Free CaO

Refined calcined alumina is commonly used in combination with high purity limestone [1317-65-3] to produce high purity calcium aluminate cement (CAC). The manufacture, properties, and appHcations of CAC from bauxite limestone, as weU as high purity CAC, has been described (104). High purity CAC sinters readily in gas-fired rotary kiln calcinations at 1600 —1700 K. CAC reactions are considered practically complete when content of free CaO is less than 0.15% andloss on ignition is less than 0.5% at 1373 K. 

Other Phases in Portland and Special Cements. In cements free lime, CaO, and periclase, MgO, hydrate to the hydroxides. The in situ reactions of larger particles of these phases can be rather slow and may not occur until the cement has hardened. These reactions then can cause deleterious expansions and even dismption of the concrete and the quantities of free CaO and MgO have to be limited. The soundness of the cement can be tested by the autoclave expansion test of Portiand cement ASTM C151 (24). 

When the kiln material is cooled it forms into crystallized clinkers. These are rather large irregular pieces of the solidified cement material. These clinkers are ground and a small amount of gypsum is added (usually about 1.5 to 3%). The gypsum prevents flash setting of the cement and also controls free CaO. This final cement product is sampled, analyzed and stored. The actual commercial cement is usually a hlend of several different cements. This blending ensures a consistent product. 

Calcium oxide is commercially obtained from limestone. The carbonate is roasted in a shaft or rotary khn at temperatures below 1,200°C untd aU CO2 is driven off. The compound is obtained as either technical, refractory or agricultural grade product. The commercial product usually contains 90 to 95% free CaO. The impurities are mostly calcium carbonate, magnesium carbonate, magnesium oxide, iron oxide and aluminum oxide. 

Anhydrite 1 high-temperature anhydrite. Produced by high-temperature l.000°C) calcining. Contains free CaO. 

For the case of purely physical absorption into liquid initially gas-free (Cao = 0), the amount of component A absorbed per unit area during the time 6 is (Section III,A,2)... 

Calcium Oxide. Lime burnt lime calx quicklime. GO mol wt 56.08. Ca 71.47%, O 28.53%. Properly stored line of commerce contains 90-95% free CaO. Commercial production from limestone W. L. Faith et al. Industrial Chemicals (John Wiley, New York, 3rd ed, 1965) pp 482-411. Ub prepn by ignition of CaCO, Ehrlich in Handbook si Preparative Inorganic Chemistry ml. 1, G. Brauer, Ed. (Academic Press, New York, 2nd ed., 1963) p 931. Review R, S. Boynton in Kirk -Othmer Encyclopedia of Chemical Ttthealogy voL 14 (Wiley-Interscience, New York, 3rd ed.. Iffl) pp 343-382. 

The calcined lime is hydrated with sufficient water to convert the free CaO into Ca(OH)z- If the free CaO content is greater than 10 to 15 %, the hard sintered lumps disintegrate into a powder. Otherwise, the lime must be ground before hydration. It may also be necessary to grind the hydrated product to achieve the required degree of fineness and setting rate. 

Natural hydraulic limes are hydrated with sufficient water to convert free CaO into Ca(OH)2, without hydrating significant amounts of the calcium silicates and aluminates (see section 16.10.1). 

A parameter which is not included in the above, but which is of relevance to applications where delayed hydration can cause expansion is the free CaO content of hydrated limes. A method of calculation is given in Annex 3, Appendix B. 

The simple formnla for the calculation of free CaO in the clinkering process by Johansen [15] was given he admitted that the limiting of the process rate factor is diffusion ... 

The influence of reducing atmosphere on phase composition and clinker properties was studies by several authors [76-81]. They state unanimously the gradual iron reduction, which leads to ferrite phase decomposition with C3A and free CaO formation. Fe can substitute Ca in all clinker phases, obviously in accordance with its solubility in individual phases. Frequently the tendency of clinker to seU-powdering, linked with Y-C2S formation is listed. 

Wachtler and Jannsen [90] studied the material phase composition in the kiln with partial precalcinahon, and without its shut down. In the material from cyclone preheater spurrite, C2F and aluminates CA and and free CaO were found. In the samples also these phases and additionally C AF were present Then the gehlenite appears, C2S and C3A. Appearing of CjA is accompanied with the disappearing of C2F and CA. When C3S appears the samples have already the composition of clinker. 

Castanet and Sorrentino [98] measured the elinkering heat in the high-temperature calorimeter of Calvet. In the case of chnker composition 4.77 % C4AF, 11.07 % C3A, 21.28% C2S, 55.7% C3S and 4% of free CaO the heat was 1876 kJ/kg of clinker. There is a difference between measured and calculated heat of 322 kJ/kg of clinker. This difference is explained. 

To sohd solutions studies the chemical. X-ray diffraction and spectroscopy (IR and NMR) methods are nsed. Chemical methods in cement chemistry consist prin-cipahy on free CaO determination in function of additive content increasing, for example MgO, which snbstitnte calcinm in solid solution. 

The lack of iron and high sihca ratio causes the sintering of clinker difficult and the mineralizers must be apphed. The fluorite is commonly used, but the more effective fluorosilicates and aluminates, for example Na3AlFg can also be applied. The CaCl2 appears in some reports, because there is an opinion of iron volatilization possibility, in the form of FeCl3 [35]. In spite of the mineralizers addition, the free CaO content in white cements is generally higher, on the level from 2 to 3 %. 

Calcium oxide is an important component of expansive cements and all the anthers are unanimous of its positive effect, causing expansion increase. The optimnm free CaO content is 3-4 % in the K type expansive cement [93]. 

Fig. 9.18 Effect of free CaO content on the K type cement expansion, (according to [93])...

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