Condensation cements

Condensation cements. Here, cement formation involves a loss of water and the condensation of two hydroxyl groups to form a bridging oxygen ... 

Paul then explains that he and his quantity surveyor want to keep open the option of using a cheaper but heavier cladding system - one made from condensed cement fibre board - as an alternative to the more expensive mirrored steel or aluminium options already specified, in case savings need to be made in the build. Adam is quick to point out that he has already done work on this. Paul insists that this won t change the layout but rather only the sizing of steel elements in order to be able to accommodate a heavier alternative if this becomes a budgetary requirement. Adam agrees to work with this. 

Refractories and Molds. Citric acid is used as a binder for refractory cements, imparting volume stabiUty and strength in ceramic materials for electrical condensers, foundry and glassmaking molds, and sand molds for metal castings (219—223). 

The mbber is then separated from its solvent by steam stripping. The viscous cement is pumped into a violently agitated vessel partly full of boiling water. The hexane flashes off and, together with water vapor, passes overhead to a condenser and to a decanter for recovery and reuse after drying. Residual unpolymerized ethylene and propylene appear at the hexane condenser as noncondensibles, and are recovered for reuse after drying. The polymer, freed from its carrier solvent, falls into the water in the form of cmmb. 

One example is silicate cement where orthosilicic acid, chemically generated in solution, condenses to form a silicic acid gel. Another is refractory cement where a cementitious product is formed by the heat treatment of an acid orthophosphate, a process which again involves condensation to form a polyphosphate. 

On mixing the cement paste, the calcium aluminosilicate glass is attacked by hydrogen ions from the poly(alkenoic acid) and decomposes with liberation of metal ions (aluminium and calcium), fluoride (if present) and silicic acid (which later condenses to form a silica gel). 

As the poly(alkenoic acid) ionizes, polymer chains unwind as the negative charge on them increases, and the viscosity of the cement paste increases. The concentration of cations increases until they condense on the polyadd chain. Desolvation occurs and insoluble salts precipitate, first as a sol which then converts to a gel. This represents the initial set. 

The two matrices in these cements are of a different nature an ionomer salt hydrogel and polyHEMA. For thermodynamic reasons, they do not interpenetrate but phase-separate as they are formed. In order to prevent phase separation, another version of resin glass polyalkenoate cement has been formulated by Mitra (1989). This is marketed as VitraBond, which we term a class II material. In these materials poly(acrylic acid), PAA, is replaced by modified PAAs. In these modified PAAs a small fraction of the pendant -COOH groups are converted to unsaturated groups by condensation reaction with a methacrylate containing a reactive terminal group. These methacrylates can be represented by the formula ... 

The fate of silicic acid is of some interest. Silicic acid polymerizes, by condensation, and finally a silica gel is formed (Wilson Mesley, 1968). The insolubilization of silicic acid has been observed to parallel closely the precipitation of phosphate (Wilson Batchelor, 1967b) and is related to an increase of pH within the cement (Kent Wilson, 1969). A low concentration of silicic acid must remain in the matrix. All this is in accord with the known aqueous chemistry of silica. 

This alternative hypothesis may explain the observational differences between different workers. Brune Smith (1982), unlike Wilson et al. (1972), found Si distributed throughout the cement but were uncertain whether it was due to Si in the matrix or the degradation of fine particles to silica gel. But Brune Smith (1982) used a normal glass powder while Wilson et al. (1972) removed fine particles to improve resolution. These differing observations are reconciled if the silicic acid which is formed migrates slightly before condensing to silica gel. 

Krajewski, K. P. (1984). Early diagenetic phosphate cements in the albian condensed glauconitic limestone of the Tatra mountains. Western Carpathians. Chemical Abstracts, 10, 114382. 

A water-soluble polymer of monoallylamine can be used in conjunction with a sulfonated polymer, such as a water-soluble lignosulfonate, condensed naphthalene sulfonate, or sulfonated vinyl aromatic polymer, to minimize fluid loss from the slurry during well cementing operations [1510,1511]. The polymer... 

A fluid loss additive useful in cementing oil and gas wells is a blend [423,424,1015] of a copolymer of acrylamide/vinyl imidazole. The second component in the blend is a copolymer of vinylpyrrolidone and the sodium salt of vinyl sulfonate. Details are given in Table 2-2. The copolymers are mixed together in the range of 20 80 to 80 20. Sodium or potassium salts or a sulfonated naphthalene formaldehyde condensate can be used as a dispersant. 

For aqueous cement slurries a copolymer of N-vinylpyrrolidone and a salt of styrenesulfonic acid has been proposed [1585]. A naphthalene sulfonic acid salt condensed with formaldehyde serves as a dispersant. 

Carbon black may serve as a low-cost additive for controlling the gas migration in cement slurries [303]. It is intended as a suitable substitute for polymer latex and silica fume and has been tested in field applications [304,1256]. The concentration of carbon black varies from 2 to 20 parts, based on the weight of the dry cement [1220]. The particle size varies from 10 to 200 nm. A surfactant is necessary for dispersion, for example, formaldehyde-condensed naphthalene sulfonate or sulfonated cumarone or indene resins. 

A dispersing agent for a cement slurry is the magnesium salt from the condensation of napthalenosulfonic acid and formaldehyde [815,816]. The additive eliminates free water, even at low temperatures and with those cements most susceptible to this phenomenon. 

Sulfoalkylated naphthol compounds are effective as dispersants in aqueous cement slurries. The compounds can also be applied in an admixture with water-soluble inorganic compounds of chromium to provide additives of increased overall effectiveness. Particularly suitable are sodium chromate or ammonium dichromate. a-Naphthol is reacted in an alkaline aqueous medium with formaldehyde to create condensation products. The aldehyde can be reacted with bisulfite to produce sulfoalkylated products [1404,1410]. 

Fig. 3.4.12 Three-dimensional rendered spin-echo image of water filled cracks in a cement paste specimen [13]. Three cracks are visible in the image a large triangular crack in the forefront, a smaller crack in the bottom left corner and a sheet-like structure at the top of the image. Water droplets can also be observed condensing on the cement paste surfaces. The measurement parameters were FOV 20 x 20 x 20 mm, acquisition points 128 x 128 x 64, nominal resolution 156 x 156 x 312 pm, echo time 2.7 ms, repetition time 500 ms and acquisition time 270 min.

The mbber cement is then sent to storage tanks where antioxidants and extenders are mixed in. The mbber cement is pumped from the storage tank to the coagulator where the mbber is precipitated with hot water under violent agitation. The solvent and unreacted monomer are first steam stripped overhead and then condensed, decanted, and recycled to the feed stage. The bottom water layer is discharged to the wastewater treatment facility. 

By achieving random dispersion of the Kaolin in the sodium silicate solution prior to formation of the silica-alumina gel, it was possible to disperse the clay crystals. They condensed somewhat perpendicular to each other and were bound together by silica-alumina gel. I therefore speculated that spray drying, during which the gel system contracts, might create a dual structure. An analogy would be a house built of cards (Kaolin), cemented together with silica-alumina gel. 

The adsorption-desorption process is of interest in many systems (such as with cement). The water vapor may condense in the pores after adsorption under certain conditions. This may be studied by analyzing the adsorption-desorption data... 

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