Cementation fronts

B) Boundary between calcite-cemented and uncemented horizons in Avalon Sandstone, showing no evidence of dissolution. Straight crystal faces of the poikilotopic calcite (left) at the boundary (centre) indicate the presence of a cementation front, rather than a dissolution front. Note that framework grains in the porous zone are coated with thin clay rims, which are absent in the cemented zone. The former is also slightly more compacted than the latter. Same locality as... 

Veneering Investments. These are phosphate bonded and contain finely ground quart2, 2irconium oxide, and/or titanium oxide to produce highly refractory, low expansion dies of fine detail. The dies are formed within impressions taken of teeth that the dentist has prepared in anticipation of covering the front surface with an aesthetic ceramic veneer. Porcelain or ceramic powders are shaped to detail on the dies and these are fired at high (- 1000° C) temperatures to produce the veneers. The veneers are then cemented to the front surface of the previously prepared teeth. 

The perforation wash tool is retrieved and the packer run in the hole with a work string, set at the desired depth and tested. An annular pressure test of 1,000 psi is usually sufficient. The packer is run with or without a tail pipe, depending on the operation to be performed. If cement is to be spotted in front of the perforations, a tail pipe that covers the length of the zone plus 10 to 15 ft must be run with the packer. 

A spearhead or breakdown fluid followed by the cement slurry is circulated downhole with the packer by-pass open. This is done to avoid the squeezing of damaging fluids ahead of the slurry. A small amount of back pressure must be applied on the annulus to prevent the slurry fall caused by U tubing. If no tail pipe has been run, the packer by-pass must be closed 2 or 3 bbl before the slurry reaches the packer. If the cement is to be spotted in front of the perforations, with the packer unset, circulation is stopped as soon as the cement covers the desired zone, the tail pipe pulled out of the cement slurry and the packer set at the desired depth. The depth at which the packer is set must be carefully decided. 

Pressure is released and returns are checked. If no returns are noticed, the packer by-pass is opened and excess cement reversed out. Washing off cement in front of perforations can be performed by releasing the packer and slowly lowering the work string during the reversing. 

The glass polyalkenoate cement uniquely combines translucency with the ability to bond to untreated tooth material and bone. Indeed, the only other cement to possess translucency is the dental silicate cement, while the zinc polycarboxylate cement is the only other adhesive cement. It is also an agent for the sustained release of fluoride. For these reasons the glass polyalkenoate cement has many applications in dentistry as well as being a candidate bone cement. Its translucency makes it a favoured material both for the restoration of front teeth and to cement translucent porcelain teeth and veneers. Its adhesive quality reduces and sometimes eliminates the need for the use of the dental drill. The release of fluoride from this cement protects neighbouring tooth material from the ravages of dental decay. New clinical techniques have been devised to exploit the unique characteristics of the material (McLean Wilson, 1977a,b,c Wilson McLean, 1988 Mount, 1990). 

The glass polyalkenoate cement was originally intended as a substitute for dental silicate cements for the aesthetic restoration of front (anterior) teeth (Wilson Kent, 1972 Knibbs, Plant Pearson, 1986a Osborne Berry, 1986 Wilson McLean, 1988). It is suitable for restoring anterior cavities in low-stress situations, that is when the restoration is completely supported by surrounding tooth material. These cavities occur on the adjacent surfaces of neighbouring teeth (class III cavities) and at the gum line (class V cavities). 

Dental silicate cement was once the most favoured of all anterior (front) tooth filling materials. Indeed, it was the only material available for the important task of aesthetic restoration from the early 1900s to the mid 1950s, when the not very successful simple acrylic resins made their appearance (Phillips, 1975). In the mid sixties there were some 40 brands available (Wilson, 1969) and Wilson et al. (1972) examined some 17 of these. Since that time the use of the cement has declined sharply. It is rarely used and today only two or three major brands are on the market. The reason for this dramatic decline after some 50 years of dominance is closely linked with the coming of modern aesthetic materials the composite resin from the mid 1960s onwards (Bowen, 1962), and the glass-ionomer cement (Wilson Kent, 1971) from the mid 1970s. 

