**10.1. Chemically bonded phosphate ceramics**

Ceramics are formed by the compaction of powders and their subsequent fusion at high to very high temperatures, ranging somewhere from ∼700° to 2000°C. Once fused, the result‐ ing ceramics are hard and dense and exhibit very good corrosion resistance [2]. Among the conventional ceramic bonds, the chemical (organic and inorganic) and hydraulic bonds were used during the preparation of ceramic materials.

Phosphate bond can be utilized for the preparation of hard and quick-setting ceramic materials, chemically bonded phosphate ceramics (PCBC) and ceramic composites [3],[4] via the reaction of metal cation with phosphate anions (**Fig. 2**). The reaction is attained by mixing a cation donor, generally an oxide (CaO [5],[6],[7],[8], MgO [2],[6],[9], ZnO [2],[8], Al2O3 [2], Fe2O3 [2],

**Fig. 2.** Potential application of chemically bonded phosphate ceramics [1].

etc.), raw materials [10],or secondary raw materials (fly ash [11] and blast furnace slag [12]) with either phosphoric acid or acid phosphate such as ammonium phosphate solution or magnesium dihydrogen phosphate (Mg(H2PO4)2·2H2O) and aluminum hydrogen phosphate (AlH3(PO4)2·H2O). Typical acid-base reaction between a metal oxide and phosphoric acid can be written as [2],[13]:

$$\text{MO}\_{\text{x}} + \text{n }\text{H}\_{3}\text{PO}\_{4} + \text{m }\text{H}\_{2}\text{O} \rightarrow \text{MH}\_{3\text{n-2x}}\text{(PO}\_{4}\text{)}\_{\text{n}} + \text{(m+x) }\text{H}\_{2}\text{O} \tag{1}$$

where *x* denotes half of valence of **M**, *n* ≥ 2*x*/3, and *m* is an arbitrary integer that decides the amount of water to be added in the reaction.

In the case of magnesium phosphate chemically bonded ceramics (MPCBC), the acido-basic **reaction 1** of orthophosporic acid with MgO yields the following products [2]:

$$\text{MgO} + 2\text{ H}\_3\text{PO}\_4 \rightarrow \text{Mg(H}\_2\text{PO}\_4\text{)}\_2 \cdot \text{H}\_2\text{O} \quad \Rightarrow \text{x} = \text{l, n} = 2, \text{ and} \text{m} = 0 \tag{2}$$

$$\begin{aligned} \text{MgO} + \text{H}\_3\text{PO}\_4 + 2 \begin{array}{c} \text{H}\_2\text{O} \rightarrow \text{Mg} \left( \text{H}\_2\text{PO}\_4 \right)\_2 \cdot 3 \text{H}\_2\text{O} \quad \Rightarrow \text{ x} = \text{l, n} = \text{l,} \\ \text{and} \text{m} = 2 \end{aligned} \tag{3}$$

$$\begin{aligned} \text{3 }\text{MgO} + 2 \text{ H}\_3\text{PO}\_4 &\to \text{Mg} \text{3(PO}\_4\text{)}\_2 + \text{3 }\text{H}\_2\text{O} \quad \Rightarrow \text{x} = \text{l, n} = \frac{2}{3}, \\\\ \text{and} \text{m} = 0 \end{aligned} \tag{4}$$

If acidic phosphate is used, the reaction proceeds as follows [2]:

**Fig. 1.** Flow chart of production of acid phosphates from phosphate ores [1].

456 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

**10.1. Chemically bonded phosphate ceramics**

used during the preparation of ceramic materials.

**Fig. 2.** Potential application of chemically bonded phosphate ceramics [1].

Ceramics are formed by the compaction of powders and their subsequent fusion at high to very high temperatures, ranging somewhere from ∼700° to 2000°C. Once fused, the result‐ ing ceramics are hard and dense and exhibit very good corrosion resistance [2]. Among the conventional ceramic bonds, the chemical (organic and inorganic) and hydraulic bonds were

