Accelerators setting time

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). 

Accelerated setting time Excess MgO Minutes 6h Sprayable coatings for construction, dental cements... 

Accelerator in Ready-Mix Concrete. Calcium chloride accelerates the set time of concrete giving it a high early strength development. It is not an antifreeze, but by using it duriag cold weather protection can proceed in a timely manner (31—34). In Russia, calcium chloride forms a component of several antifreeze admixtures (33). Reviews of the concerns and possible remedies of calcium chloride corrosion problems in concrete are available (21,35). There is no consensus on what the safe levels of calcium chloride in concrete are. 

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. 

Impression plasters are manufactured from the finest finishing plasters, selected for color and purity. Setting time accelerators, setting expansion control agents, fillers, flavors, colors, or other special modifying agents may be added, eg, starch, to cause disintegration of the plaster impression when it is boiled. 

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

Most of the properties of a dental silicate cement are affected by preparative variables, particularly the powder/liquid ratio (Jorgensen, 1963 Wilson Batchelor, 1967b). Increase in the powder/liquid ratio accelerates set and increases strength and resistance to erosion (Figure 6.21). Temperature and, to a lesser extent, humidity during mixing have some effect, but chiefly they affect setting time. 

In this case acetic add and acetate are reaction promoters. The reaction is greatly accelerated and the setting time of cements is shortened (Table 9.1). 

Often it is necessary to influence the setting time, either by accelerators or by retarders. If a cement is to be placed into a shallow depth, then acceleration of setting will be desirable to avoid unnecessary waiting times. On the other hand, in a deep formation more open time is required, which may require the addition of retarders. 

Accelerating admixtures based on calcium chloride, formate, nitrate, and thiocyanate have no significant effect on the workability, air content, mix stability, or water-cement ratio of concretes into which they are incorporated. The only properties of plastic concrete which are affected are the heat evolution and setting time. 

Fig. 5.20 Effect of temperature on setting time with accelerators. 

Accelerating admixtures reduce both the initial and final setting time of mortar sieved from concrete mixes, determined by ASTM C403 68 or BS 5075 (1975). Typical results are given in Table 5.1 for a 300 kg m cement content mix with a compacting factor of 0.85 0.02 [24] at normal ambient temperature. 

Setting characteristics Li2C03 accelerates both initial and final set times at dosages exceeding 1% by... 

Corrosion inhibitors, like calcium nitrite, are accelerators and depending on their dosage may cause rapid slump loss and shorten the setting time when added at the batch plant. 

The choice of a particular option or options is determined by the desired set time and rate strength development. Accelerated time of setting is usually achieved by increasing the heat of hydration of the concrete mixture. Selection of appropriate chemical admixtures enables the production of cold-weather concrete mixtures with both accelerated setting and early-age strength development characteristics, similar to that obtained with plain... 

As we have already seen, there will be settings where the pattern of differences between treatment groups does not conform to proportional hazards, where the hazard ratio is not a constant, single value. Such situations are best handled by using an alternative model to incorporate baseline factors. The accelerated failure time model is an analysis of variance technique which models the survival time itself, but on the log scale ... 

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