Cement setting

Gerinne, n. running, flowing channel, gutter, gerinnen, v.i. coagulate, curd, curdle, congeal, jelly, gel (Cement) set. 

The setting reaction for the great majority of acid-base cements takes place in water. (The exceptions based on o-phenols are described in Chapter 9.) This reaction does not usually proceed with formation of a precipitate but rather yields a substance which entrains all of the water used to prepare the original cement paste. Water thus acts as both solvent and component in the formation of these cements. It is also one of the reaction products, being formed in the acid-base reaction as the cements set. 

The cement sets as the result of an acid-base reaction between a zinc oxide dental powder and a poly(alkenoic acid). The pH increases and an insoluble amorphous salt is formed which acts as the cement matrix. A general account of the gelation processes is given in Section 5.4. 

The zinc polycarboxylate cement sets within a few minutes of mixing and hardens rapidly. Strength is substantially developed within an hour. However, even when fully hardened the cement exhibits marked plastic behaviour. Its most important property is its ability to bond permanently to untreated dentine and enamel. 

The glass polyalkenoate cement sets rapidly within a few minutes to form a translucent body, which when young behaves like a thermoplastic material. Setting time (37 °C) recorded for cements mixed very thickly for restorative work varied from 2-75 to 4-7 minutes, and for the more thinly mixed luting agents from 4-5 to 6-25 minutes. Properties are summarized in Table 5.15. 

Komrska Satava (1970) showed that these accounts apply only to the reaction between pure zinc oxide and phosphoric acid. They found that the setting reaction was profoundly modified by the presence of aluminium ions. Crystallite formation was inhibited and the cement set to an amorphous mass. Only later (7 to 14 days) did XRD analysis reveal that the mass had crystallized directly to hopeite. Servais Cartz (1971) and Cartz, Servais Rossi (1972) confirmed the importance of aluminium. In its absence they found that the reaction produced a mass of hopeite crystallites with little mechanical strength. In its presence an amorphous matrix was formed. The amorphous matrix was stable, it did not crystallize in the bulk and hopeite crystals only grew from its surface under moist conditions. Thus, the picture grew of a surface matrix with some tendency for surface crystallization. 

Crisp et al. (1978) were able to detect the formation of crystallites both on the surface and in the bulk of the reaction product. In the absence of aluminium the reaction between zinc oxide and phosphoric add was very rapid and the cement set in less than two minutes. Hopeite was formed, within minutes, both at the surface and in the bulk of the reaction mass. It was doubted whether this mass constituted a true cement. 

The addition of STPP (1-7%) acted as a retarder and increased compressive strength (mortar II). Less heat and ammonia were evolved and the cement set more slowly in 10 minutes. The paste hardened in 30 to 60 minutes. Traces of ADP persisted for 30 minutes but no STPP was detected in the reaction products. Struvite, the main hydration product, schertelite and dittmarite all appeared within 5 minutes. Struvite continued to increase in amount as the cement aged schertelite disappeared after 3 hours and dittmarite after a week. Stercorite was found only during the first 7 hours. 

Sugama Kukacka (1983b) described cements based on magnesium oxide and a 56% aqueous solution of ammonium polyphosphate (APP). The po wder was a fine magnesium oxide that had been calcined above 1300 °C and had a surface area of 1 to 5 m g . The reaction was strongly exothermic the cements set within 3 minutes and developed an early strength of 13-8 MPa after 1 hour and over 20 MPa after 5 hours. 

Table 9.2. Effect of ZnO ignition temperature on cement setting time (Prosser Wilson, 1982)...

The liquids used were 1 1 mixtures of EBA-HV and liquid methacrylate which also contained dihydroxyethyl-p-toluidine as the accelerator. Both mono- and di-methacrylates were used. The benzoyl peroxide initiator was included in the EBA zinc oxide/silanized (1 1) glass powder. These polymer cements set 5 to 10 minutes after mixing. Since there is a substantial amount of monomer in the liquid (50%) the contribution of the polymer to the strength of the cement must be considerable. Brauer Stansbury (1984b) suggested that the two matrices, the polymer matrix and the salt matrix, may be interpenetrating but separation of the two phases is likely. 

These cements set in 3-5 to 56 minutes (at 37 °C). Infrared spectroscopy showed that as the cement set there was loss of acid carbonyl groups and OH groups associated with calcium hydroxide, and simultaneously formation of ionic carboxylate groups and hydrogen-bonded OH groups. 

An alternative electrical method that has been used in the study of glass-ionomer cements has been the measurement of dielectric properties. Tay Braden (1981, 1984) measured the resistance and capacitance of setting cements at various times from mixing. From the results obtained, relative permittivity and resistivity were calculated. In general, as these cements set, their resistivity was found to fall rapidly, then to rise again. Both these results and the results of relative permittivity measurements were consistent with the cements comprising highly ionic and polar structures. 

The senior author first became interested in acid-base cements in 1964 when he undertook to examine the deficiencies of the dental silicate cement with a view to improving performance. At that time there was much concern by both dental surgeon and patient at the failure of this aesthetic material which was used to restore front teeth. Indeed, at the time, one correspondent commenting on this problem to a newspaper remarked that although mankind had solved the problem of nuclear energy the same could not be said of the restoration of front teeth. At the time it was supposed that the dental silicate cement was, as its name implied, a silicate cement which set by the formation of silica gel. Structural studies at the Laboratory of the Government Chemist (LGC) soon proved that this view was incorrect and that the cement set by formation of an amorphous aluminium phosphate salt. Thus we became aware of and intrigued by a class of materials that set by an acid-base reaction. It appeared that there was endless scope for the formulation of novel materials based on this concept. And so it proved. 

I. Livsey and R. Shaunak. Cement setting retarding agents. Patent GB 2182031, 1987. 

K. A. Rodrigues. Cement set retarding additives, compositions and methods. Patent US 5341881, 1994. 

Lignosulfonates and lignosulfonate derivatives are used extensively as cement set time retarders (20, 21). Many of the same additives used in drilling muds are used in cement slurries and spacer fluids for similar purposes. 

Keywords Heavy metals, cement, setting time, cadmium, waste management References ... 

Magnesia cement is largely composed of magnesium oxide (MgO). In practice, the MgO is mixed with fillers and rocks and an aqueous solution of magnesium chloride. This cement sets up (hardens) within 2-8 h and is employed for flooring in special circumstances. 

Ca-P ionic cements Bone substitute Improvement of cement setting. 

Ions released into the matrix as the cement sets may interact with the organic part of the matrix. Metal ions, such as Ca + and AP+, may be chelated by car-boxylate groups, either on the polymer or on the tartaric acid additive. These have been considered in reasonable detail in the literature [230]. What has received far less attention is the possibility that fluoride ions might interact with carboxylic acid groups, either to modify the setting reaction or to become relatively securely anchored within the set cement. This possibility was raised in a review published in 1998 [230], but has not been followed up subsequently. It is based on the well-established observation that fluoride ion will form extremely strong hydrogen bonds with carboxylic acids in aqueous solution. They are of the type ... 

Sodium silicate cemcnl does not withstand bases, bui is resistant to acids except hydrofluoric. This cement sets lo a very rigid solid, so lhal when subjected to mechanical shock or lo temperature change il is liable to crack. 

High content of cement setting retarders like sulfates, halides... 

Compounds which prevent cement setting like sugar, lignine sludge... 

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