Phosphate bond can be utilized for the preparation of hard and quick-setting ceramic materials, chemically bonded phosphate ceramics (PCBC) and ceramic composites [3],[4] via the reaction of metal cation with phosphate anions (**Fig. 2**). The reaction is attained by mixing a cation donor, generally an oxide (CaO [5],[6],[7],[8], MgO [2],[6],[9], ZnO [2],[8], Al2O3 [2], Fe2O3 [2],

$$\text{MgO} + \text{Mg} \left(\text{H}\_2\text{PO}\_4\right)\_2 \to 2\text{ MgHPO}\_4 + \text{H}\_2\text{O} \tag{5}$$

and similar reaction can be also written for the reaction of Al(H2PO4)3 with AlO3/2:

$$\text{2 AlO}\_{3/2} + \text{Al} \left(\text{H}\_2\text{PO}\_4\right)\_3 \rightarrow \text{3 AlPO}\_4 + \text{3 H}\_2\text{O} \tag{6}$$

Reactions **5** and **6** indicate that partially acidic phosphate salt is only an intermediate phase. The dissociation of orthophosporic acid and the pH stability range of formed ionic species are given by **Eqs. 11**–**13** in **Chapter 9**. General dissociation reaction for acidic dihydrogen phosphate and its dissociation constant in the form of *pK*dis (*pK*dis = -*log K*dis) can be written as follows:

$$\text{M(H}\_2\text{PO}\_4\text{)}\_\text{a} \leftrightarrow \text{M}^{n\ast} + \text{n } \left(\text{H}\_2\text{PO}\_4\right)^{\text{n}-}\tag{7}$$

and

$$\text{pK}\_{\text{dis}} = -\log\left(\left[\text{M}^{n\*}\right]\left[\text{H}\_2\text{PO}\_4\right]^{n-}\right) \tag{8}$$

The values of *pK*dis for KH2PO4, (NH4)H2PO4, Mg(H2PO)2·2H2O and Ca(H2PO4)2·H2O are 0.15, -0.69, 2.97 and 1.15, respectively [2].

The preparation scheme for the synthesis of aluminum phosphate binder is shown in **Fig. 3**. The evolution of chemical composition of phosphate binder with Al:P ratio of 1.4:3 with temperature is introduced by **Table 1**.

**Fig. 3.** Schematic procedure for the synthesis of aluminum phosphate binder [14].

Initially, the motivation for the development of these ceramic materials was the preparation of dental cements. Phosphate chemically bonded1 ceramics (PCBC) find recently the applica‐ tion in diverse fields (**Fig. 2**), which include structural ceramics, refractory materials [15], toxic, radioactive [16],[17],[18],[19] or asbestos-containing2 [20],[21],[22] waste management [23], oil drilling and bioceramics, pigments [1], etc. [2].

#### **10.1.1. Ceramics and refractories**

The broad term "ceramics" refers to any of a large family of materials that are usually inorganic and require high temperatures in their processing or manufacture. They are generally classified into glass, whitewares, including artware and structural ceramics, and refractories. In general,

<sup>1</sup> Chemical bonding as a means of solidification is very widely observed in nature. The formation of sedimentary rocks, such as carbonate rocks, lateritic soils and solidification of desert soils, are examples of this process [1].


**Table 1.** The alteration of composition of phosphate binder with increasing temperature [14].

( ) ( )<sup>n</sup> <sup>n</sup> M H PO M n H PO 2 4 <sup>n</sup> 2 4

458 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

dis 2 4 pK log M H PO - <sup>+</sup> = - é ù

and


temperature is introduced by **Table 1**.

**Fig. 3.** Schematic procedure for the synthesis of aluminum phosphate binder [14].

of dental cements. Phosphate chemically bonded1

radioactive [16],[17],[18],[19] or asbestos-containing2

drilling and bioceramics, pigments [1], etc. [2].

**10.1.1. Ceramics and refractories**

+ -

( [ ] ) <sup>n</sup> <sup>n</sup>

The values of *pK*dis for KH2PO4, (NH4)H2PO4, Mg(H2PO)2·2H2O and Ca(H2PO4)2·H2O are 0.15,

The preparation scheme for the synthesis of aluminum phosphate binder is shown in **Fig. 3**. The evolution of chemical composition of phosphate binder with Al:P ratio of 1.4:3 with

Initially, the motivation for the development of these ceramic materials was the preparation

tion in diverse fields (**Fig. 2**), which include structural ceramics, refractory materials [15], toxic,

The broad term "ceramics" refers to any of a large family of materials that are usually inorganic and require high temperatures in their processing or manufacture. They are generally classified into glass, whitewares, including artware and structural ceramics, and refractories. In general,

<sup>1</sup> Chemical bonding as a means of solidification is very widely observed in nature. The formation of sedimentary rocks,

such as carbonate rocks, lateritic soils and solidification of desert soils, are examples of this process [1].

