Calcium phosphate cements (CPCs) are a clinical alternative to traditional bioceramics because they are easy to handle and shape, they mold themselves well to the contours of defective surfaces, and set in situ in the bone cavity to form a solid restoration [1]. Since they were developed in the mid-1980s, CPCs have also attracted great interest due to their chemical similarity to the mineral phase of bone tissue and their good osteoconductivity [2].

One of the most important formulations is based on α-tricalcium phosphate [α-Ca₃(PO₄)₂; α-TCP], which sets in situ and forms a calcium-deficient hydroxyapatite [Ca₉(HPO₄)(PO₄)₅(OH); CDHA] when hydrated [3]. However, it is not very strong under compression [4] and its mechanical strength is low when compared to that of cortical bone [5] limiting its application to areas subjected to low mechanical loads [6].

In view of the excellent bioresorbability of CDHA, researchers have focused their efforts on overcoming the mechanical weakness of calcium phosphate cements by using different fillers, fibers and reinforcing additives that lead to the formation of various multiphase composites, based on the idea that the filler in the matrix may eliminate crack propagation [7]. Nevertheless, the presence of fillers prevents bone ingrowths into pores and produces a denser cement with a slower resorption rate and hence a slower bone substitution [8]. Therefore, it is difficult to increase the strength of these cements without negatively affecting other properties.

In the late 1990s, the Swedish company Doxa Certex AB proposed the use of calcium aluminate cements (CACs) as dental restorative materials in place of amalgam [9], and today the use of CACs has extended to several orthopedic applications [10,11]. The calcium aluminate system has two inherent features that make it suitable for load-bearing applications: fast setting and high consumption and turnover of water during the setting and reaction. The high water turnover gives the system a potentially high strength, several times that of normal CPCs. Moreover, CACs have good corrosion resistance in contact with body fluids and are biocompatible since the amount of Al ion leakage is very low [12,13]. In these materials, the main phase commonly used is monocalcium aluminate [CaAl₂O₄, CA] due to its optimal reaction rate compared with the other phases.

Although some calcium phosphates, as β-tricalcium phosphate, are used in combination with CACs in order to induce some biological activity in the resultant composites [14], the use of CA as a reinforced additive of traditional CPCs is not documented. Thus, the aim of this work was to design and study new α-TCP/CA formulations intended for biomedical applications. To this end, the chemical, mechanical and biological properties of α-TCP/CA cement after aging in simulated body fluid were investigated.

Figure 1 shows the XRD pattern of CA where the presence of monoclinic CaAl₂O₄ (JCPDS 2310-36) as main phase, in addition to dicalcium aluminate [CaAl₄O₇, CA₂] (JCPDS 2310-37) was found. The specific surface area of the powder was 9.12 m²/g and a slight increase to 11.88 m²/g was achieved after grinding.

Figure 2 shows the initial and final setting time of α-TCP and composites containing different CA mass%. For a-TCP-based cement the initial and final setting times were higher than those reported in the literature for similar compositions [22]. With the addition of CA, the setting times increased, this increase being directly proportional to the amount of CA added. There were no significant differences in the final setting times of composites containing CA.

Figures 3-5 show the powder XRD patterns of composites before and after soaking in SBF for 7 and 14 days. For all times and all formulations, the characteristic peaks of β-TCP (JCPDS 09-0169), which appears as a seconddary phase in a-TCP powder (JCPDS 29-0359), were detected. After 24 h setting (Figure 3), for a-TCP-based cement, mainly peaks of CDHA (JCPDS 46-0905) were observed. With the addition of CA, diffraction patterns were very different from those of a-TCP-based cement and apparently, only unreacted peaks of a-TCP in addition to β-TCP were present.

After 7 days of soaking (Figure 4) the intensity of a-TCP lines decreased in relation to set cements and CDHA lines appeared.

Fourteen days after, the hydration reaction seemed to be complete for a-TCP, whereas a great amount of unreacted a-TCP, in addition to CDHA, could be observed for

composites containing CA (Figure 5). There were no peaks of CA and no proof of the presence of Ca₃Al₂O₆·6H₂O (C₃AH₆) (JCPDS 24-0217) or Al₂O₃·3H₂O (AH₃) (JCPDS 29-0041), the most likely phases during hydration of CA.

Figure 6 shows the SEM micrographs of the surface of composites after soaking in SBF. For conventional CPC (Figure 6(A)), a superficial layer of CDHA with a globular shape similar to some bioactive materials was deposited within 14 days [23].

Some bacterial contamination by Bacillis and Cocci colonies, represented by spherical and rod-shaped holes, were also observed [24].

For 5CA and 10CA (Figures 6(B) and (C)) small round shaped particles, spherulites-like cristals, of hydroxyapatite, were beginning to deposit on top of the leaf-like intermediary structure since the early stages (about 1day of soaking). Evidence of the formation of a new product containing phosphorus was formed on the surface of composites was confirmed by EDS analysis (Figure 7).

Figure 8 shows the compressive strength and porosity of a-TCP and a-TCP/CA composites before and after