The Mullite presence in ceramic materials is always regarded as a favourable advantageous property because it improves the product properties by its great hardness and low coefficient of expansion. The Mullite generation takes place between 1000oC and 1200oC after the producion of a glassy melt in the biscuit and it increases further between 1200oC and […]

Since all the fired clay contains high proportion of Silica it is obvious that its presence in the free Crystalline state must always be expected beside Mullite. The presence of Silica can be demonstrated by X-ray methods. The study on a dilatometer occasionally discloses a well developed Cristobalite effect at its conversion temperature of about […]

The heating effect on Kaolin may be followed by using Electron Microscope. Upto 1100oC we can not observe the breakdown of the hexagonal lamellae. Above 1100oC the fine needles begin to separate and that is the first nuclei of Mullite. When we heat it further we can observe the well formed needles of Mullite scattered […]

The excess Silica in the body materials acts as a strong flux. Clays which contain excess quantity of Silica easily sinters. Since the ceramic materials are not melted in the course of manufacturing, the Aluminum Silicates produced in them arise due to the result of reactions in the solid state. In slagged fire clay bricks, […]

The alumino silicates gained importance in the ceramic field not only as raw material for ceramic bodies but also as new materials often found in the fired wares, were they occur in the form of acicular crystals. They are usually felted together in a state of disorder and occur preferentially as local agglomerations. They were […]

Free silica in the form of cristobalite has been observed in the glass.
Sillimanite is most suitable as a raw material for refractory shapes, since normally it does not need to be pre-fired. With the transformation of other forms to mullite powerful stresses are associated which render pre-firing essential before using the minerals in the […]

The greatest heat evaluation was observed for andalusite followed by kyanite and sillimanite. Sillimanite is the most stable form of these minerals. All of them become instable on heating to a white heat. They are then transformed into a new silicate, 3Al2O3.2SiO2 which is called mullite, and a glassy melt.
Mullite is the most stable aluminum […]

The energy contents of different polymorphous but chemically identical compounds in a system can be determined by comparing their heats of chemical reaction. The most instable form will have the highest heat of chemical reaction where as the most stable form will have the lowest heat of chemical reactions. Since generally the glass obtained by […]

Three naturally occurring important ceramic minerals of silicates of alumina have been intensively studied due to their prime importance in ceramic fields. They all have the formula Al2O3.SiO2 but differ in crystalline character and physical properties. The following are the three minerals:
1. Sillimanite
2. Andalusite
3. Kyanite
The important properties in ceramics for the three minerals studied were
1. The conditions under which […]

At about 900oC to 1000oC an exothermic reaction takes place while heating the clays. Two reactions are considered possible causes of the evolution of heat.
1. Conversion of alumina from the gamma to alpha form.
2. Combination of gamma alumina and silica to form sillimanite or mullite.
It takes place with strong evolution of heat. The second reaction is theoretically […]