Flash-setting cements

If the formation has openings too large for the cement to plug them, much of the cement, pumped down the hole, may be lost. In such cases, it is necessary to first plug these openings by means of flash-setting cements. CBS formulations can be designed to flash set... 

When the kiln material is cooled it forms into crystallized clinkers. These are rather large irregular pieces of the solidified cement material. These clinkers are ground and a small amount of gypsum is added (usually about 1.5 to 3%). The gypsum prevents flash setting of the cement and also controls free CaO. This final cement product is sampled, analyzed and stored. The actual commercial cement is usually a hlend of several different cements. This blending ensures a consistent product. 

According to Locher and Richartz [18], as well as Sprung [44] the formation of syngenite can be also the reason of false set. This phase is produced in cement pastes with higher amount of K2SO4 and hemihydrate. The formation of syngenite causes not only flash set but also leads to the consumption of sulphates, and further to the rapid C3A reaction with water. As a consequence, the stiffening of paste takes place [45]. 

It has long been recognized that, even in the absence of calcium sulfate, flash setting of cement may be prevented and a cement suspension of flowing consistency obtained by combining finely groimd Portland clinker with an alkali metal carbonate or hydrogen... 

False Set. Premature, but temporary, stiffening of portland cement resulting from overheating of the added gypsum during grinding of the clinker (cf. flash set). 

Flash Set. Premature and permanent stiffening of portland cement that has not been adequately retarded cf. false set. Flash Wall. A continuous wall of refractory brickwork built inside a downdraught kiln in front of the fireboxes its purpose is to direct the hot gases towards the roof of the kiln and to prevent the flames from impinging directly on the setting. 

In Portland cement, the hydration of the C3A phase is controlled by the addition of gypsum. The flash set is thus avoided. The C3A phase reacts with gypsiun in a few minutes to form ettringite as follows ... 

Ramachandran, V. S., and Beaudoin, J. J., Handbook of Analytical Techniques in Concrete Science and Technology, William Andrew Publishing, Norwich, New York, p. 964 (2001) Michaux, M., and Nelson, E. B., Flash Set Behavior of Oil Well Cement Slurries Containing Lignosulfonates, 9th Int. Congr. Cements, New Delhi, India, 4 584-590 (1992)... 

The use of g3 sum to control setting in Portland cement accounts for considerable quantities of the use of this material.1 Control of the reaction rate of tricalcium aluminate (C3A),the constituent of cement that reacts most rapidly with water, is most commonly achieved through the addition of gypsum (sometimes hemihydrate is also used) to commercial Portland cement. This material is normally added to the cement clinker (in amounts of approximately 2 to 3%) before grinding. The cement manufacturers usually specify a sulfurtrioxide content of about 36%. Excess sulfate in the form of hemihydrate can cause flash set in portland cement. 

Asbestos fibers plus 30-45% special Flashing cement (hard setting) (asbestos)... 

The slot in the interfaces for passage of the belt is formed by two "L"-shaped sapphire pieces which are attached to the stainless-steel flange or vacuum closure bar by epoxy cement. The belts used are either 0.05 or 0.075 mm thick and the slot tolerance is set to be 0.075 mm greater than the belt thickness (i.e., either 0.125 or 0.15 mm). The belt width is 0.317 cm and the slot width is 0.325 cm. A ribbon 0.32 cm wide travelling at a speed of 2.5 cm/sec will carry away a liquid film 0.2 mm thick from a solvent flow of 1 ml/min and if the solvent film can be evaporated without loss of solute, then the ribbon will transport virtually 100% of the solute into the mass spectrometer. Sample utilization will then depend only on the efficiency of the flash vaporization step. In practice, some sample is lost by spray processes and the flash vaporization cannot be fully efficient for all compounds. Nevertheless, yields in the range of 25-40% have been attained with an LC/MS ribbon interface system. It follows that the quantity of column eluent taken from the ribbon will be twenty times greater than that taken by the wire and provide significantly improved sensitivity. 

Uses Neutralizer for agric. chems. grinding aid, strength improver, and set accelerator for concrete and cement metal forming lubricant thixotropic additive for paints/pigments demulsifier for petroleum/coal Properties Liq. sp.gr. 1.12 dens. 9.35 Ib/gal b.p, 372 C( 4.48 mm Hg) flash pt. (Seta) 198 C 87% TEA 13% higher Alkasil HNM 1223-15 [Rhodia Novecare]... 

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