clay was also applied as an admixture. In addition, the Vuppor organic admixture 6 was used to modify the pore H igh f rost resistance is one of the signs of good quality structure of test specimens. in clay roo ng tile products. T he present paper deals with the question of whether the water absorption of clay roo ng tiles aVects f rost resistance. T he results PREPARATION OF SPECIMENSshow that clay roo ng tiles with high water absorption Mineral admixtures were added to the brick clay in doses can be just as f rost proof as tiles with lower absorption.of 1 or 10 wt-% referred to the weight of dried clay, along M edian pore radius is found to be an important factor with Vuppor admixture at a constant dose of 1 wt-% of in determining f rost resistance: increasing the median dried admixture to dried clay. Test specimens (100Ö pore radius increases frost resistance. BCT /561 50Ö 20 mm) were manufactured from a plastic mass (upsetting height 24 mm according to PfeVerkorn test), held T he author is in the Department of Materials for 48 h at laboratory temperature (20°C), and then dried Engineering, Faculty of Civil Engineering, Slovak at 110°C to constant weight. Specimens were red in an University of T echnology, R adlinského 11, SK -813 68 electric laboratory furnace at 900°C with a dwell time of 1 h. Bratislava, Slovak R epublic ( sveda@svf.stuba.sk). M anuscript absorption N V (wt-%) is de ned as 100(m n m s )/m s , where the Institute of M aterials, M inerals and M ining.m n is the saturated weight of the specimen after 4 h submersion in boiling water and m s is the dry weight of the specimen.

The aim of the article is to find the optimal fluxing agent for porcelain body regarding to the possibility of the lowest firing temperature. Sintering behaviour of dry pressed test samples made from the mixture of kaolin and three different types of industrially milled feldspar rocks, bone ash and quartz sand with similar granulometry as dependence of water absorption on the firing temperature was investigated. The most intensive fluxing agent for the sintering is bone ash-the mixture containing bone ash (20wt.%) showed sintering temperature 1200 • C. That is about 50 • C lower compared with the most intensive feldspar based fluxing agent-potassium feldspar rock containing 75% of pure microcline.

No abstract

T he present paper extends previous findings already published where the relationships among total porosity, bulk density, compressive strength, and thermal conductivity were analysed. T he paper brings more accuracy into some original mathematical relationships and takes into account two additional physical properties (pore volume and the median pore radius). It has been found that the relationships between physical properties and a dose of any admixture added to brick clay could be better described using a quadratic function. T he relationships between pairs of physical characteristics can be described by three types of mathematical function (linear, exponential, and power). T he amount of admixture used, the firing temperature, and type of clay aVect the value of the coeYcients but not the type of functions.BCT /371

This study investigated the effects of three pore-forming agents on the properties of the fired clay body applied in the production of lightweight bricks for the building envelopes. Test samples were made from clay raw material already containing two combustible pore-forming agents (sawdust and cellulose sludge). A part of this research was focused on studying the influence of adding two combustible pore-forming agents (molasses and cornstarch) and a chemical additive Vuppor to the claw raw material. Testing of the material properties showed that although the three samples had almost the same pore volume, their thermal conductivities varied. These findings led to an important conclusion. The pore size of 1–200 µm (filled with not only air but also biomass ash) reduced the thermal conductivity, and conversely, an increase in the small pore size less than 1 µm increased the thermal conductivity of the fired clay body.