Development of engobe coatings based on alkaline kaolins

Abstract

The properties of alkaline kaolins as promising raw materials during manufacturing engobe coatings for construction ceramics were studied. The chemical and mineralogical composition and peculiarities of thermal transformations of kaolins were determined; the compositions of engobe coatings were developed, their rheological properties were studied and physical and ceramic indicators after annealing were determined; the peculiarities of the formation of the coating structure were established. The relevance of expansion of the raw material base is very high, since existing formulations of ceramic masses and coatings include mainly high-quality clays, kaolins, feldspars, quartz sands and others. Natural supplies of such raw materials are getting rapidly exhausted, which negatively affects the results of production. Thus, the search for alternative raw materials should take into account not only its availability, but also the ability to ensure high quality of ceramic products. The research results revealed that alkaline kaolins are complex raw material containing kaolinite, quartz and feldspar minerals (microcline or albite), that is why they can replace the listed materials that are introduced into the composition of engobes by separate components. To correct the rheological properties of the developed engobe coatings, it is possible to use traditional electrolytes – rheotan and liquid glass – in the amount of up to 0.7 % by weight. During the thermal treatment, alkaline kaolins sinter actively at the temperatures of 1,100–1,150 °C and contribute to the intensive formation of the strong stone-like engobe layer with water absorption of 3–6 % on the product surface. This coating, in addition to creating a decorative effect, increases the durability of construction products on average by 30–35 %. The obtained data have both scientific and practical significance, as they allowed substantiating the expediency of using alkaline kaolins as complex raw material in the production of engobe ceramics. This makes it possible to reduce the number of separate components in the composition of coating and to intensify sintering of engobe, which generally leads to the improvement of product quality