Sintering behaviour of calcium exchanged low silica zeolites synthesized from kaolin

The novel route for the synthesis of aluminosilicate ceramics through zeolite precursors is found to have certain advantages over the traditional methods using oxides, kaolinite etc. and the non-conventional use of sol–gel precursors. The published work in this area consists of sintering the ion exchanged low silica zeolites prepared from commercially available zeolites. Generally, zeolites are manufactured by crystallisation of sodium aluminosilicate gel prepared from different aluminium and silica sources. The present paper deals with the ion exchange and sintering of calcium exchanged zeolites A and X synthesised from a locally available china clay. Kaolinite is a combined source of silica and alumina and the zeolites prepared from it are always contaminated with trace amounts of Fe, Ti, Ca, Mg etc. which are originally present in the naturally occurring clay as ancillary minerals. A systematic study has been done on the calcium exchange of kaolin based zeolites A and X, sintering of these modified zeolites at various temperatures and measurement of properties of the sintered products. The effect of rate of heating on the properties was also investigated. It has been found that the zeolite structure is not lost during ion exchange and remains somewhat the same on sintering up to 750°C. Further heating results in the crystallisation of anorthite resulting in an anorthite–glass composite material. A firing rate of 3°C/min resulted in a material with large volume expansion, high porosity and water absorption and low density. Reducing the rate of heating to 1°C/min, a normal sintering behaviour was observed to give products with high shrinkage and density and good hermeticity. However, the strength of the products was found to be quite low. For a comparative study, the two zeolites were synthesised from sodium aluminate and sodium silicate and the ion exchange and sintering were carried out in a similar way. The preliminary results showed that there are some differences in the sintering behaviour indicating the possible influence of the origin of the zeolites.