Slags additive

Ferrosihcon production is a nearly slag-free process. Most of the impurities introduced into the furnace via the raw materials and any other sources are transferred to the product. In order to produce high purity grades of ferrosihcon, the tapped ahoy is refined by treating with gas mixtures and slag additions. The principle of ferrosihcon refining is quite similar to that of sihcon. Details on the thermochemistry of ferrosihcon refining and practical examples are available (24). 

Because most of the ash is removed as molten slag, addition of a bottom grate is not necessary.7 However, small TDF is required, because much of the combustion must occur in suspension.7 TDF that is too large to combust completely can get carried over into the boiler or dust collection system, and cause blockage problems.9 Therefore, particle size may inversely determine the amount of TDF that can be used in a cyclone boiler.11 Three cyclone-fired boilers at utilities have burned 1M x 1" TDF in test operation, one at the 2 percent, one at the 5 percent, and one at up to a 10 percent level.3,9,12 One pulp and paper mill plans the use of TDF in a cyclone-fired hog-fuel boiler.13... 

This technique reduces the amount of slags and dust that need disposal and limits VOC-emissions. The energy consumption is reduced (by 10 to 15 %) due to the reduced amount of slag. Additionally the extracted airflow may be reduced. 

As it has been shown by Roy [80] the alkahs content in the pore solution of cement paste was significantly lower at 50% slag addition (Fig. 6.35). It should be tmderlined that this phenomenon is observed irrespectively of the alkali content in mineral additions, which can be higher than in cement. Similarly, the sodium and potassirrm content decrease in pore solution occirrs in case of fly ash cement paste [115]. The effect of fly ash is, however, not clear, because in some cases Diamond [129], as well as Glasser and Marr [130], observed the increase of these ions concentration in the liquid phase of cement paste. However, these authors are of the opinion that silica firme arrd, after longer period of time, fly ash, reduce the sodium and potassium soluble corttpotmd content [129, 130]. 

Fig. 6.35 Alkali content in the pore solution of cement paste, 50% of slag addition, (according to [80]) 1—cement with 65 % of slag addition 0.97 % Na O in clinker, 3 eement without slag, 0.97 % NajO, in clinker 1,4 cement with 5 % of slag addition, 0.97 % Na O. in clinker 7 slag as in 1, 5—cement without slag, 1.47 % Na O. in cUnker, 6 cement with 50 % of slag, slag as in 1, clinker as in 5, 7—cement with 50 % of two slags (35 % as in 7,15 % of the seeond), elinker as in 7...

The granulated blastfurnace slag and siliceous fly ash addition will have a similar effect on the ability of chloride ions binding in cement pastes, because they will also decrease the C/S ratio in C-S-H gel. Uchikawa and Okamura [210] report the following C/S ratio 1.7 for Portland cement, 1.6 in case of 40% slag addition and 1.2 at 40% of sihceous fly ash addition. Simultaneously, these additions will cause the aluminates content decrease in favor of calcium silicate hydrates. 

Many research were focused on the effect of fly ash, siUca fume and granulated blast furnace slag addition on the concrete freeze-thaw resistance. All these additions improve the freeze-thaw resistance, as compared with the concrete from Portland cement, however, on condition that all these eoncretes are air entrainment [80], Freeze-thaw resistance of concrete with fly ash ean be lowered in the case when it has high coal content. The limit value, increased recently to 9% (according to the European standard EN 197-1 2002/A3 2007), is too high and the 5% level should be maintained, as it was reeommended by the elder directives (previous standard EN 197-1 2002). The effeet of non-eombusted coal on the appUcabilily of fly ash is diseussed in details in Seet 7.4. 

Uchikawa [18] determined the composition of C-S-H phase in the Portland cement paste and in the pastes with 40% fly ash and slag addition after 4 years of hydration. The results are given in Table 8.1. 

The addition of fly ash to cement results in the formation of decreased amounts of calcium hydroxide in the hydration product. This is attributable to the dilution effect and to the consumption of calciiun hydroxide by the pozzolanic reaction with the fly ash. In Fig. 1, the amount of calcium hydroxide formed at different times of hydration in cement containing fly ash is given. The amount of Ca(OH)2 estimated by TG was found to be lower in samples containing fly ash. With the increase in the amount of fly ash, less calcium hydroxide was formed because of the pozzolanic reaction and dilution effect. Even at 60% fly ash, some lime was present in the mortar, and the pH was found to be 13.5. At this pH value, the passivity of steel is assured. It can also be observed that there is more lime at 60% fly ash than at 75% slag addition. 

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