Cement consistency

Cements are commonly made by heating a mixture of limestone and clay to about 1700 C. The product is ground with gypsum. Chemically cements consist of a mixture of calcium silicates and aluminates with some sulphate present. World production 1976 730 megatonnes. 

The compressive strength of polycarboxylate cements at cementing consistency is 55—85 MPa (8,000—12,000 psi). Typical diametral tensile strength ranges from 8—12 MPa (1160 1740 psi). The solubiHty and disintegration in distilled water after 7 days at 37°C is 0.04—0.08 wt %, and is not reflected in clinical performance. 

Rubber latex cement Rubber latex cement consists of mixtures of sand and other fillers which are gauged with rubber latex solution. These cements are suitable for dilute acid conditions and are particularly useful in conditions where dilute acid alternates with water or dilute alkalis. They remain very slightly resilient and adhere very well to stoneware. They are not of course... 

Fully hardened cements have brittle characteristics (Williams Smith, 1971 Skibell Shannon, 1973) and show little creep under load (Wilson Lewis, 1980). When mixed to a luting (cementation) consistency, their compressive strength reaches 70 to 131 MPa after 24 hours (Wilson, 1975b) depending on brand (Table 6.4). There is little subsequent increase in strength (Paffenbarger, Sweeney Isaacs, 1933 Smith, 1977). 

In the subsequent hardening phase, precipitation and hydration continue. The set cement consists, essentially, of partly-reacted glass particles embedded in an aluminium phosphate gel. The morphology of the filler particles is one where a glass core is sheathed by silica gel. 

The set cement consists of zinc oxide particles bonded together by a loose matrix of zinc eugenolate (Wilson, Clinton Miller, 1973). Electron-micrographs show that the zinc oxide particles are covered by zinc eugenolate (Figure 9.5a). 

Antifreezing agents for cement consist mainly of salts such as calcium chloride, magnesium chloride, sodium chloride, and soda. Calcium chloride is highly corrosive and very restricted in use. Some salts, especially potassium chloride, will affect the curing time of cement. The latter chemical is in fact used to increase the pot life of cement. Likewise, alcohol freezing-point depressants, such as ethylene glycol, can be also included in the composition [1022]. 

Common (dry) cement consists of anhydrous crystalline calcium silicates (the major ones being tricalcium silicate, CasSiOs, and (3-dicalcium silicate, Ca2Si04), lime (CaO, 60%), and alumina (a complex aluminum silicate, 5%). While cement is widely used and has been studied in good detail, its structure and the process whereby it is formed are not completely known. This is due to at least two factors. First, its three-dimensional arrangement of various... 

Cementation consists in the formation of a surface alloy with a less reactive metal. For zinc, alloying can be effected with mercury (amalgamation), copper, silver, nickel.12 The reactivity of a cemented metal can be explained considering that the supporting metal plays only the role of an electron reservoir, the true chemistry is effected by the superficial additional metal. Zinc can also be activated by washing with aqueous ammonium chloride,13 or by reacting the powder with trimethyl-chlorosilane.14 In this latter case, Barbier reactions were effected even at 0°C in short times. The activation mechanism was not determined. 

Wilson et al. [25] analyzed various brands of commercial cements and specified their possible composition, properties, and microstructure. Wilson et al. report the most representative and comprehensive data on commercial porcelain dental cements. These cements consist of powdered alumina-lime-silica glass mixed with phosphoric acid that formed a hard and translucent product. The starter glass powder consists of 31.5-41.6 wt% silica, 27.2-29.1 wt% alumina, 7.7-9.0wt% calcium oxide, 7.7-11.2 wt% sodium oxide, 13.3-22 wt% fluorine and small amounts of phosphorous and zinc oxides. Often very small amounts of magnesium and strontium oxides are also present. 

Alumina cement consists of calcium oxide and aluminum oxide compounds (see Fig. 5.3-1), mainly CA, CA2 and C12A7 (the abbreviations are explained under Table 5.3-6). 

Alumina cement is used in refractory pastes at temperatures up to 1700°C. Rapid setting cement consists of mixtures of alumina cement and Portland cement (used e.g. in sealing water leaks). 

Two other types of operations involve cement slurries. They are used to plug back a well from a deeper to a shallower depth for a number of reasons abandonment, whip stocking, lost circulation, or shutting olf water. Squeeze cementing consists of forcing a column of cement slurry... 

Aluminous cement consists predominantly of calcium aluminate. 

A typical two package polyester cement consists of an unsaturated polyester (41), cellulose acetate butyrate or polyvinyl acetate (1.8) and N,N-dimethylaniline (1) dissolved in styrene (18). This liquid (binder) is intimately mixed with a filler consisting of graded silica or solid glass spheres (15.5), talc (23), titanium dioxide (1.0) and benzoyl peroxide (1.0). 

---