Soils used for manufacturing burnt building materials contain compressed carbonate grains, most often calcite or dolomite, of various sizes, which coagulate from leaking solutions when the partial pressure of CO2 is decreased. During processing in the brick industry, these grains (mainly the coarser ones, called loess nodules in practice) have an adverse effect on the quality of the final products.
They calcinate during heat treatment (burning) while calcium oxide (CaO) and magnesium oxide (MgO) are formed. These grains also shrink because of the smaller molar volume of the oxides formed by burning. The amount of free calcium oxide and magnesium oxide depends on the raw material used, the level of size reduction and the burning conditions characterized by the so-called firing curve. If building materials with higher contents of these oxides are used in conditions where they are exposed to moisture action, CaO and MgO are hydrated and the hydration speed depends on the burning conditions which determine the surface and thus the activity of burnt lime grains. Hydroxides (calcium and magnesium hydroxides) are formed by hydration and this process is connected with a volumetric expansion of the hydrating grain. The stress that causes this expansion in a building material body reduces its mechanical strength.
New types of products are appearing now and they combine thin-walled building materials with cavities filled with foam-silicate and polystyrene based insulating materials which are treated under hydrothermal conditions. The action of these conditions, particularly moist steam at elevated temperatures, significantly intensifies the hydration processes in the oxide grains contained in the body. Strong alkali solutions reacting with the body phases are also formed by water condensation in the body and they appear as inactive during slow hydration under normal conditions. Both these factors, i.e. the increasing hydration speed and the formation of new-forming phases, significantly increase the adverse effect of carbonates in a raw material on the properties and life of the burnt body.
In their research, doc. Ing. Petr Ptáček, Ph.D., Ing. Tomáš Opravil, PhD., and Ing. František Šoukal, PhD. from the Faculty of Chemistry focused on eliminating these adverse effects by reducing the size of the lime grains, because with their additional hydration in the body, finer grains generate lower stress. Due to their larger surface, they also react more easily during burning and the presence of finely dispersing carbonates in a raw material may even have a positive effect since they help to eliminate organic substances from the body and reduce the possibility of so-called black cores forming.
The subject of solving this is in treating the working mixture for manufacturing burnt building materials by adding inorganic and/or organic compounds of Zn2+ and/or B3+ ion of at least 0.25% of the mixture weight. This treatment effects the phase composition of products in such a way that the temperature range of the formation of carbonate calcination products is reduced and there are fewer of them in the final product which reduces their undesired effect. More details can be obtained from the records of the Industrial Property Office where the patent is published.
Author of the paper: author collective from the Faculty of Chemistry
Source of the introductory picture: commons.wikimedia.org
http://commons.wikimedia.org/wiki/File:Stapel_bakstenen_-_Pile_of_bricks_2005_Fruggo.jpg
