Shrinkage carbonation

The substitution of carbon microspheres for phenol ones reduces the carbonization shrinkage and material losses1,39). The strength properties of the final material are, however, lower (Table 16), This seems to be caused by a weakening of the adhesion of the binder to the carbonized filler (compared to the uncarbonized one)75). 

There are two further requirements for a suitable matrix precursor. Firstly, the carbonization shrinkage of the matrix should not damage the carbon fiber of the skeleton. Secondly, the porosity formed by carbonization of the precursor should be open... 

Table I. Carbonization Shrinkage and Translation of Fiber Strength In Unidirectional C/C Composites Fabricated with Polyphenyleneacetylene (HA 43) as Matrix Precursor (20)...

Lea [39] is accepting the hypothesis that carbonation shrinkage can be explained by decrease of the non-evaporable water content in cement gel. However, Powers (according to Lea [3 9]) attributed this shrinkage to the dissolution of Ca(OH)2 in the zone of shrinkage stress caused by drying, and precipitation of calcium carbonate in the zones of lower stress. 

The following properties are altered as a consequence of concrete carbonation shrinkage (see Sect. 5.3.2), strength, porosity, susceptibility to deformations, and resistance to the environmental impact. However, the pH of pore solution in concrete decreases and the passive film on steel is deteriorated therefore, the reinforcement is exposed to corrosion. This corrosion is probably the most frequent reason of concrete deterioration, because the mst formation causes the surrotmding concrete to crack and spall (see Sect. 6.4.11.). 

The carbonation shrinkage starts much later according to circumstances that is, it develops according to the amount of CO2 in the atmosphere, and is less important as it concerns only external layers of concrete elements. Carbonation shrinkage may be reduced by the addition of SF (Persson 1998). 

Carbonation shrinkage. Carbon dioxide attacks concrete and produces ... 

Calcium carbonate formation is accompanied by a decrease in the water content and increase in the weight of concrete, thus causing the cracking of the surface. Carbonation shrinkage decreases the cover to reinforcement and increases the risk of ingress of corrosive species. Most important however, is the fall in pH as the concrete becomes less... 

Another form of deterioration in concrete involves the chemical reaction between the products of cement hydration and carbon dioxide. This reaction decreases the pH of the solution, and this may lead to the corrosion of the embedded steel. Carbonation shrinkage is another phenomenon that has been documented. Maslehuddin, et al., examined by DTA/TG, the products formed in mortar specimens exposed to CO2 at 55-75°C for 54 weeks. Some were contaminated with chloride and sulfate ions. The amount of Ca(OH)2 and CaC03 formed in several cement mixes containing fly ash, silica fume, and slag was determined (Table 9). In general, the amount of Ca(OH)2 is lower in samples exposed to CO2, and the amount of CaC03 in samples exposed to CO2 is higher. There is also an accelerated carbonation in contaminated specimens. 

Sulfur Compounds. Various gas streams are treated by molecular sieves to remove sulfur contaminants. In the desulfurization of wellhead natural gas, the unit is designed to remove sulfur compounds selectively, but not carbon dioxide, which would occur in Hquid scmbbing processes. Molecular sieve treatment offers advantages over Hquid scmbbing processes in reduced equipment size because the acid gas load is smaller in production economics because there is no gas shrinkage (leaving CO2 in the residue gas) and in the fact that the gas is also fliUy dehydrated, alleviating the need for downstream dehydration. 

Anthracite is calcined at appreciably higher temperatures (1800—2000°C). The higher calcining temperatures for anthracite are necessary to complete most of the shrinkage and to increase the electrical conductivity of the product for use in either Soderberg or prebaked carbon electrodes for aluminum, siHcon, or phosphoms manufacture. 

Two types of carbon electrodes are in widespread use. Prebaked carbon electrodes (Fig. 5) are those made from a mixture of carbonaceous particles and a coal-tar pitch binder. The electrode is formed by extmsion or mol ding from a heated plasticlike mix and subsequently baked. Final bake temperature is sufficient to carbonize the binder, ie, about 850°C. At this temperature the binder is set, all volatiles have left, and a significant portion of the product shrinkage has occurred. 

The shrinkage in demand has resulted in a restmcturing of the carbon black-industry. Several of the principal multinational oil companies have left the business including Ashland, Cities Service Co., Phillips, and Conoco. Some plants have changed ownership. In the United States this has increased the production capacities of Degussa, Sid Richardson, and Huber. Today s U.S. industry consists of six principal producers. Rated capacities of the six U.S. manufacturers is shown in Table 13. Cabot Corp. and Columbian Chemicals are the leading producers, followed by Degussa, Sid Richardson, J. M. Huber Corp., and Witco. A survey of the future markets and present stmcture of the carbon black industry has been presented (1). 

Siilfuric acid from iron pyrites Paint pigments roasting of metallic oxides Refractory clays calcination of refractory clay to reduce shrinkage Foundry sand removal of carbon from used foundry sand Fullers earth calcination of fuller s-earth material... 

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