Fig. 1 shows the TG-DSC of the starting reagents at the specific molar ratio of 2:1. Four endothermal events can be identified during heating. The two first are associated to the reagents decomposition, first CaHPO4 produces calcium pyrophosphate (Ca2P2O7) at 439°C and second CaCO3 produces CaO at 776°C. These two products will react in the region between 780 and 1100°C to produce β-TCP as follows: CaO+Ca2P2O7→β-Ca3(PO4)2, however such event is not identified in the DSC, since it is a process with low interchange of energy and is under the sensitivity of the technique and then, thermal mechanical analysis (TMA) is recommended instead [26]. The third event occurs at 1288°C and is attributed to the β→α phase transformation, crystalline network changes from rhombohedral to monoclinic. Then, the fourth event at 1460°C where α→α′ is presented, here a hexagonal cell distribution appears. However, during the cooling process only one exothermic peak is visible, which is related to the reconversion of α′→α at 1440°C. Both heating and cooling processes were performed at 10°C/min and no reconversion of α→β is identified, it can be confirmed that cooling at such rate inside a furnace with an air atmosphere will not lead TCP to the non-desired β phase. The total mass loss of 19.8% is associated to water evaporation (0.2wt.%) and to CaHPO4 and CaCO3 decompositions (4.3 and 15.3wt.% respectively). Further phase transformation events are not related with mass loss as only crystalline network is changing. The thermal processing of the powder mixture promotes a minor sintering of particles but no melting occurs in the analyzed temperature range. Table 1 summarizes all identified thermal events in the TG-DSC assay.

Fig. 1.

TG-DSC curve of the starting reagents mixture (2CaHPO4:CaCO3). DSC during heating (red line) and cooling (blue line). Mass loss (black dash line).

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Some researchers have demonstrated that impurities in the starting reagents as magnesium (Mg) and strontium (Sr) are β stabilizers and silicon (Si) is α stabilizer at high temperatures processing. XRF shown in Table 2 indicates that the chemical composition of the reagents was mainly P2O5 and CaO. The low contents of MgO (0.13wt.%), SrO (0.02wt.%) and SiO2 (0.01wt.%) cannot be considered as phase stabilizers impurities as values are below the ones established by others researchers [13,15,23,28,29]. TG-DSC and chemical content corroborate that quenching is not needed when α-TCP is the only phase desired via thermal processing. As stated by Monma and Goto [30], high energy is required to promote the β→α phase transformation and such energy is also required for reconversion, meaning that the cooling rate of 10°C/min is fast enough to avoid high content of β-TCP in the final desired phosphate.

Particle size distribution (PSD) of the three TCP samples is shown in Fig. 2. It is clear that all TCPs present a bimodal and similar distribution. In Fig. 2a, TCP-F shows a PSD of d10: 1.5μm, d50: 9.1μm, d90: 26.2μm; while the PSD for TCP-A shows in Fig. 2b present a slightly lower amount of big particles being d10: 1.6μm, d50: 8.5μm, d90: 22.4μm and Fig. 2c shows the PSD for TCP-S, which present d10: 2.0μm, d50: 10.4μm and d90: 24.7μm. These similar PSD indicate that the milling and sieving process affected equally all TCPs. As shown in several works, particle size distribution affects reactivity of TCP and further the microstructure of the derived CPC [16–18,31–33], such d50 ranging 8.5 and 10.4μm will lead to plate like CDHA crystals [34], and further no reactivity differences will occur due to the similar particle size distribution.

Fig. 2.

Particle size distributions (columns) of (a) TCP-F, (b) TCP-A and (c) TCP-S, cumulative volume percentage (dash line) and its respective d10, d50 and d90 identification.

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Elemental composition and Ca/P ratio of the three TCPs samples is presented in Table 3, indicating similar Ca/P ratio around 1.5, which is a characteristic value of pure TCP and is the appropriate one to promote CDHA (Ca/P=1.5) precipitation after conversion of TCP [35]. All TCPs present mainly CaO and P2O5 with a sum of weight percentage (wt.%) higher than 98%. The remaining oxides are as follows, TCP-F (SiO2: 0.58; Al2O3: 0.21; Fe2O3: 0.15; MgO: 0.1, ZrO2: 0.16wt.%, other oxides below 0.1wt.%), TCP-A (Al2O3: 0.44; Na2O: 0.15wt.%, other oxides below 0.1wt.%) and TCP-F (SiO2: 0.15; Al2O3: 0.29; Na2O: 0.13; ZrO2: 0.3wt.%, other oxides below 0.1wt.%).

FTIR-DRIFT TCPs spectra shown in Fig. 3 present the characteristics bands of the PO4− ion. The most intense bands are obtained at 1058, 1013 and 984cm−1 for the antisymmetric PO stretching bond (ν3), at 954cm−1 for the symmetric PO stretching bond (ν1) and a doublet at 560–590cm−1 for the antisymmetric PO bending bond (ν4) [11]. The main difference is seen in the low wavenumber doublet, where greater distortion is observed as the cooling method increases. The distortion of the phosphate ion is attributed to a lower order of crystallinity in TCPs [36,37], which is in consistent as a faster cooling method will lead to a less crystalline material.

