The NATURE of PORTLAND CEMENT CLINKER Afodern Afethods
of Research as A p p l i e d
to
its Sfudy
H . G. FISK* Fundamenfal equilibrium studies on the systems limesilica, and lime-alumina-silica, have shmn what compounds are possible in portland cement clinker. Petrographic and X-ray studies have proued the existence of these compounds in commercial cement clinkers. Synthetic studies on the pure compounds and mixtures of them have indkted their functions in portland cement.
The principles of phase equilibria as applied to the portland cement regions of the lime-silica and the limealumina-silica systems are briefy rmkwed. Theapplication of thermo-chemical, petrographic, and X-ray crystal analysis methods to the solution of the problem of the constitutia of portland cement clinker i s indicated.
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the calcination is carried out in large cylindrical kilns which are limed with refractories. These rotary kilns are inclined at a slight angle from the horizontal so that the burden or charge in the kiln is carried continuously forward through the hot zone, and discharged at the lower end as clinker. The clinker thus produced consists of varying sized pellets which are composed of a fine-grained (microscopic) mass of minerals. Due to a number of fundamental studies our knowledge of the chemistry of the formation of clinker is well advanced. As a result of researches on the pure compounds prepared synthetically, and mixtures of these, it is now fairly well understood which compounds are responsible for the various properties of cement.
ORTLAND cement is the product obtained by linely grinding clinker and adding to it a small percentage of powdered gypsum. Portland cement owes its properties of setting and hardening to the presence of the silicates and alumiuates or ferrites of calcium; which compounds react with water. When portland cement is mixed with properly sized and proportioned sand, aggregate, and the correct amount of water, and allowed to harden the resulting product is our well-known building and structural material, concrete. Clinker is manufactured by calcining a properly proportioned and intimately associated mixture of calcareous and argillaceous constituents. In this country
Section
I
Chemistry of P o r t l a n d Cemenf Clinker Formationfrom the Standpoint of Equilibrium Diagrams In the formation of clmker from its raw m a t e r i a l s limestone and clayey material or slag-new chemical compounds result, and these compounds have chemical and physical properties decidedly diierent from those of the original raw materials. The changes have been brought about through the agency of heat. In fact, high temperatures, around 1450°C., are necessary to complete the reactions. Silicate chemistryt is closely allied to mineralogy, as many of the compounds formed in silicate reactions occur in nature as minerals and others not found in nature have definite physical properties, and can best be investigated and studied by the methods of determinative and synthetic mineralogy. The application of physical chemistry to the science of mineralogy and
the chemical problems of geology has been very valuable, and is essential in the study of silicate chemistry. In addition to the well-known laws of chemistry, such as the laws of the conservation of matter and energy, and the mass law, there are two general principles of fundamental importance in silicate chemistry. The first of these is called Ostwald's rule and states that a substance tends to pass from a less stable to a more stable condition through successive changes of decreasing instability. The second of these laws is known as the principle of Le Chatelier which states that three factors or conditions govern a system. These factors are concentration (or composition), temperature, and pressure. Further, the principle of Le Chatelier states that if one of these factors is changed, the other factors will tend to adjust themselves so as to * Cement Co.* offset or counteract the effect of the change. In silicate BuEingon, Indiana. t ~h~~ branch of chemistry dealing with -ctions and prac- chemistry the small changes in pressure ordinarily esses involving the compounds of silicon (and in general reencountered have such a slight effect that for most actions which are carried out in the dry s t a t e t h a t is, those purposes pressure may be and its effect in watv salutions do not play an important part) is somenot be taken into account in the following discussion. times called "silicate chemistry."
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In considering silicate systems it is necessary to distinguish between phases and components. A phase is a homogeneous, physically distmct, mechanically separable portion of a system. There may be more than one solid phase in a system, and a t times more than one liquid phase (liquid immiscibility), but never more than one gas phase. The components of a system are the constituents, the concentration or amount of which can undergo independent variation in the different phases.
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ther rise in temperature until it melts to a glass a t about 1713°C. If a suitable flux is present, quartz can be converted by prolonged heating between 870°C. and 1470°C. to another form of silica known as tridymite. These are not the only changes which take place in the silica minerals with changes in temperature, but they illustrate the effect of that single variable, temperature, on a system of one-component. The changes in the silica minerals are very important in silica brick which is widely employed in the steel industry.
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