« + (7)

ë û (8)

ceramics (PCBC) find recently the applica‐

[20],[21],[22] waste management [23], oil

the term "glass" refers to an amorphous solid with non-directional properties, characterized by its transparency, hardness, rigidity at ordinary temperatures and capacity for plastic working at elevated temperatures. Major commercial uses of glass include plate or "float" glass, as used for windows and windshields; glass tubing, or formed shapes, used for electric lighting envelopes; and glass containers such as tumblers and bottles. Whiteware is charac‐ terized by a crystalline matrix held together by a glassy phase and usually covered by a glazed coating. Major classifications of whiteware are ceramic tiles, glazed and unglazed, for floors, walls and external use; sanitaryware in the form of toilets and lavatories; and tableware, from earthenware to fine china [24].

Kaolin is major raw material used for the fabrication of conventional ceramics. It is obtained from the alteration of granitoid rocks3 [25]. It consists mostly of kaolinite and a small amount of impurities such as quartz, micas and other phyllosilicates. Firing of kaolinite induces numerous complex structural and microstructural transformations leading to the formation

<sup>2</sup> SEM picture of thin long fibrous structure of chrysotile (H4Mg3Si2O9 [20],[21],[22]) asbestos under the magnification of 98× (a), 5000× (b) and 10,000× (c).

of mullite and silica (cristobalite) phase. Mullite phase is characterized by some advanta‐ geous properties such as good corrosion resistance, low dilatation coefficient, good creep and thermal shock resistances, thermal stability and high strength. These advantages make this phase favorable for different applications. Orthophosphoric acid reacts with aluminum from kaolin to provide new compounds, which are Al(H2PO4)3 at room temperature and AlPO4 when heated to temperatures above 800°C [10].

Approximately 70% of all refractories used in industry are in the form of bricks, which are cast in the shapes such as straights, soaps, splits, arches, wedges, keys, skews, jambs or other special and frequently patented shapes. Most industrial refractories are composed of metal oxides or of carbon, graphite or silicon carbide. Mostrefractory bricks are shaped by combining the sizegraded refractory aggregate with a small amount of moisture and casting in a dry press. The important properties of any refractory, including its high-temperature strength, depend on its mineral makeup, the particle-size distribution of minerals and the way these materials react at high temperatures and in furnace environments. When a refractory is chosen for a particu‐ lar service, the service conditions must be considered in the design. In proper selection of a refractory, these factors, together with an economic balance, must be considered so that the refractory ultimately produces the lowest cost per unit weight of product per unit weight of refractory consumed [26].

**Fig. 4.** Cutting plane throughout the block of chemically phosphate bonded refractory material containing large grains of calcined bauxite (a) and electron microscopy (SEM) picture of ceramic body (b).

The materials must be chosen with generous safety margins in the temperature capability and the refractory construction system, whether brick, monolithic or fiber, should be suitable not only for the operating conditions but also for the type of equipment concerned and the construction conditions. Refractory concretes and castables (monolithic refractories) are especially suitable for these small burner quarls. Fibers and refractory ceramic fibers (glass,

<sup>3</sup> Granular crystalline rock consisting essentially from quartz, orthoclase-feldspar and mica. Usually is light gray, white or light in color [25].

mineral, ceramic fibers and whiskers) become an important construction as well as insula‐ tion material, although the brickwork has the longest history of the development [27].

Aluminosilicate refractories are manufactured using refractory clays, sillimanite minerals, bauxite (**Fig. 4**) and mixtures of alumina and silica sand. They will refer, somewhat arbitrari‐ ly, to common crystalline compounds with melting temperatures of at least 1500°C. The major categories of traditional refractories are fire clays, high alumina and silica. The choice of material for traditional refractory applications, as well as for advanced material applications, was and is based on balancing the cost and the performance lifetime [28].

Industries involved in steel melting and casting of special alloys are always interested in using the maintenance-free, functional refractories to achieve energy-efficient metal processing. They seek for the development of new and advanced thermal ceramics for protective ther‐ mal insulation linings and molten metal handling crucibles. Conventionally, porous to dense Al2O3, SiO2, MgO, ZrO2, SiC4 [29] and fireclay ceramic bodies were used as thermal insula‐ tion refractory liners [30]. Phosphate bond can also be utilized in manufacturing of refracto‐ ry and wear-resistant and of protective coatings on metal [14] or ceramic surfaces [31]. Intensive development of ceramic materials has increased the availability of well-character‐ ized engineering ceramics capable of the utilization over the range of temperatures and atmospheres [32].

