Ceramics Based on Sodium Rhenanite CaNaPO₄, Obtained via Firing of Composite Cement-Salt Stone

Abstract

Ceramics based on rhenanite CaNaPO₄ with density of 0.94 g/cm³ and compressive strength of 10.3 MPa was obtained via firing at 900 °C of composite cement-salt stone prepared from a hardening powder mixture of calcium citrate tetrahydrate Ca₃(C₆H₅O₇)₂∙4H₂O and sodium dihydrogen phosphate NaH₂PO₄. The phase composition of the obtained samples of cement–salt stone was represented by monetite CaHPO₄, unreacted sodium dihydrogen phosphate and calcium citrate tetrahydrate. According to the XRD data, the phase composition of the ceramic samples after annealing in the temperature range of 500–700 °C was mainly represented by the β-CaNaPO₄ phase. It was found that after an annealing at temperature of 900 °C, the phase composition of ceramics was presented with the only phase of β-CaNaPO₄. It was demonstrated that an increase in the annealing temperature led to an increase in the grain size from 1 μm after annealing at 500 °C to 5 μm after annealing at 900 °C. Obtained ceramic material based on CaNaPO₄ could be important for regenerative treatments of bone tissue defects.

1. Introduction

One of the important areas of modern inorganic materials science is the development of biomaterials based on calcium phosphates that could be used to replace or treat damaged bone tissue [1,2]. Ideally, the implant should gradually dissolve in the body’s environment, while performing its supporting functions, and new bone tissue should form in its place. In this regard, the key characteristic of the material is its ability to resorb in the body’s environment. The traditionally used hydroxyapatite (HA), Ca₁₀(PO₄)₆(OH)₂, has the lowest solubility among calcium phosphates [3]. In the case of a regenerative approach to the treatment of bone tissue, bioresorbable phases are introduced into the composition of materials for bone implants which, compared to HA, have greater resorption, namely tricalcium phosphate β–Ca₃(PO₄)₂ (Ca/P = 1.5) [4,5,6], calcium pyrophosphate Ca₂P₂O₇ (Ca/P = 1) [7,8,9], tromelite Ca₄P₆O₁₉ (Ca/P = 0.66) [10,11], calcium polyphosphate Ca(PO₃)₂ (Ca/P = 0.5) [12], Na-substituted tricalcium phosphate Ca₁₀Na(PO₄)₇, K-substituted tricalcium phosphate Ca₁₀K(PO₄)₇, sodium rhenanite CaNaPO₄, and potassium rhenanite CaKPO₄ [13,14]. A necessary element of any strategy for improving the solubility of a compound with an ionic nature of the chemical bond is lowering the energy of the crystal lattice. Consistent implementation of this approach leads to two directions of increasing the resorption of calcium phosphate materials, as follows: (a) transition to calcium phosphates with a Ca/P ratio lower than that of HA; (b) modification of the chemical composition associated with the replacement of the Ca²⁺ cation in the phosphate structure.

Thus, ceramics based on rhenanite CaMPO₄, where (M = Na or K) are of great interest for different biomedical applications [15,16].

A known method for producing rhenanite CaNaPO₄, is the crystallization of glass in the system SiO₂-Al₂O₃-Na₂O-K₂O-P₂O₅-F [17] or in SiO₂-CaO-Na₂O-P₂O₅-F-K₂O [18]. However, this method does not allow to obtain rhenanite CaNaPO₄ as the only phase.

There is a method of processing phosphate ores using sodium carbonate or potassium chloride at temperatures of 300–900 °C, when the formation of rhenanite CaNaPO₄ occurs only when the phosphate stone interacts with sodium carbonate [19]. This method is not suitable for the synthesis of rhenanite CaNaPO₄ for medical uses.

It is well-known that rhenanite CaNaPO₄ powder can be synthesized by heating a mixture of Na₂CO₃ and Ca₂P₂O₇ at 1000 °C for 10 h [20]. The solid-phase synthesis of rhenanite CaNaPO₄ carried out in this way, as is typical for any solid-phase synthesis, gives a powder with low sintering activity which will require higher temperatures to form ceramics based on it.

The ref. [21] shows a method for obtaining a material based on rhenanite CaNaPO₄ from a charge containing sodium salt (sodium bicarbonate NaHCO₃) and calcium phosphate – monetite CaHPO₄. This process includes pressing the initial charge and firing at 1300 °C for 16 h. The main disadvantages of this method are the high temperature of the reaction and duration of the synthesis.

