**10.2. Oil-well cement**

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

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

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].

A variety of cements are used in modern clinical dentistry, such as glass ionomers, zinc phosphate and zinc polycarboxylate [38],[39]. Dental zinc phosphate cement is primarily used forthe cementation ofindirectrestorations, such as crown and bridges. It has the longestrecord of any cement, approximate 100 years, and has remained popular throughout this time. Zinc phosphate cements are also considered the strongest among the dental cements. However, it is also applied for temporary fillings, cavity bases and buildings of teeth beneath crowns. Zinc phosphate cement is primarily in contact with the pulp-dentin system and in certain cases (e.g. temporary fillings) with the gingiva. A variety of cementing materials are currently used

Phosphoric acid-based cements originated from OSTERMANN´S formula from 1832, which was composed of calcium oxide and anhydrous phosphoric acid. In 1902, FLECK established a

The powder is mainly a mixture of zinc oxide and up to 13% magnesium oxide. The liquid is an aqueous solution of phosphoric acid containing 38 – 59% H3PO4, 30 – 55% water and 0 – 10% zinc. Aluminum is essential to the cement-forming reaction, and zinc moderates the reaction between powder and liquid, allowing adequate working time and sufficient quanti‐ ty of powder to be added for optimum properties of cement. When the powder is mixed with liquid, phosphoric acid attacks the surface of particles, dissolving zinc oxide, which releases zinc ions into the solution. Aluminum in the liquid reacts with phosphoric acid to form zinc aluminophosphate gel on remaining portion of particles. Thus, the cement reveals a cored structure consisting primarily of non-reacted zinc oxide particle core embedded in a cohe‐ sive amorphous matrix of zinc aluminophosphate (glasslike phosphate). Aluminum phos‐

<sup>5</sup> The word 'luting' implies the use of a molded or moldable article to seal a space or to cement two components together

formula that is similar to that being in use today [40],[41],[42],[43],[44].

agents, but zinc phosphate cement has been used for many decades.

prepared.

**10.1.2. Dental phosphate cement**

as the bases and luting5

[44].

The main application of the cement in an oil well is to stabilize the steel casing in the bore‐ hole and to protect it from corrosion. The cement is pumped through the borehole and is pushed upwards through the annulus between the casing and the formation. The cement is exposed to the temperature and pressure gradients of the borehole [47].

Various types of oil-well cements are distinguished. The most important is the phase compo‐ sition of cement, primarily the C3A phase content, which causes quick paste thickening. The main feature of oil-well cement is that it must remain sufficiently fluid for a long period, required for its pumping to deep well. Simultaneously, the temperature in the borehole increases with depth6 [48],[49]. The types and properties of oil-well cements specified by the American Petroleum Institute (API) are introduced by Clinkers containing limestone loess, diatomite, pyritic ash and sand modified with gypsum and apatite were used for the manu‐ facture of heat resistant oil-well cements. Apatite is also a good stabilizer for high belite cements [50].


**Table 2** Types and properties of oil-well cements [49].
