Cements slow-setting

Zinc Oxide—Eugenol Cements. Zinc oxide—eugenol cements have many uses in dentistry. The admixture of powdered zinc oxide [1314-13-2] and Hquid eugenol [97-33-OJ, fotins a bland, easily mixed paste having exceUent working time but slow-setting characteristics. 

Post-operative sensitivity has occasionally been reported when the glass-ionomer cement has been used as a luting agent. This observation is more than anecdotal, but the reason for it is unknown. It is not connected with pulpal irritation but may be related to hydraulic pressures (Pameijer Stanley, 1988). The indication is that sensitivity is related to clinical technique and is exacerbated if certain slow-setting glass-ionomer cements are used, especially if they are mixed too thinly. 

Calculate the overall Ca and Si contents (expressed as CaO and Si02) of a Portland cement clinker that has 55% alite, 30% belite, 5% alumi-nate phase, and 10% ferrite (assume ideal compositions for the latter two). Would you expect this to behave as a fast or a slow setting cement ... 

Polystyrene Although polystyrene is usually bonded by solvent cementing, it can be bonded with vinyl acetate/vinyl chloride solution adhesives, acrylics, polyurethanes, unsaturated polyesters, epoxies, urea-formaldehyde, rubber-base adhesives, polyamide (Versamid-base), polymethylmethacrylate, and cyanoacrylates. The adhesives should be medium-to-heavy viscosity and room-temperature and contact-pressure curing. An excellent source is a Monsanto Company technical information bulletin which recommends particular commercial adhesives for bonding polystyrene to a number of different surfaces. Adhesives are recommended in the fast-, medium-, and slow-setting ranges (10). 

In recent years, a new type of glass-ionomer material has become available that overcomes this problem of the weakening effect of additional fillers designed to enhance the bioactivity of the cement. Known as the glass carbomer , it overcomes the problem of slow setting by specifying the need to cure with the aid of heat from dental cure lamps. 

The test determines the mixing stability of bitumen emulsions with cement. It applies to overstabilised cationic bitumen emulsions and to slow-setting and overstabilised anionic bitumen emulsions. 

A polymeric filling material which at present is still little used but has major potential when the properties are made more acceptable, is the glass ionomer (10) cement, known by the acronym ASPA - aluminosilicate glass, very similar to the glass of the dental silicate cement and an aqueous polyacrylic acid solution as the setting medium (Fig, 9). Its use currently is practically limited to cervical erosion repair. The more extensive application of this material has been hampered by the lack of esthetic appearance due to opacity and rather slow set. 

Concrete, Mortar, and Plaster. Citric acid and citrate salts are used as admixtures in concrete, mortar, and plaster formulations to retard setting times and reduce the amount of water requited to make a workable mixture (172—180). The citrate ion slows the hydration of Portland cement and acts as a dispersant, reducing the viscosity of the system (181). At levels below 0.1%, citrates accelerate the setting rate while at 0.2—0.4% the set rate is retarded. High early strength and improved frost resistance have been reported when adding citrate to concrete, mortar, and plaster. 

Two matrices are formed a metal polyacrylate salt and a polymer. There is a lack of water in the system because some of it has been replaced by HEM A, and lack of water in glass polyalkenoate cements is known to slow down the ionomer add-base reaction (Hornsby, 1977). Thus, the initial set of these materials results from the polymerization of HEMA and not the characteristic acid-base reaction of glass-ionomer cements. The later reaction serves only to harden and strengthen the already formed matrix. 

Setting of the cements occurs by neutralisation, and involves initial formation of calcium or strontium polyacrylate and later formation of aluminium polyacrylate. There is also evidence for a later, slow reaction involving the ion-depleted... 

Cements that harden by the loss of solvent generally are to be avoided because the solvent can be lost only be diffusion thru the expl. Diffusion may be slow and the solvent may modify the properties of the expl. Two types of cement that have been used for this purpose are catalytic setting cements, like epoxy resins, and contact cements. Compatibility of the materials to be used should be checked. Compatibility of epoxy resins with most explosives depends upon the catalyst or hardener used (Ref 8). Data regarding bond strengths and other pertinent properties also have been compiled (Refs 5 6)... 

The dry product is ground to a powder and then a little calcium sulfate (CaS04) is added to slow down the setting rate of the cement. When water is added to the mixture, slow complex chemical changes occur, resulting in the formation of a hard interlocking mass of crystals of hydrated calcium aluminate and silicate. 

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