Dental silicate cement is used for the aesthetic restoration of anterior (front) teeth because it is translucent and so can be made to colour-match tooth enamel. It is prepared by introducing powder into the liquid gradually in order to dissipate heat, although the exotherm is not so great... 

Although these cements have high compressive strength, their low flexural and tensile strengths coupled with brittleness and lack of toughness makes them suitable only for low-stress anterior (front teeth) restorations. 

Dental silicate cement is solely used for restoring anterior (front) teeth. It is probably the strongest purely inorganic cement and develops its strength rapidly. Although satisfactory in areas of the mouth washed by saliva it is... 

The senior author first became interested in acid-base cements in 1964 when he undertook to examine the deficiencies of the dental silicate cement with a view to improving performance. At that time there was much concern by both dental surgeon and patient at the failure of this aesthetic material which was used to restore front teeth. Indeed, at the time, one correspondent commenting on this problem to a newspaper remarked that although mankind had solved the problem of nuclear energy the same could not be said of the restoration of front teeth. At the time it was supposed that the dental silicate cement was, as its name implied, a silicate cement which set by the formation of silica gel. Structural studies at the Laboratory of the Government Chemist (LGC) soon proved that this view was incorrect and that the cement set by formation of an amorphous aluminium phosphate salt. Thus we became aware of and intrigued by a class of materials that set by an acid-base reaction. It appeared that there was endless scope for the formulation of novel materials based on this concept. And so it proved. 

Eight months after injection began in the second injection well, wastes had leaked upward into the adjacent shallow monitoring well. The leak apparently was caused by the dissolution of the cement grout around the casing. In June 1972,13 months after injection began in the second well, the waste front reached the deep monitoring well located 450 m (1500 ft) northwest of the injection well. Waste injection ended in December 1972. As of 1977, the wastes were treated in a surface facility.170... 

The great difficulty with such fire-side, multitube boilers is overheating the hot-side tubesheet. This tubesheet is exposed to the full temperature of the combustion gases, but is difficult to keep cool with the boiler s feedwater. Also, the ends of the tubes, where they are sealed in the tubesheet, are hard to keep cool. To protect these ends from the direct radiant heat in the combustion chamber, ferrule inserts, about lV2 in long, are cemented into the front end of each tube. 

Limestones vary greatly in color and texture, the latter ranging front dense and hard limestone, e.g.. marble or travertine, which can be suwed and polished, to soft, friable forms, e.g., chalk and marl. Chalk is a very fine-grained white limestone, while marl is an impure deposition product that contains clay and sand. Texture, hardness, and porosity appear to be functions of the degree of cementation and consolidation during the formation of these materials. Color variations arise from the presence of impurities. Some impurities, such as sulfur and phosphorus, make limestone unattractive for metallurgical uses. 

The lamp is mounted securely at its ends and connected to a source of direct current preferably 220 volts or 500 volts through suitable resistances and a switch. The switch is closed and a small Bunsen flame is played on the capillary between A and B. The mercury vaporizes and the arc strikes and is localized between the two bulbs. As soon as the lamp starts, water is turned on so that it flows down around the lamp in a large stream. The cold water keeps the cement intact and preserves the lamp. The energy is so intense that without water-cooling the quartz would be melted immediately. Hard water may be used provided that it passes over the lamp so rapidly that there is not time to heat up the water and deposit lime. The mounting of the lamp may be varied for different purposes. The lamp may be placed in front of a quartz window in a copper tank which is partially drained until the lamp is started. After starting, the tank is allowed to fill to an outlet near the top thus covering the lamp. 

In a second set of experiments, the rate of penetration of a brine solution into a cement sample was measured as a function of soaking time. The sample, which had a w/c ratio of 0.4, was heated in an oven at 105°C for 4 days to remove all evaporable water, and then sealed on all sides except the end face which was dipped in a saturated salt solution. The resulting 1-D profiles illustrated in Fig. 20 show the progression of the 23Na front into the sample with prolonged soaking times. 

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