Cement setting times

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

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 ... 

Mixing lidocaine with bone cement is also foimd to increase the cement setting time [ 1 ]. 

Set regulator Calcium sulfate (gypsum, bassanite, anhydrite) added to Portland clinker to control cement setting times. 

To control the degree of moisture evaporation and setting time, freshly poured concrete is sprayed with solvent solutions of aromatic, dicyclopentadiene, or aHphatic resins (see Cement). 

In concrete, triethanolamine accelerates set time and increases early set strength (41—43). These ate often formulated as admixtures (44), for later addition to the concrete mixtures. Compared to calcium chloride, another common set accelerator, triethanolamine is less corrosive to steel-reinforcing materials, and gives a concrete that is more resistant to creep under stress (45). Triethanolamine can also neutralize any acid in the concrete and forms a salt with chlorides. Improvement of mechanical properties, whiteness, and more even distribution of iron impurities in the mixture of portland cements, can be effected by addition of 2% triethanolamine (46). Triethanolamine bottoms and alkanolamine soaps can also be used in these type appUcations. Waterproofing or sealing concrete can be accompUshed by using formulations containing triethanolamine (47,48). 

Fegulated-set cement, called jet cement in Japan, is formulated to yield a controlled short setting time, <1 h, and very eady strength (73). It is a modified cement that can be manufactured in a conventional Pordand cement kiln. It incorporates set control and eady strength development components. 

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. 

It is often necessary to either accelerate, or retard the thickening and setting time of a cement slurry. 

All of the gluconates can be used as a cement or plaster additive where they retard setting times whilst increasing the strength and water resistance of the materials. 

The properties of these cements - the fluidity of the mix, the working and setting times of the cement paste, and the strength of the set cement - are affected by a number of factors. These include the composition of the powder, the concentration, molecular mass and type of the polyacid, the... 

Attempts have been made to improve the mechanical properties of these cements by adding reinforcing fillers (Lawrence Smith, 1973 Brown Combe, 1973 Barton et al, 1975). Lawrence Smith (1973) examined alumina, stainless steel fibre, zinc silicate and zinc phosphate. The most effective filler was found to be alumina powder. When added to zinc oxide powder in a 3 2 ratio, compressive strength was increased by 80 % and tensile strength by 100 % (cements were mixed at a powder/liquid ratio of 2 1). Because of the dilution of the zinc oxide, setting time (at 37 °C) was increased by about 100%. As far as is known, this invention has not been exploited commercially. 

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. 

Crisp and coworkers found that the development of surface crystallinity was related to the speed of set. The faster the reaction, the shorter was the inhibition period before surface crystallization took place. When the setting time of a cement was between two and three minutes, surface crystallinity developed in a few minutes. When it was seven minutes, surface crystallinity was delayed by three hours. The reaction rate was affected by the chemical composition and physical state of the cement components. Well-ignited zinc oxide, the presence of magnesium in the... 

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