Rapid set

Good mouldability, with easy flow and rapid setting. 

Commercial Rapid Set citrus pectin (864 mg galacturonic acid per g, degree of methylation 73) was from SBI (Beaupte, France). 

The bulk of the cement is extremely porous as the fractured surface of a specimen shows (Figure 6.3c). The pores are 0-5 pm in diameter and more abundant in the depth of the cement. The porosity arises from excess unbound water which separates out as globules in the cement and is trapped by the rapid setting. Subsequent diffusion of these globules leaves the cement porous. This makes the cement permeable to dyes (Wisth, 1972). 

The ZOE cement is easy to mix and a greater amount of powder can be incorporated into this cement (5 1 by mass) than any other, where even 4 1 by mass is unusual. Because the ZOE cement is sensitive to moisture it can be formulated to have a long working time under normal room conditions (23 °C, relative humidity 50 %) and a rapid set once placed in the warm and moist conditions of the mouth. This is a considerable clinical advantage, making it convenient to use. The cement can be used in a war pack for use on the battlefield. Nevertheless, sensitiveness to humidity can give rise to problems in use under tropical conditions. 

Dissolve 57 g. of dry malonic acid in 92-5 ml. of dry pyridine contained in a 500 ml. round-bottomed flask, cool the solution in ice, and add 57 g. (70 ml.) of freshly-distilled n-heptaldehyde (oenanthol) with stirring or vigorous shaking. After a part of the aldehyde has been added, the mixture rapidly sets to a mass of crystals. Insert a cotton wool (or calcium chloride) tube into the mouth of the flask and allow the mixture to stand at room temperature for 60 hours with frequent shaking. Finally, warm the mixture on a water bath until the evolution of carbon dioxide ceases (about 8 hours) and then pour into an equal volume of water. Separate the oily layer and shake it with 150 ml. of 25 per cent hydrochloric acid to remove pyridine. Dis.solve the product in benzene, wash with water, dry with anhydrous magnesium sulphate, and distil under reduced pressure. Collect the a -nonenoic acid at 130-132°/2 mm. The yield is 62 g. 

Examples of retarders are shown in Table 10-11. They are added to prevent cement from setting too rapidly. These additives are also referred to as set retarders. Cements with retarders to prevent rapid setting may be used at the high-temperature and high-pressure environments of deep wells. Common retarders are lignosulfonate and certain carbohydrate derivatives, such as welan gum, xanthan gum, cellulose, and polyanionic cellulose. 

Phenolic-isocyanates (phenolic-urethanes). The binder is supplied in three parts a phenolic resin in an organic solvent (0.8%), methylene diphenyl diisocyanate (MDI) (0.5%), and a liquid amine catalyst. When mixed with sand, the amine causes a reaction between the resin and the MDI, forming urethane bonds, which rapidly set the mixture. The speed of setting is controlled by the type of catalyst. The optimum cure temperature is 25 to 30°C. Compression strength is typically over 4000kPa (600psi). 

Incomplete displacement of fluid from the annular space can result in gaps in the cement sheath through which fluids from different formations can intermingle. In this situation, a "squeeze cementing" treatment is required to plug these gaps. Portland cement or rapidly setting sodium silicate slurries can be used in this operation. 

Calcium chloride does not react significantly with cement pastes for a period of 2-6 h [1, 5] after mixing, although rapid setting can occur in this period. 

Reports of incompatibility between Portland cement and chemical admixtures have increased over the past 10 years. Rapid set, accelerated stiffening, increase in time of set, lack of water-reduction are some of the reported problems. Such effects produced by the interaction of the composition of the cement and that of the chemical admixtures has been often referred to as cement-admixture incompatibility . This is discussed below. 

The soluble sulfate versus time curve for the cement containing the natural anhydrite is radically changed when CLS is present [130, 131]. The rate of solution of natural anhydrite, which is much slower than that of gypsum or calcium sulfate hemihydrate, is further retarded in the presence of chemical admixtures, which leads to a sulfate-starved system in the concrete, often producing rapid set and an increase in rate of concrete slump loss (Fig. 7.42). Apparently the adsorption of the lignosulfonate by the natural anhydride plus the rapid reaction between the soluble SO3 and the... 

An alternative to silicate-based Portland cement is the calcium aluminate cement, ciment fondu, which originated with the Lafarge company in France in 1908. Ciment fondu is typically made by heating limestone with bauxite, which is mainly AIO(OH) but contains much iron oxide (see Section 17.2). As noted above, calcium aluminate hydrates and hardens much more rapidly than alite, and so ciment fondu, either as such or mixed with Portland cement, can be used whenever a rapidly setting cement is required, for example, for construction at low temperatures. Concretes made from aluminate cements remain serviceable at higher temperatures than Portland cements and so are used to make cast refractories for pyrometal-lurgical applications. 

According to this viewpoint, the hydroxide ion rapidly sets up an equilibrium with the amidocobalt complex. The rate-determining step is the dissociation of this complex (Eq. 13.40), but since its concentration depends on the hydroxide km concentration through equilibrium, the reaction rate is proportional to the hydroxide ion concentration. 

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