## 4 Conclusions and Recommendations  
In this research, the stiffening effect of fillers was investigated based on mastic rheology and the application of micromechanics models. The rheological properties were measured with a dynamic shear rheometer (DSR). The models include the Maron-Pierce Model, Nielsen Model, Mooney Equation, Krieger-Dougherty Model, Chong et al. Model, and Robinson Model.  
The results show that the same volume content of the filler has a different effective volume in mastic. This will result in different stiffening effects. The fillers increase the stiffening effect of the composite, especially at high temperatures. The fillers with similar effective volume and packing show similar behaviour. The acidic fillers result in lower viscosity, while basic filler results in higher viscosity in mastic. The stiffening effect is dependent on the filler type.  
Micromechanics-based models can predict stiffening potential from the properties of the individual ingredient properties. The models produced good results at low volume concentrations with the exception of the Chong model. Maron-Pierce model underestimated the stiffening potential of granite mastic at higher concentrations. In comparison to other models, the values \(\phi_{\text{m}}\) and \(\text{K}_{\text{E}}\) in the Mooney model are significantly different for limestone and granite, respectively. In future studies, we recommend adjusting the \(\text{K}_{\text{E}}\) and \(\phi_{\text{m}}\) to account for variations of particle shape, size, surface texture, dispersion, and agglomeration, etc.  
It is difficult to precisely model the mastic data with any single model due to the presence of complex stiffening effects beyond volume filling. The stiffening effect is complicated by the presence of the adsorption layer. This increases the effective volume of filler and will eventually reduce the maximum packing \(\phi_{\text{m}}\) in suspension. Although models somewhat accurately predicted the stiffening potential of fillers, they only account for volume filling and exclude the physicochemical interactions.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Model**} & \multicolumn{2}{c}{**Limestone Mastic**} & \multicolumn{2}{c}{**Granite Mastic**} \\ \cline{2-5}  & \(\varphi_{\text{m}}\) & \(\text{K}_{\text{E}}\) & \(\varphi_{\text{m}}\) & \(\text{K}_{\text{E}}\) \\ \hline
**[PERSON]–[PERSON]** & 0.77 & - & 0.82 & - \\
**[PERSON]–[PERSON]** & 0.47 & 3.39 & 0.46 & 3.01 \\
**[PERSON]** & 0.79 & 2.02 & 0.49 & 1.43 \\
**[PERSON]** & 0.49 & 5.24 & 0.41 & 3.27 \\
**[PERSON]** & 0.48 & 5.66 & 0.40 & 3.01 \\
**[PERSON]** & 0.53 & 4.91 & 0.42 & 3.13 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Micromechanics models parameters.  
**Author Contributions:** Conceptualisation, A.R.; formal analysis, A.R. and A.M.; investigation, A.R. and A.M.; resources, A.R. and A.M.; writing--original draft preparation, A.R., A.M. and N.I.M.Y.; writing--review and editing, A.R., G.A., N.T. and N.I.M.Y.; supervision, G.A. and N.T.; funding acquisition, N.I.M.Y. All authors have read and agreed to the published version of the manuscript.  
Universiti Kebangsaan Malaysia supported paying the Article Processing Charges (APC) of this publication (DIP-2020-003 and Dana Pecutan Penerbitan FFAB 2021).  
Not applicable.  
Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.  
All data used in this research can be provided upon request.  
The authors would like to thank Universiti Kebangsaan Malaysia for their financial support.  
The authors declare no conflict of interest.