**4. Conclusions**

The work provides an original way to obtain CaSiO3-HAp porous ceramics with the wet and solid-phase synthesis strategies being combined. The method involves sol-gel synthesis (template) of the starting raw material in the form of an amorphous composite material based on xonotlite and its subsequent spark plasma sintering yielding ceramic wollastonite. TGA, XRD, and EDX showed that the HAp phase formation in the resulting ceramics can be initiated by the solid-phase interaction of the reaction mixture (CaO and CaHPO4) in the spark plasma heating by the "in situ" reaction directly at the moment of sintering of the amorphous xonotlite. It has been determined that the optimal temperature for the SPS-RS to obtain crystalline wollastonite from the amorphous xonotlite, as well as to initiate the reaction "in situ" with the formation of HAp, is 900 ◦C.

Nitrogen physisorption and mercury porosimetry allowed to reveal that the pore size and volume depend on the type and quantity of the template introduced at the different stages of the synthesis, leaving pores after the thermal-oxidative treatment of the ceramics. It is shown that latex introduced in the sol-gel synthesis allows to form pores of about 100 nm, while carbon pore-forming agents added prior to SPS-RS contribute to the formation of macropores of 1 μm and larger. We determined

that the use of latex and 10 wt% carbon fiber is considered the optima, as porous (19.7%) and dense (RD 80%) biocomposite ceramics is formed that exhibit the required mechanical strength (<sup>σ</sup>cs. 111 MPa), which is within the values of the normal strength of natural bone (110–120 MPa). The use of graphite powder should be carefully adjusted due to its impact on the other important physical and mechanical characteristics of ceramics.

The results of the microbiological test revealed that the (20 and 50 wt%) HAp-containing ceramic samples show the bacteria *Pseudomonas aeruginosa* being covered with a thick layer of alginate, with the latter not being observed on pristine wollastonite. Alginate appearance indicates the protective response of the bacteria to a negative environmental impact. In this regard, in terms of the risk of infection, composite HAp-containing ceramics is more attractive for biomedical applications.

**Author Contributions:** Conceptualization, E.P.; data curation, V.T.; formal analysis, A.P.; investigation, V.M., E.M. and Y.S.; methodology, O.S. and A.B.; project administration, V.A.; resources, A.F.; software, Y.S.; validation, I.B.; visualization, T.K.; writing—review and editing, E.P. and A.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** The investigation was financially supported by Russian Science Foundation (project No. 18-73-10107).

**Acknowledgments:** Equipment of CUC "Far Eastern center of structural investigation" (Institute of chemistry FEB RAS, Vladivostok, Russia), also interdisciplinary CUC in the field of nanotechnologies and new functional materials and CUC "Laboratory of mechanical tests and structural studies of materials" (FEFU, Vladivostok, Russia) used in the research is gratefully acknowledged. We thank Andrei Vladimirovich Shevel'kov, Head of Laboratory of Directed Inorganic Synthesis, Chair of Inorganic Chemistry, Chemical Faculty, Lomonosov Moscow State University, Moscow, Russia, and also Aleksandr Sergeevich Tyablikov for their help in performing experiments.

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