**4. Discussion**

The results showed that the design of binders with SCMs can be optimized by considering their physical and mechanical characteristics. HCFA had reduced fineness and increased water demand compared to LFS and LF, which rendered pastes with OPC substituted with HCFA less workable. Combining OPC with HCFA and LFS or LF in ternary pastes showed a solution to this problem. Furthermore, considering packing density can aid the design process and help design pastes with either the lowest w/cm ratio at maximum solid concentration for strength optimization or with the w/cm ratio for the required consistency. Fineness has been shown to influence the packing density and consistency of pastes greatly, but reactivity and synergy between binders also seems to be very important for compressive strength development. The reactivity of HCFA seemed to contribute more to strength development compared to the finer LFS and LF, despite the increased water demand. Regarding fineness, the use of SCMs coarser than cement can still lead to equal packing densities but grinding the SCMs to higher fineness could lead to further improvements. The ternary mixtures with OPC, HCFA, and LFS or LF, however, showed packing densities similar to that of 100% OPC paste. HCFA with LF in ternary pastes with OPC showed good synergy in cement-based binders, reaching the strength development of the reference paste, despite having a higher w/cm ratio at the 30% replacement rate, which was not the case when LFS was used as the third constituent. Overall, it seems that designing ternary binders with suitable SCMs by considering particle packing could compensate for strength loss and result in equal performance with reduced cement content. **Funding:** The present research was funded by the Research Committee of the Aristotle University of Thessaloniki, Greece.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** The author would like to thank Eirini-Chrysanthi Tsardaka for her help in preparing the pastes and Eleni Pavlidou for her help with SEM.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