Fig. 3.

FTIR spectra of the TCPs cooled by the three different methods.

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Fig. 4a shows the X-ray diffractogram of the TCPs samples obtained by the three cooling methods, where identified peaks are mainly related to α-TCP and only one peak is related to β-TCP (2θ: 27.8°) in accordance with patterns JCPD 9-348 and JCPD 9-169 respectively. This result indicates that the alpha phase is highly obtained in the three TCPs and that even by cooling the TCP at a low rate of 10°C/min will lead to the desired CPC precursor. This result is in accordance with those described by others authors [9,12,13,21], which indicate that in specific conditions where beta stabilizers as magnesium or strontium oxides are not present in the reagents a low cooling rate is enough to avoid the phase transformation from the alpha to the beta TCP. However, the only identified difference among the TCPs is a broadening in the base of the peaks as the cooling method is faster. Hence, the TCP cooled inside the furnace presents the sharpest peaks; such phenomenon is attributed to a difference in crystallinity, and agrees with the result obtained by FTIR, confirming that the faster the cooling method, the less crystalline the TCP is.

Fig. 4.

XRD of a) the three TCPs samples (left column) and b) its derived CPCs (right column). Phases identified by patterns JCPD 9-432 (CDHA), 9-348 (α-TCP) and 9-169 (β-TCP).

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Fig. 4b shows XRD spectra of the TCP derived CPCs after 7 days of setting at 37°C in Ringer's solution. It has been identified that the final CPCs present peaks associated to CDHA, some to α-TCP and to traces of β-TCP by comparison with XRD patterns JCPD 9-432, 9-348 and 9-169 respectively. As the setting process starts with α-TCP dissolution and followed by CDHA precipitation, it is clear that α-TCP conversion in CPC-F and CPC-S is not culminated after 7 days of setting as given that principal peak at 31.7° (2θ) is still expressed. In CPC-A it is notorious that almost full conversion was obtained. As some β-TCP was detected in all TCPs previous to the cements preparation (Fig. 4a), such phase outstands more in the CPCs as its solubility is much lower than α-TCP's one.

Table 4 shows the phases quantification of each CPC sample. CPC-F and CPC-S have the lowest amounts of CDHA (81.4 and 84.8wt.% respectively), whilst CPC-A has the highest (91.7wt.%). This indicates that TCP-A medium crystallinity in comparison with the other two TCPs is the most appropriated to favor the α-TCP dissolution and the subsequent CDHA precipitation. However, it was expected that less crystalline TCP showed higher reactivity of α-TCP and a higher amount of CDHA in the final CPC, as some works state that amorphous CaPs are highly reactive [37,38].

The FTIR spectra shown in Fig. 5 exhibits the three samples TCP derived CPC. It is clear that no differences among the CPCs is presented, indicating that the CPCs are similar in terms of chemical bonds; PO43− is identified by 569, 590, 962cm−1 bands and a broad band between 1000 and 1100cm−1. HPO42− is identified by bands at 870 and 1218cm−1. H2O and CO32− absorption are also identified by 1650 and 1993cm−1 bands respectively. OH− band at 3570cm−1 indicates that formation of hydroxyapatite is present in the CPCs. Presence of HPO42− bands and the absorption of H2O and CO32− on CPCs is common when the setting is performed by immersion on Ringer's solution, indicating that the hydroxyapatite obtained is a calcium deficient type, CDHA [39,40].

Fig. 5.

FTIR spectra of TCP derived CPC-F, CPC-A and CPC-S samples after setting for 7 days at 37°C in Ringer's solution.

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The differences between the FTIR spectra of TCPs (Fig. 3) with its respective CPCs (Fig. 5) show that conversion of the starting phosphate is occurring in favorable way to promote CDHA precipitation.

In Fig. 6, the SEM images show the morphology of the three CPCs samples in their fracture surfaces produced by compression tests, where at low magnification (500×) CPC-F and CPC-S have similar topography and cements seem highly dense. However, in CPC-A it is clear that higher porosity was obtained, which may explain why CDHA content was higher, since water could enter more easily inside the sample and promote higher dissolution of the starting α-TCP. At higher magnification (10,000×) characteristic CDHA fine plate like crystals may be observed in all cements. More ordered crystals are present in CPC-F and more disordered on CPC-A and CPC-F, perhaps due to the higher reactivity of their TCPs precursors. Despite these low morphological differences, CPCs maximum compressive strengths (Fig. 7) evidence no statistical difference among them three. However, average values seem to decrease as CPC is derived from faster cooled TCP; CPC-F (18±6MPa), CPC-A (15±6MPa) and CPC-S (11±6MPa), which would be explained by the higher ordered CDHA that implies greater crystals entanglement and then, higher mechanical resistance of the CPC [41–44].

Fig. 6.

SEM images of CPC-F, CPC-A and CPC-S fracture surfaces. Large images at 500× (scale bar 50μm), inserted images at 10,000× (scale bar 1μm).

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Fig. 7.

Compressive strength of the three CPCs samples after 7 days setting in Ringer's solution at 37°C.

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