The progressive changes in ceramic raw materials during firing processes are
a complex area. This is partly due to the large number of raw material
characteristics, primarily mineral composition, and partly to the relatively
inadequate particle distribution in the unfired clay body. The most important
starting point is always the optimal raw material composition which should
give appropriate physical and mechanical characteristics to the final
products after firing processes and should provide an efficient and
economical production. The paper analyzes the influence of some additives
(fly ashes and waste glass materials) on the development of the ceramic
roofing tile microstructure during the thermal treatment. The analyzed raw
material mixtures were: the standard raw material mixture (from Kanjiza,
Northern part of Serbia) and the modified one, i.e. the mixture of the
standard raw material and corresponding additive. The silica phase obtained
during the thermal collapse of the clay minerals in the presence of the glass
additive bounded better CaO and MgO components released from the carbonates.
The crystalline phases like plagioclases were performed in a considerable
quantity and the products with new physical characteristics were formed.
The processes of simultaneous moisture and heat transfer, which are often nonstationary, and the distinct nature and properties of the material to be dried complicate the description of the drying process. The theory of moisture migration and modeling of drying process has been the subject of many studies. Three theories, the difusion, the capillary low, and the evaporation-condensation, have won general recognition for the explanation of moisture transfer in porous media. This study has several objectives. The irst one was to present a new method for calculation of the variable efective difusivity as well as to identify diferent drying mechanisms and its exact transitions during isothermal drying of clay tiles. The second and main objectives were to analyze all obtained isothermal data, to create a link with the comprehensive theory of moisture migration during drying, and to set up the non-isothermal drying process. The procedure was based on the principle of controlling the mass transport during the drying process. Proposed regimes were consisted from several isothermal segments. Isothermal segments were selected and speciicated in accordance with the clay raw material nature and the moisture migration theory.
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