The Phase Systems Important for Cement Chemistry

The chemical composition of clinker is complex, however, it is easy to notice that the sum of four components, CaO, Si02, AI2O3 and Fe203 is as a rule higher than 95 %. The processes of Portland cement clinker phases ciystallization in the four components system can thus be presented. It is especially justified that MgO is not forming own compounds in the rich in calcium part of five-components system, but solid solutions with remaining clinker phases, or is present as periclase. However, the four-components system is seldom used, because the use of ternary systems is much more convenient. They are usually presented as the horizontal projection. 

In order to understand this the most important ternary system in cement chemistry the two forming its sides two-eomponents systerrrs should be discussed Ca0-Si02 and Ca0-Al203. 

Analogically, from the melt of the composition given by point S (Fig. 2.11), after achieving the liquidus temperature (T,), phase CaO is crystallizing, and the melt composition is changing along the liquidus crrrve till the point M This composition the melt will attain at 2070 °C. At this temperature the meritectic reaction starts  

It will proceed at constant temperature, as an exothermic reaction. The system will be invariable, as a result of Gibbs s phase rule.  

One can reminds that the phase rule gives the equation  

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The most frequently we use the ternary system horizontal projection, on which the most important isotherms are marked. On the Fig. 2.14 the section of this system is presented, on which, apart of earlier discussed two-component compounds, mull-ite, without practical importance for cement chemistry, is shown. However, it is present in siliceous fly ash, but it belongs to inert compounds does not reacting with calcium hydroxide in water. Furthermore two ternary phases are formed in... 

In heterogeneous systems where several materials are present the existence and variability of these substances can be determined by the application of the phase rules. For the Ammonia Laboratory the existence of phases was especially important to the chemistry of fertilisers, which were often mixtures of various salts. In September 1920, Ernst JSnecke, then professor at the Hannover Technische Hochschule, joined the staff of the Ammonia Laboratory to study the phenomenon from its theoretical side. He began with investigations on ammonium sulphate and nitrate, and the sodium chloride/ammonium nitrate system. His group members (in 1922 four scientists) did more technical work in cement and salt chemistry. In Germany, theoretical work on the existence of phases did not play a major role at the universities. With the work of Janecke, I.G. Farben had access to the theoretical underpinning of the important fertiliser business. Janecke retired in 1935. ... 

In principle, all the concepts of surface chemistry previously described are also expected to be vahd for cement particles. However, the highly dissolving and reactive nature of such particles introduces a dynamic and complex feature to the particle-water system. This makes it difficult to interpret the data obtained from cement-containing suspensions, which is probably the main reason for the paucity of publications available in the literature that focus on the interactions between additives and high-alumina cement particles. The concepts described herein are therefore based on sffidies performed in aqueous pastes of calcium silicate (Portland) cement, which have been more extensively researched in recent years due to their considerable importance for concrete technology (47-50). Although calcium aluminate and calcium silicate cements are known to display distinct reaction rates and form different hydration products, similar interactions between the cementious phase and the chemical additives (admixtures) are expected to occur in aqueous suspensions. 

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