Refractory castables can be classified according to different aspects including the content of calcium, binder source, overall chemical composition, bulk density, application method and others. Binders commonly used in monolithic products may be of various classes [33]:


of mullite and silica (cristobalite) phase. Mullite phase is characterized by some advanta‐ geous properties such as good corrosion resistance, low dilatation coefficient, good creep and thermal shock resistances, thermal stability and high strength. These advantages make this phase favorable for different applications. Orthophosphoric acid reacts with aluminum from kaolin to provide new compounds, which are Al(H2PO4)3 at room temperature and AlPO4

Approximately 70% of all refractories used in industry are in the form of bricks, which are cast in the shapes such as straights, soaps, splits, arches, wedges, keys, skews, jambs or other special and frequently patented shapes. Most industrial refractories are composed of metal oxides or of carbon, graphite or silicon carbide. Mostrefractory bricks are shaped by combining the sizegraded refractory aggregate with a small amount of moisture and casting in a dry press. The important properties of any refractory, including its high-temperature strength, depend on its mineral makeup, the particle-size distribution of minerals and the way these materials react at high temperatures and in furnace environments. When a refractory is chosen for a particu‐ lar service, the service conditions must be considered in the design. In proper selection of a refractory, these factors, together with an economic balance, must be considered so that the refractory ultimately produces the lowest cost per unit weight of product per unit weight of

**Fig. 4.** Cutting plane throughout the block of chemically phosphate bonded refractory material containing large grains

The materials must be chosen with generous safety margins in the temperature capability and the refractory construction system, whether brick, monolithic or fiber, should be suitable not only for the operating conditions but also for the type of equipment concerned and the construction conditions. Refractory concretes and castables (monolithic refractories) are especially suitable for these small burner quarls. Fibers and refractory ceramic fibers (glass,

<sup>3</sup> Granular crystalline rock consisting essentially from quartz, orthoclase-feldspar and mica. Usually is light gray, white

of calcined bauxite (a) and electron microscopy (SEM) picture of ceramic body (b).

when heated to temperatures above 800°C [10].

460 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

refractory consumed [26].

or light in color [25].

Regarding the chemical ones, the bond strength can be provided by the addition of phos‐ phates (dry or solution) or by in situ generation of phosphates (formed via the reaction with added H3PO4) in the refractory structure. The reaction of Al2O3 or Al(OH)3 with orthophos‐ phoric acid can be described by reactions [33]:

$$26\text{ H}\_3\text{PO}\_4 + \text{Al}\_2\text{O}\_3 \rightarrow 2\text{ Al} \text{(H}\_2\text{PO}\_4\text{)}\_3 + 3\text{ H}\_2\text{O}\tag{9}$$

$$\rm 3\ H\_3PO\_4 + Al(OH)\_3 \to Al(H\_2PO\_4)\_3 + 3\ H\_2O \tag{10}$$

A cold-setting refractory material was developed by HIPEDINGER et al [36] via the magnesiaphosphate reaction. A cement paste based on alumina, silica fume, magnesia and orthophos‐ phoric acid or monoaluminum phosphate was designed to form cordierite-mullite during

<sup>4</sup> It was proved that H3PO4 is effective binder for SiC [29].

heating. This cement paste set at room temperature and MgHPO4·3H2O phase (newberyite) was observed, but amorphous phases were predominant. Two exothermic effects were detected during the setting process corresponding to the acido-basic reaction of magnesia with phosphates and to the formation of bonding hydrates. At 1100°C, *c*-AlPO4 was formed by the reaction of alumina with orthophosphoric acid or monoaluminum phosphate. At 1350°C, the dominant crystalline phases were cordierite and mullite. A refractory concrete with ob‐ tained cement paste and a cordierite-mullite aggregate (scrap refractory material) was prepared.

The acid phosphate impregnation, with ozone pretreatment, improves the oxidation resist‐ ance of carbon materials (polycrystalline graphite and pitch-based carbon fiber), as shown by the weight measurement in air up to 1500°C. The impregnation involves using phosphoric acid and dissolved aluminum hydroxide in the molar ratio of 12:1 and results in rough, white and hard aluminum metaphosphate coating of the weight of about 20% of that of the carbon before the treatment. Without ozone pretreatment, the impregnation is not effective. Without aluminum hydroxide, the impregnation even degrades the oxidation resistance of carbon [37].
