Asbestos cement process

In classical asbestos cement processes, a slurry of asbestos fibers and cement is produced and deposited as a fibrous fleece in continuously operating cylindrical sieves. This fleece is then compressed under 200 bar pressure and allowed to set in molds for 24 to 48 h. This process can be accelerated by steam hardening. 

Acids/alkalis Plastics fabrication Rubber-synthetic manufacturing Cement processing and manufacturing Asbestos and associated operations Fermentation industry Electroplating industry... 

Using a predictive model developed from mesothelioma data from studies of asbestos insulation workers (Peto et al. 1982), asbestos textile workers (Peto 1980), amosite factory workers (Seidman 1984), and asbestos-cement workers (Finkelstein 1983), EPA (1986a) estimated that continuous lifetime exposure to air containing 0.0001 f/mL of asbestos would result in about 2-3 cases of mesothelioma per 100,000 persons. The corresponding cumulative lifetime exposures associated with excess risks of 10 " -10 are shown in Figure 3-1. Cumulative exposure levels of 0.031, 0.0031, 0.00031, and 0.000031 f-yr/mL represent excess mesothelioma risks of 10" , 10 , 10, and 10 ", respectively. Appendix D provides further details on the derivation of these risk estimates. Currently (in 2001), EPA is in the process of reviewing their cancer risk estimates for asbestos fibers. 

By far the largest quantity of asbestos is utilized in asbestos composites for reinforcing inorganic (cement) and organic (PVC, rubber, duromers) binders. Whereas the consumption in Western industrialized countries has declined steeply (95% in the USA banned in FRG since 1993), more than 2 10 t of asbestos was processed worldwide in 1997 mainly to building materials made from asbestos cement, which were utilized in Asian and Eastern European countries and in developing countries. 

Asbestos cement materials are widely used due to their good mechanical properties, low thermal conductivity and resistance to weathering, frost and rotting. The slurry of asbestos and cement can also be processed directly using a spray process, as so-called asbestos spray for fire-retarding layers as well as for thermal (hot and cold) and sound insulation. 

Asbestos wastes may be solidified prior to their landfill burial. This may be achieved by a cementing process such as that using pozzolanic concrete, which contains fly ash or kiln dust mixed with lime, water, and additives (Peters and Peters 1980). Other processes for solidification include thermoplastic and polymeric processes. In the former, a binder such as paraffin, polyethylene, or bitumen is used. In the latter, polyester, polybutadiene, or polyvinyl chloride is used to trap the asbestos fibers or particles over a spongy polymeric matrix. The solidified waste should be disposed of in a licensed hazardous waste dump or disposal site. 

Very often, corrosive vapors are a by-product of an industrial or chemical process. Ducts must be used to carry these vapors safely from the area. The type of duct used is determined by the vapor. Heavier-gauge metal may be sufficient, although a protective coating or special lining may be required in the ducts. Stainless steel, asbestos cement, and plastic linings have been used with success, depending on the corrosive vapor. 

In Europe, asbestos -cement pipes were made by mixing fibers with Portland Cement and then allowing the cement to cure at ambient temperatures. In the United States the process was similar, except that rather than curing at low temperatures, pipes were cured in steam pressured autoclaves, at temperatures near 170 C, for about ten hours. This process makes very high quality pipes. I was astounded that this process could be operated economically, but it was and the product is superior. 

The first widely used manufactured composite in modern times was asbestos cement, which was developed in about 1900 with the invention of the Hatschek process. Now, fibres of various kinds are used to reinforce a number of different materials, such as epoxies, plastics and ceramics. Here we will concentrate on the use of fibre reinforcement in materials made with hydraulic cement binders. 

Shotcreting Using a modification of normal shotcreting techniques, it has been found possible to produce steel and polypropylene fibre shotcretes, for use particularly for lining of tunnels, and for stabilization of rock slopes. With this method, too, relatively high volumes of fibres can be added to the mix. Pulp type processes For asbestos cement replacements (cellulose or other fibres are used as a replacement for the asbestos), the fibres are dispersed in a cement slurry, which is then dewatered to produce thin sheet materials. These can be built up to the required thickness by layering. This process yields fibre contents of typically from 9% to over 20% by volume. 

The absence of CH-rich zones in the vicinity of the fibres was reported by Akers and Garrett [41] for asbestos-cement composites and by Bentur and Akers [42] for cellulose FRC composites produced by the Hatscheck process. This may be the result of the affinity of these fibres for the cement particles, and the processing treatment which involves dewatering, both of which lead to a system with very little bleeding, and probably reduce the extent of formation of water-filled spaces around the fibres in the fresh mix. This is reflected in the nature of the fibre-matrix bond failure in asbestos composites, the cement matrix was sometimes seen to be sticking to the asbestos fibre bundle. This suggests that a strong interface was... 

