Conversion high alumina cement

DTG curve peak heights may be used for quantitative purposes since when no mass-loss is occurring, dm/dt = 0. During a mass-loss reaction, ilmttk > 0, hence the DTG peak is proportional to the mass-loss of the sample. This approach has been used to analyze natural rubber and butadiene rubber in a blend (118) and also the degree of conversion of high alumina cement (119). 

Midgley, H.G. (1980) The relationship between cement clinker composition and strength recovery of hydrating high alumina cement during conversion, in Proceedings 7th ICCC, Paris, Vol. 3, pp. V68-70. 

Figure 9. DTA curve for a partially converted high-alumina cement concrete from a test drilling Degree of conversion =100 y/(,v+y)...

The particular temperatures at which peaks are observed can be used for the identification of components in a mixture, and the size of a particular peak can be used for quantitative evaluation. Examples include the determination of quartz in clays and analysis of the constituents of polymer blends, both of which would be difficult to achieve by other means. DTA has been used to determine the presence and degree of conversion of high-alumina cement concrete (HAC), a material subject to severe loss of strength under certain atmospheric conditions due to solid-state reactions in the cement matrix. Figure 9 shows a DTA curve for partially converted HAC together with the way the data are used for a conversion measurement. 

Monocalcium aluminate, a major component of high alumina cement, hydrates to metastable products CAHjo, C4AHj3, and C2AHg which eventually convert to the stable cubic aluminum hydrate, CjAH. In the presence of 2-4% SMF, SNF, or modified lignosulfonate, the DTA results have shown that the degree of conversion of CAHjo and C2AHg phases to the cubic phase is marginally retarded. 

Estimates of the degree of conversion of high alumina cement (HAC) concrete can be made from thermal analysis. Midgleyl" provides an expression for the degree of conversion based on the simplified equation 3CAHjo CjAH + 2AH3 + 18H. It is ... 

Figure 8. DTA curves for high alumina cements of different degrees of conversion. Heating rate 20°C/min.t l...

Bentsen, S., Selvod, S., and Sandberg, B. (1996) Effeet of mierosihea on conversion of high alumina eement, in Calcium Aluminate Cements (ed. R.J.Manghabai), E. F.N. Spot , London, pp. 294-298. 

Carbon dioxide dissolves in the pore solution of cement paste, producing CO3", which reacts with Ca to produce CaCOj. The OH and Ca ions required by these reactions are obtained by the dissolution of CH and decomposition of the hydrated silicate and aluminate phases. C4AH. is quickly converted into C ACH, and ultimately into CaCOj and hydrous alumina monosulphate and ettringite yield CaCOj, hydrous alumina and gypsum. C-S-H is decalcified, initially by lowering of its Ca/Si ratio, and ultimately by conversion into a highly porous, hydrous form of silica. The... 

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