Rhenanites are very widely used to obtain phosphate fertilizers. Here, the ‘’Rhenania process’’ should be mentioned, which is a well-known procedure used in the fertilizer industry to obtain soluble phosphate materials [22]. In this process, the natural mineral fluorapatite Ca₅(PO₄)₃F is mixed with soda Na₂CO₃ and silicon dioxide SiO₂ while the molar ratio of Na₂CO₃ /P₂O₅ is fixed at 1.0 (Equation (1)). The SiO₂ is added to prevent the occurrence of free CaO in the sintered product. These powder mixtures are then crushed and calcined in a rotary kiln at around 1000–1200 °C for several hours. Rhenanite, a highly soluble CaNaPO₄, is the major phase in the final product of the Rhenania process [22].

Ca₅(PO₄)₃F + 2Na₂CO₃ + SiO₂ = 3CaNaPO₄ + Ca₂SiO₄ + 2CO₂ + NaF.

(1)

Furthermore, CaNaPO₄ can also be obtained via firing a mixture of CaO, H₃PO₄ and Na₂CO₃ [22] at 1100 °C. Other starting components can be used for rhenanite CaNaPO₄ preparation (Equation (2)):

2CaCO₃ + Na₂CO₃ + 2(NH₄)₂HPO₄ → 2CaNaPO₄ + 3CO₂↑+ 4NH₃↑+ 3H₂O (t ~ 900 °C)

(2)

Alternatively, sodium rhenanite CaNaPO₄ can be obtained by cement technology with subsequent firing. The work [23] shows a method for obtaining CaNaPO₄ from brushite cement, prepared by the reaction of β-tricalcium phosphate, monocalcium phosphate monohydrate (Equation (3)), and a highly alkaline bioactive glass (composition (wt.%): SiO₂-50, Na₂O-25, CaO-20, P₂O₅–5).

Ca₃(PO₄)₂ + Ca(H₂PO₄)₂∙H₂O + 7H₂O → 4CaHPO₄∙2H₂O

(3)

After mixing brushite with bioactive glass, firing was carried out at high temperatures. With an increase in the temperature to 700–800 degrees, CaNaPO₄, and β-Ca₃(PO₄)₂ phases were obtained, which indicates that the transition of bioactive glass to a viscoplastic state has occurred. The formation of the CaNaPO₄ phase occurs due to thermochemical interactions between Na₂O and CaO from the glass matrix and β-Ca₂P₂O₇.

Therefore, the aim of this work was to obtain bioresorbable ceramics based on rhenanite CaNaPO₄ by firing of a composite cement-salt stone prepared from powder mixture of calcium citrate Ca₃(C₆H₅O₇)₂∙4H₂O and sodium dihydrogen phosphate NaH₂PO₄.

2. Materials and Methods

2.1. Initial Reagents and Synthesis

Calcium citrate tetrahydrate Ca₃(C₆H₅O₇)₂·4H₂O (CAS No. 5785-44-4, puriss. p.a. ≥ 85%) and sodium dihydrogen phosphate NaH₂PO₄ (CAS No. 7558-80-7, puriss. ≥ 99%) were purchased from Sigma Aldrich.

2.2. Preparation of the Sodium Rhenanite Ceramics

The following Equation (4) was used to calculate the composition of the powder mixture:

Ca₃(C₆H₅O₇)₂∙4H₂O + 3NaH₂PO₄ → 3CaNaPO₄ + 2H₃C₆H₅O₇ + 4H₂O

(4)

The initial mixture, consisted of powders of calcium citrate tetrahydrate Ca₃(C₆H₅O₇)₂·4H₂O and sodium dihydrogen phosphate NaH₂PO_4, were used in a molar ratio corresponding to Equation (4), which were previously homogenized in a planetary mill in an acetone medium for 15 min. The resulting powder mixture was mixed with water at a water/solid ratio (W/T) = 0.5. The resulting paste dough was mixed in a porcelain bowl for 30 s. The latex mold with sizes of 10 × 10 × 30 mm was filled with prepared paste and left to harden in air for a day. The cement-salt stone samples formed as a result of hardening was fired for 2 h in the range of 500–900 °C.

2.3. Characterization

2.3.1. XRD

Here, X-Ray diffraction (XRD) analysis was conducted using a Rigaku D/Max-2500 (Rigaku, Tokyo, Japan) with a rotating anode (Cu-Ka radiation), with an angle interval (2Ѳ) of 2–70°. Phase analysis was performed using the ICDD PDF2 database [24].

