The increased demands in terms of the quality of vertically perforated bricks call for the brick industry to manufacture products with very good thermal insulation properties and sufficient strength. Mixed pore-forming is gaining in significance to increase the thermal insulation of vertically perforated bricks with low body bulk densities. This is achieved by the combined use of organic pore-forming agents (swellable paper-making residues, fine sawdust) and inorganic void and pore formers (moler earth, foam glass balls, zeolite, aluminium hydroxide and glass or mineral fibres) as well as the addition of mineral-phase and glass-phase forming additives. Prerequisite for changes in the raw material composition is that the firing process is adapted to the raw material mix. This is the only way that the sequence of exothermic and endothermic combustion and decomposition reactions on the one hand and the body forming sintering reactions to achieve the required product properties on the other hand can be guaranteed also under fast firing conditions. For its technical realizability, the fast firing of lightweight vertically perforated bricks requires an optimation of the fast firing process leading to a situation where the inner surfaces of the brick can take part in the heat and mass transfer – especially in the case of raw material mixtures with a high energy load.
The research project pursues the following question: How can the formation of the fired clay body, respectively the most important product properties (e.g. body bulk density, strength and thermal conductivity, be influenced through extremely fast heating up times and therefore altogether through shorter total firing times? For this purpose, for highly porous green bricks made of raw material mixtures with different mineralogical composition, the firing behaviour (such as expansion and shrinkage processes) and the endothermic and exothermic reactions aredetermined under fast firing conditions. Furthermore its influence on the product properties is determined in comparison with the conventional tunnel kiln.
3 Results and conclusions
In fast firing (S) of vertically perforated bricks, a smaller volume shrinkage occurs in comparison to conventional tunnel kiln firing (N) through the concluding sintering processes. This is expressed in a slightly higher porosity and a lower body bulk density. After firing, the brick body has a pore system with a comparatively higher fraction of smaller pore diameters and a larger specific pore surface. The formation of crystalline mineral phases is reduced and the fraction of X-ray amorphous phases is increased. The latter contain both minerals with disordered lattice structure as well as glass phases occurring through the formation of partial melts. Through higher firing temperatures and correspondingly also slightly longer firing times in the upper temperature range, the formation of crystalline mineral phases is increased and the pore distribution shifts in the direction of larger pore diameters. The differences in the fractional quantities of mineral phases and amorphous constituents are shown in »1.
The thermal insulation of the bricks fired in significantly shorter times tends to be higher. This is indicated by comparatively lower heat conductivities, which can be verified unanimously for all mixtures tested here. On the basis of the heat conductivities determined through individual brick measurement on conventionally fired bricks, the increase of the thermal insulation amounts to 4 to 8%, in individual cases even 12%.
In fast firing, through the development of reaction gases, reducing atmospheres in the green brick are evoked verifiably in partial areas of the preheating section, which accelerate the sintering process. These processes determine the physical properties of the body, which, in comparison to oxidizing firing, can already be achieved at low temperatures. At least, through the acceleration of the reactions under reducing conditions, the only short dwell times in the temperature range of the sintering process during fast firing are compensated, in comparison to the longer firing times of a conventional firing process. This is one of the causes for the fact that the compressive strengths of the bricks deviate only negligibly from those of the conventionally fired bricks. In individual cases even higher compressive strengths are achieved.
When implementing the determined relations between the type of pore and void formers and the heat conductivity as well as the effect of fast firing on the thermal insulation of the bricks, the following must be taken into account: The heat transport responsible for the heat conductivity through the vertically perforated brick takes place mainly via three transport mechanisms. These are: The heat conduction through the body structure forming the brick skeleton, the heat conduction through the air in the perforation chambers and the heat radiation from web to web through the perforation chambers. The effects of these mechanisms are, in terms of magnitude, approximately equivalent, so that the optimization of the heat conductivity of the body by measures relating to the raw materials and the firing technology can only exert an influence of approximately one third on the heat conductivity of the bricks.
According to expectations, the thermal insulation of the bricks, which was improved within the scope of this project by means of fast firing, is not to be described as unusually high. However it creates security in the fulfilment of the requirements for thermal insulation properties of vertically perforated bricks. The combined use of suitable organic pore-forming agents and inorganic lightweight materials as well as a firing process adjusted to the raw material mixture create the prerequisites for the production of bricks with extremely low heat conductivities and sufficiently high strengths.
This project was conducted by the Research Alliance of the Brick and Tile Industry Regd (FGZ). Under project number AiF 15252 N it was funded by the Federal Ministry of Economics and Technology (BMWi) through the German Federation of Industrial Research Associations “Otto von Guericke” Regd (AiF) and implemented by the Brick and Tile Research Institute Essen Regd (IZF).
For a handling fee, the 88 page long final report is available from the Research Alliance of the Brick and Tile Industry Regd in Berlin.