 vol.%, especially in the frequency domain (Figure 6). At both concentrations, the values of G\({}^{\prime}\) and G\({}^{\prime\prime}\) in the low-frequency range, where particle-particle behaviour is dominant, were lower compared to Al and 316L materials. However, at 60 vol.%, the behaviour of GB composite significantly changed. G\({}^{\prime}\) and G\({}^{\prime\prime}\) increased significantly compared to 316L and Al materials. To illustrate the extent of the G\({}^{\prime}\) increase in the area of particle-particle dominated behaviour, the values of G\({}^{\prime}\) at 30 vol.% loading and frequency \(\approx\)0.1 rad/s were compared to the values G\({}^{\prime}\) at 60 vol.% (again at \(\approx\)0.1 rad/s). The storage modulus of GB composite material increased about 3500 times, while the increase of Al and 316L composite materials was between 140 and 180 times. The G\({}^{\prime}\) values of Ti composite material increased the least, about 60 times. The different behaviour of composite materials can be partially explained with the ATR-FTIR results and the occurrence of physical or chemical bonding. On the one hand, it was shown that GB composite materials interact only physically with the binder system. FTIR results indicate the adsorption of Si-O surface groups with the open ring of the carbonyl group of MAPP component of the binder system.