2.3.2. SEM

The microstructure of the ceramic materials was studied using a LEO SUPRA 50VP (Carl Zeiss, Jena, Germany) scanning electron microscope (SEM) with an acceleration voltage of 21 kV. The images were recorded using an Everhart–Thornley secondary electron detector (SE2). The SEM specimens were prepared by depositing a small amount of the samples onto an aluminum substrate followed by coating the surface with a chromium layer to avoid the charging effects. The surface of the cement-stone and ceramic samples was coated with a layer of chromium (up to 15 nm).

2.3.3. Thermal Analysis

Thermal analysis (TA) was carried out using a NETZSCH STA 409 PC Luxx thermal analyzer (NETZSCH, Selb, Germany), in the temperature range of 40–1000 °C. The composition of the gas phase formed upon decomposition of samples was studied using a QMS 403C Aëolos quadrupole mass spectrometer (NETZSCH, Germany) coupled to the NETZSCH STA 409 PC Luxx thermal analyzer. Mass spectra (MS) were recorded for m/Z = 18 (H₂O), as well as for m/Z = 44 (CO₂).

2.3.4. Determination of Strength Properties

The bending and compressive strengths of ceramic samples in form of balks were determined using universal testing machines LFV 10-T50 (Switzerland) and P-05, respectively.

3. Results and Discussion

The XRD patterns of the initial components and cement-salt stone based on calcium citrate and sodium dihydrogen phosphate are shown in Figure 1.

The Equation (5) reflects the reaction of phase composition of cement-salt stone formation:

Ca₃(C₆H₅O₇)₂∙4H₂O + 3NaH₂PO₄ → 3CaHPO₄∙+ Na₂HC₆H₅O₇ + NaH₂C₆H₅O₇+ 4H₂O

(5)

According to the XRD data (Figure 1), the main phases in the cement-salt stone based on calcium citrate tetrahydrate and sodium dihydrogen phosphate are monetite CaHPO₄, unreacted sodium dihydrogen phosphate, and calcium citrate tetrahydrate. The presence of unreacted components shows that the reaction has not been fully completed in the conditions given here. There are also peaks that probably correspond to the acidic calcium citrate salts, such as Na₂HC₆H₅O₇ and NaH₂C₆H₅O₇.

The microstructure studies of the cement-salt stone support the results of the XRD data. The micrograph of the sample demonstrated in Figure 2 shows small crystals of monetite CaHPO₄ with plate-like morphology and particles of sodium dihydrogen phosphate NaH₂PO₄ of spheric shapes. The CaHPO₄ crystals are most likely formed on the surface of the less soluble calcium citrate. The CaHPO₄ crystals have a size of less than 2 μm due to the action of C₆H₅O₇³⁻ which slows down the reaction and inhibits the growth of calcium hydrogen phosphate crystals.

According to the simultaneous thermal analysis (Figure 3), the total weight loss of the powder mixture when heated to 1000 °C was 44 %.

Three peaks can be observed on the mass spectrum curve for m/Z = 18 (H₂O) in the range of 50–300 °C. In this temperature range, thermal decomposition of calcium citrate tetrahydrate Ca₃(C₆H₅O₇)₂.4H₂O with the formation of anhydrous calcium citrate Ca₃(C₆H₅O₇)₂ is possible. On the mass spectrum curve for m/Z = 44, there is a peak in the range of 435–495 °C, reflecting the release of CO₂. Thermal decomposition of anhydrous calcium citrate Ca₃(C₆H₅O₇)₂ with the formation of calcium carbonate CaCO₃ occurs with heat release according to the following Equation (6):

Ca₃(C₆H₅O₇)₂→3CaCO₃ + 5H₂O + 9C

(6)

The carbon formed as a result of this transformation (Equation (6)) could not remain in elemental form at such a high temperature (above 340 °C), especially in the presence of atmospheric oxygen. Therefore, it must have turned into CO and/or CO₂. In the temperature range of 450–650 °C, the following process is observed: calcium carbonate CaCO₃ transforms into calcium oxide and CO₂, as follows (Equation (7)):

CaCO₃ → CaO + CO₂

(7)

During the heat treatment, the products obtained during the acid-base reaction thermally decomposed, and products of thermal decomposition interacted with each other to form ceramics. The processes taking place during heating can be described by the following equations:

2CaHPO₄ → Ca₂P₂O₇ + H₂O

(8)

Na₂HC₆H₅O₇ → Na₂CO₃ + C₅H₄O₃ + H₂O

(9)

NaH₂PO₄ → NaPO₃ + H₂O

(10)

4CaCO₃ + 6CaHPO₄ → Ca₁₀(PO₄)₆(OH)₂ + 4CO₂↑ + 2H₂O

(11)

Ca₁₀(PO₄)₆(OH)₂ + NaPO₃ → CaNaPO₄ + 3Ca₃(PO₄)₂ + H₂O

(12)

