Calcium phosphate-silicates solubilities

Solubilities of the Glasses of Calcium Phosphate-Silicates in Water in the Presence of Ion-Exchange Resin (36)... 

Because calcium oxide is a fairly reactive powder, it forms calcium hydroxide when in contact with water. This reaction is exothermic and hence heats water during formation of the hydroxide. Because of this excess heat, it cannot directly be used to form phosphate ceramics by reacting it with an acid phosphate solution and must be used in a less soluble form as sparsely soluble silicate or hydrophosphate. In spite of this difficulty, because human bones contain calcium phosphate, there have been sufficient efforts in developing methods of forming biocompatible CBPCs of calcium phosphate by using partially soluble phosphates of calcium rather than using oxide itself. A similar approach may also be taken if one uses partially soluble silicate or aluminate of calcium. These routes are discussed in Chapter 13. 

Since calcium oxide is more than sparsely soluble and its reaction with phosphoric acid or a soluble phosphate is highly exothermic, researchers have used less soluble salts of calcium to react with the phosphates and form a phosphate ceramic [4-12]. In the acidic medium of the phosphate solutions, the salts of calcium dissolve slowly and release Ca (aq) into the solution, which subsequently reacts with phosphate anions and forms calcium phosphates. The best calcium minerals for forming CBPCs are combination of oxides of calcium and insoluble oxides such as silica or alumina, e.g., calcium silicate (CaSi03) and calcium aluminate (CaAl204), or even a phosphate of calcium such as tetracalcium phosphate (Ca4(P04)2 0). These minerals are reacted with acid phosphate salts to form phosphate cements. 

Note in Fig. 13.2 that the solubility of monocalcium silicate is higher than that of the corresponding aluminate at any pH > 3. In the acidic region that is of interest for forming CBPCs, both may be considered as sparsely soluble, and if they are reacted with a phosphate salt, ceramics may be formed. Thus, monocalcium silicate and aluminate are starter minerals to form calcium phosphate ceramics. 

Calcium oxide is the main ingredient in conventional portland cements. Since limestone is the most abundant mineral in nature, it has been easy to produce portland cement at a low cost. The high solubility of calcium oxide makes it difficult to produce phosphate-based cements. However, calcium oxide can be converted to compounds such as silicates, aluminates, or even hydrophosphates, which then can be used in an acid-base reaction with phosphate, forming CBPCs. The cost of phosphates and conversion to the correct mineral forms add to the manufacturing cost, and hence calcium phosphate cements are more expensive than conventional cements. For this reason, their use has been largely limited to dental and other biomedical applications. Calcium phosphate cements have found application as structural materials, but only when wollastonite is used as an admixture in magnesium phosphate cements. Because calcium phosphates are also bone minerals, they are indispensable in biomaterial applications and hence form a class of useful CBPCs that cannot be substituted by any other. 

This group includes materials such as various clay minerals including kaolinites and montmorillonites soluble sulfates and sulfuric acid sodium silicates calcium aluminate cements soluble phosphates, polyphosphates, and phosphoric acid and borates and boric acid. 

The setting reaction of dental silicate cement was not understood until 1970. An early opinion, that of Steenbock (quoted by Voelker, 1916a,b), was that setting was due to the formation of calcium and aluminium phosphates. Later, Ray (1934) attributed setting to the gelation of silicic acid, and this became the received opinion (Skinner Phillips, 1960). Wilson Batchelor (1968) disagreed and concluded from a study of the acid solubility that the dental silicate cement matrix could not be composed of silica gel but instead could be a silico-phosphate gel. However, infrared spectroscopy failed to detect the presence of P-O-Si and P-O-P bonds (Wilson Mesley, 1968). 

Fused Calcium Magnesium Phosphate. In a process developed by TVA, a mixture of phosphate rock and olivine or serpentine (magnesium silicate) is fused in an electric furnace.21 The molten product is quenched with water and used in a finely divided state as a fertilizer. The product, a calcium magnesium phosphate (CMP) glass, contains about 20 percent P205 and 15 percent MgO. Over 90 percent of the product is soluble in citric acid. 

The solubility of basic slag has been found to increase with increasing calcium silicate. Some of the constituents other than phosphate have value on certain soils which happen to be deficient in these constituents. 

Leach-Resistant Chemicals. Insoluble Complexes. Leach-resistant fire retardants can be formed by reacting soluble salts with metal salts to form insoluble, metallic salt complexes. Sodium silicate reacted with calcium chloride formed an insoluble, hydrated calcium silicate (95). Application of a 20% diammonium phosphate solution, followed by a 20% magnesium sulfate solution, has been proposed as a ready-to-use treatment for wood roofs (96). This combination forms an insoluble magnesium ammonium phosphate and is recommended for roofs that are 5 years old or older. Test results indicate that this treatment provides increased flame-spread protection. 

Inorganic anodic and mixed Cl present water-soluble salts of ammonium, alkali (sodium, potassium), alkaline-earth (calcium, strontium, magnesium, barium) and other (zinc, lead) metals containing active anions in their molecules (nitrates, nitrites, chromates, carbonates, phosphates, molybdates, silicates). ... 

Several possible calcium and aluminium phosphates exist and they differ in their solubility in aqueous media and also in their resistance to acid attack. Among the factors determining which of these products are formed are powderiliquid ratio of the cement and concentration of the phosphoric acid solution. As a result, this material was easy to prepare in a soluble or acid-sensitive state. Incorrect metering of the powder to liquid components increased the solubility of the set cement, and leaving the bottle of phosphoric acid solution open to the air led to uptake of moisture from the atmosphere, with a corresponding reduction in acid concentration. This resulted in an increase in the proportion of more soluble metal salts in the set cement. These factors combined to make the dental silicate difficult to use in the clinic and gave the material a reputation for unreliability [8]. 

---