Asbestos-cement was the first FRC composite in modern times, and was used extensively as a cladding material, for roofing and wall units as well as pipes. The asbestos fibres are made of natural crystalline fibrous minerals, consisting of bundles of filaments (sometimes known as fibrils), with individual filaments being as thin as 0.1 /tm or less. In the manufacture of the actual composite, the bundles tend to be split up during the processing procedure, though considerable portions of the reinforcement remain in the form of small fibre bundles. 

The final stage of the production process involves curing, which can be based on room temperature treatment in a tunnel in which moist conditions are maintained, or higher temperature steam curing to accelerate the hardening process. Autoclave curing is also common in the asbestos-cement industry, and in such instances part of the cement is replaced with finely ground silica. Replacement of part of the cement with low-cost inert fillers or fly ash is also sometimes used, for economic reasons. 

I n the asbestos-cement industry it is common to use a blend of different types of asbestos fibres, to optimize the characteristics of the mix in its green (processing) stage, as well as the strength in the hardened state. Some of the asbestos fibres are chosen for their processing characteristics, while others are present because of their efficiency in enhancing strength. 

When considering the properties of asbestos-cement composites in relation to the production processes, the orientation effect must also be taken into account. The data reported by Allen indicate that the strength in the transverse direction is only about 60-85% of that in the longitudinal direction, with similar ratios reported for the ultimate elongation. Zevin and Zevin [9,10] determined the orientation of the fibres by X-ray methods. They then calculated the strength ratio between the longitudinal and transverse directions, which was found to compare favourably with... 

Table 9.4 Effect of fibre processing on the properties of asbestos-cement composites ...

Table 9.5 Longitudinal to transverse strength ratio in asbestos-cement composites produced by different processes (after Zevin and Zevin [ 10])...

The two main approaches for dealing with the mechanics of asbestos-cement composites are based on composite materials concepts (rule of mixtures) and fracture mechanics, Microstructural characterization of the composite has usually been carried out in conjunction with these studies, to determine the numerical parameters required for the modelling, such as the aspect ratio, and to resolve the pull-out and fracture processes during failure. The results of these microstructural studies wi II be reviewed first, since they provide the background required for the modelling of the processes which control the mechanical performance of the hardened composite,... 

Fracture mechanics has been applied to model some of the fracture characteristics asbestos-cement composites, especially those related to crack bridging by the fibres. Mai [12] analysed the behaviour of both notched beams in flexural loading and double cant lever beams (DCB) to determine the critical stress intensity factor, /Cc and the specific work of fracture, WF, which measures the average fracture energy per apparent unit crack surface over the entire fracture process. The effect of fibre content on these parameters is shown in Figure 9.15. The trends resemble those observed for the effect of fibre content on strength (Figure 9.14). 

S.A.S Akers and G.G. Garrett, The influence of processing parameters on the strength and toughness of asbestos cement composites , int J. Cem. Comp. Ltwt. Conor. 8, 1986, 93-100. 

Cellulose-pulp FRC composites with flexural strengths in excess of 20 MPa can be readily produced [18,20,24-27], with about 10% fibres by mass, using processes applied in the asbestos-cement industry. From the point of view of... 

Applicability Most hazardous waste slurried in water can be mixed directly with cement, and the suspended solids will be incorporated into the rigid matrices of the hardened concrete. This process is especially effective for waste with high levels of toxic metals since at the pH of the cement mixture, most multivalent cations are converted into insoluble hydroxides or carbonates. Metal ions also may be incorporated into the crystalline structure of the cement minerals that form. Materials in the waste (such as sulfides, asbestos, latex and solid plastic wastes) may actually increase the strength and stability of the waste concrete. It is also effective for high-volume, low-toxic, radioactive wastes. 

When used as substitutes for asbestos fibers, plant fibers and manmade cellulose fibers show comparable characteristic values in a cement matrix, but at lower costs. As with plastic composites, these values are essentially dependent on the properties of the fiber and the adhesion between fiber and matrix. Distinctly higher values for strength and. stiffness of the composites can be achieved by a chemical modification of the fiber surface (acrylic and polystyrene treatment [74]), usually produced by the Hatschek-process 75-77J. Tests by Coutts et al. [76] and Coutts [77,78] on wood fiber cement (soft-, and hardwood fibers) show that already at a fiber content of 8-10 wt%, a maximum of strengthening is achieved (Fig. 22). 

Asbestos fibers are found worldwide in many products as reinforcement in cement water pipes and the inert and durable mesh material used in filtration processes of chemicals and petroleum, for example. However, asbestos is not the only inorganic fiber in use today. Synthetic inorganic fibers abound. Glass fibers have replaced copper wire in some intercontinental telephone cables. Fiberglas (a trade name) has become the insulation material of choice in construction. Carbon and graphite fiber composites are favored materials for tennis racket frames and golf clubs. Fibrous inorganic materials have become commonplace in our everyday lives. 

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