Na₂CO₃ + Ca₂P₂O₇ → 2CaNaPO₄ + CO₂↑

(13)

CaCO₃ + Ca₂P₂O₇ → Ca₃(PO₄)₂ + CO₂↑

(14)

2NaPO₃ + CaCO₃ → CaNa₂P₂O₇ + CO₂↑

(15)

Ca₃(PO₄)₂ + CaNa₂P₂O₇ + Na₂CO₃ → 4CaNaPO₄ + CO₂↑

(16)

NaPO₃ + CaCO₃ → CaNaPO₄ + CO₂↑

(17)

During the heat treatment of cement-salt stone at temperatures of 500 and 700 °C (Figure 4), in addition to the target phase β-CaNaPO₄, hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ was formed. This phase was formed due to the interaction of monetite CaHPO₄ with calcium carbonate CaCO₃ (Equation (11)) which was the product of the calcium citrate Ca₃(C₆H₅O₇)₂ decomposition (Equation (6)). At 700 °C, in addition to β-CaNaPO₄ and Ca₁₀(PO₄)₆(OH)₂, phases of double calcium-sodium pyrophosphate CaNa₂P₂O₇ and β-Ca₃(PO₄)₂ phases were formed. The formation of the CaNa₂P₂O₇ phase was due to the interaction of the NaPO₃ melt with calcium oxide CaCO₃ (Equation (15)). The β-Ca₃(PO₄)₂ phase was formed as a result of the interaction of CaCO₃ with Ca₂P₂O₇ (Equation (14)). At 900 °C, only the target phase β-CaNaPO₄ was found.

The geometric density of ceramic materials is shown in Figure 5. After firing at 500 °C, the density of ceramic samples was 0.56 g/cm³. The density of ceramics has decreased compared to the density of cement-salt stone. The decrease in the density of the samples was due to a decrease in the mass of the sample, because of the decomposition of the components of the cement-salt stone during heating (Figure 6). With an increase in firing temperature from 700 °C to 900 °C, the ceramic density increased from 0.68 g/cm³ to 0.94 g/cm³ or from 21.8 % to 30.2% relatively to the density of β-CaNaPO₄ equal to 3.11 g/cm³ (Figure 7). The shrinkage of the samples was 2.7% and 17.8% at 500 °C and 900 °C, respectively.

The SEM images of the samples shown in Figure 8 clearly demonstrate that Na-rhenanite crystals grow from 1 to 5 μm substantially as the firing temperature increases from 700 °C to 900 °C.

Figure 9 shows the temperature dependence of compressive and bending strengths of ceramic materials. The compressive strength of ceramic samples (Figure 9) increase from 3.5 to 10.3 MPa with increasing temperature from 500 °C to 900 °C, This compressive strength increasement is associated with the process of liquid-phase sintering, leading to the formation of more durable contacts between grains.

4. Conclusions

In the present work, an approach to obtaining bioresorbable ceramics with a phase composition represented by β-CaNaPO4 was described. This approach involved the preparation of a powder mixture with a given molar ratio of Na:Ca:P = 1, which was capable of entering into a chemical reaction; molding samples of cement-salt stone; and firing samples of cement-salt stone to obtain ceramics.

Samples of cement-salt stone were prepared from a powder mixture with a molar ratio of Na:Ca:P = 1, including calcium citrate tetrahydrate Ca₃(C₆H₅O₇)₂^∙4H₂O and sodium dihydrogen phosphate NaH₂PO₄. The phase composition of cement-salt stone samples based on Ca₃(C₆H₅O₇)₂^∙4H₂O and NaH₂PO₄ was represented mainly by monetite CaHPO₄, as well as unreacted NaH₂PO₄ and Ca₃(C₆H₅O₇)₂^∙4H₂O. Heat treatment of the obtained cement-salt stone at a temperature of 500°C led to the formation of β-CaNaPO₄ and Ca₁₀(PO₄)₆(OH)₂ phases. At 700 °C, in addition to β-CaNaPO₄ and Ca₁₀(PO₄)₆(OH)₂, the phases of double calcium-sodium pyrophosphate Na₂CaP₂O₇ and β-Ca₃(PO₄)₂ were formed. According to the XRD data, after firing at 900°C, the resulting ceramics contained the only β-CaNaPO₄ phase. It was shown that, as the temperature increased, the shrinkage and density of ceramic samples increased. Thus, ceramic material with density of 0.94 g/cm³ developed here consisting of biocompatible and bioresorbable β-CaNaPO₄ phase can be used in regenerative methods for the treatment of bone tissue defects.