Expansive type inorganic binders (cements)

Today, in ceramic forming processes two types of binders are used. These are organic and inorganic binders. Cement is an inorganic type binder and is used widely in ceramics, refractories, concrete productions and road filling operations. Several chemical reactions occuring in time in these materials could cause various damages. The most preminent among these is the delayed ettringite formation (DEF) which can result in huge volumetric expansion and thus cracking. Ettringite containing a very high percentage of chemical water is a calcium aluminate sulfo hydrate compound and forms by a topo-chemical reaction in internal voids perpendicular to void suface and cause a large increases in apparent volume and generates huge stresses which finally lead to cracking of these materials. In order to prevent defectes associated with the DEF, a great deal of reseach have been done on the mechanism of ettringite formation in recent years.On the other hand, in casting industry one of the most difficult task is the production of parts with sufficient dimensional accuracy. Most metals and their alloys exibit shrinkage during the course of solidification and further cooling period to room temperature. This phenomena results in a size reduction of parts produced by casting processes. Generally, shrinkage allowances are given to models, but in some cases such as dental and body implants the only way to meet the solidification shrinkage is the usage of the thermal and setting expansion techniques. Mechanism of thermal expansion has long been used in the foundry industry by using minerals such as quartz and cristoballite which show inversion and volumetric expansion at elavated temperatures whereas usage of setting expansion is a rather new phenomenon and is nowadays widely practiced by dental and body implant producers and jewellerry industry.In the present work, an attempt has been made to produce a special type of cement bonded investment powder which expands significantly on setting to meet the thermal shrinkage of metals and alloys taking place in the course of solidification.Experiments were carried out in three stages; in the first step, tricalcium aluminate (C3A) was produced. This compound plays a great role in the formation of high volume hydrates during the setting process. CaCO3 and Al2O3 was mixed homogeneously (the weight ratio is 3:1) and then subjected to calcination (1200-1300oC) and to clinkering operation at high temparetures (1350oC) for five hours. In the second stage, an expansive type mixture was produced by mixing C3A, quartz and plaster of paris. Small amounts of various control additives, such as wetting agents, accelerators, inhibitors and antisettling agents were also used to impart various properties. In the final stage of investigation, a number of invesment casting molds were produced from various invesment powders with vastly different setting properties. An Al-Si type Etial-140 alloy was melted and then poured into these molds and dimensional changes were determined for each casting.In conclusion, it is believed that a new and expansive type cement bonded investment powder which can be used succesfully in producing cast parts with great dimensional accuracy was developed.