## 2 _Introduction_  
Over the past four decades, there has been a major advance in the development of medical materials, and this includes the innovation of ceramic materials for skeletal repair and reconstruction [1]. These so-called \"bio-ceramic materials\", which are known for their biocompatibility [2], bioinert, and bioactive [3], are a special kind of biomaterial that is used to treat, augment, repair, or replace the diseased or damaged hard tissue of the human body [3; 4]. There is currently widespread research being conducted in the development of bio-ceramic materials; such studies are motivated not only by the beneficial technical properties of the materials such as phase stability and bioactivity but also by the potential to reduce energy consumption in the fabrication of bio-ceramic products [5] that are also effective in their action, minimizing the process and being economical [6]. Other research additionally incorporated bio-ceramic material with other materials such as an iron oxide to improve the synthesis methods to produce the HAp with a magnetic core [7]. Hence, in order to reduce energy consumption during the fabrication of bio-ceramics, there is a need to explore a new technique to ensure that the densification of ceramic materials can be achieved at lower sintering temperatures. Among the various forms of biocearamics, calcium phosphate, particularly hydroxyapatite (Ca\({}_{10}\)(PO\({}_{4}\))\({}_{6}\)(OH)\({}_{2}\); HAp), has received great attention due to its chemical and crystallographic structure which has similar mineral components to those of bones and teeth of vertebrates [8; 9; 10; 11]. Furthermore, HAp is resistant to X-ray radiation and UV irradiation without visible aging and structural damage that may be implied in certain clinical treatments [12; 13]. [PERSON] et al. summarize the current knowledge of HAp in medicine and dentistry from the perspective of their use in the manufacture of transitional implants for guided bone regeneration [14] and their works have also been supported by other researchers [15; 16; 17]. Meanwhile, HAp nanoparticles have become a matter of interest in biomedical research not only in bone management but also in the development of cancer drug delivery systems [18; 19; 20] as well as in future perspectives in cosmetics and oral care products [21; 22]. In addition, research conducted in the past has shown that the pure HAp of high crystallinity can be replicated through conventional sintering techniques [23]. The densification of hydroxyapatite is highly dependent on the sintering temperature, and the optimized sintering temperature is commonly reported within the range of 1200-1300 \({}^{\circ}\)C [24; 25; 26; 27; 28]. However, HAp tends to decompose at this high temperature. A previous study reported that the phase transformation which occurs at high sintering temperatures will finally lead to the formation of \(\alpha\)-tricalcium (\(\alpha\)-TCP) and calcium oxide at 1400 \({}^{\circ}\)C, accompanied by a nanopore formation. The nanopores that disperse in the \(\alpha\)-TCP grains are the main factor leading to low density and decreased mechanical strength of the specimens [29]. Meanwhile, research conducted at low sintering temperatures of \(\sim\)800 \({}^{\circ}\)C exhibited slow dihydroxylation behavior and promoted low densification resulting in poor mechanical properties [30]. In order to maintain the conventional sintering technique in producing HAp ceramics at a lower sintering temperature, a new approach to activate the HAp with alkaline activation prior to the sintering process could be considered. This process is called \"geopolymerization\". A previous study has shown that sintered kaolin ceramics at 900 \({}^{\circ}\)C can be achieved by initiating the geopolymerization and that this results in a good performance of compressive strength [31].