1 Starting situation
Clay masonry bricks are construction materials with a long tradition. The further development of their product properties is the subject of a wide range of efforts. To confirm the sustainability of these construction materials, the focus is now on their recyclability as a new aspect. This applies especially to the lightweight clay bricks filled with thermal insulation materials that are used for exterior walls. As interior plaster, gypsum-based construction materials are often used on account of their construction physics properties. With regard to recycling, the mineral thermal insulation materials constitute impurities, while the gypsum is undesirable both in respect of application and water management perspectives. In earlier investigations, it could be proven that thermal insulation materials can be separated from the coarsely comminuted bricks by means of air separation . For the particle sizes < 8 mm, that is not possible. For gypsum plasters, no suitable, mechanical separation process is available. Their removal prior to deconstruction is feasible, but a very laborious and costly process.
One approach that enables the recycling of gypsum-containing clay brick waste is its utilization as a raw material for the production of lightweight aggregate with product properties corresponding to those of standardized lightweight aggregates like expanded clays. The approach builds on the authors’ own studies on the utilization of brick-containing construction waste , , , . According to this, masonry rubble with a brick content of at least 50 mass% can be processed in a thermal process to lightweight aggregate. The starting material is crushed, ground, doped with an expansion agent and granulated on a pelletizing disk. During subsequent thermal treatment, the green granulates are stabilized and simultaneously expanded at temperatures from 1 150 to 1 250 °C. The requirement for the expansion agent is that, in the temperature range of melt phase formation, this decomposes with the release of a gaseous component.
In the following, studies are reported in which firstly a model wall consisting of vertically perforated clay bricks with gypsum plaster was used as starting material for the production of lightweight aggregate. Secondly, clay bricks filled with thermal insulating materials were used with the addition of gypsum as secondary raw material.
2 Current knowledge
Current knowledge on the genesis of lightweight aggregates is based primarily on granulates made from clays. The expansion process can only take place if a sufficient quantity of melt phase with appropriate viscosity is present in the temperature range in which the expansion gas is released. An approximate estimation of the potential of a starting material with regard to melting phase formation is possible on the basis of the content of the oxides SiO2, Al2O3, Fe2O3, CaO, MgO, K2O and Na2O. The viscosity of the melt depends on the ratio of the content of fluxing agent to the content of free quartz. Different chemical reactions are responsible for the formation of the expansion gas. Identified as the primary source of expansion gas source are the redox reactions of iron oxides with organic carbon, when these take place in the temperature range of the pyroplastic state.
To verify the suitability of clay raw materials and that of waste for the production of lightweight aggregates, generally the ternary diagram published by Riley in 1953 and supplemented by White in 1960 is used , . The Riley diagram is based on measurements of the bloating, that is expansion behaviour of 39 clays from the Midwestern United States. The lightweight granulates produced from these exhibit densities between 290 and 990 kg/m³. To better represent the specific role of iron oxides during the expansion process, this diagram was further developed by Cougny to a quaternary diagram in 1990 .
The idea of separating the gypsum with this process is based on fundamental investigations into the thermal decomposition of calcium sulphate. From the investigations conducted at beginning of the previous century, which remain our current level of knowledge even today, it is known is that the thermal dissociation of anhydrite only takes place above 1 200°C . In the presence of other oxides, the decomposition is shifted to lower temperatures:
Dissociation of anhydrite in the presence of silicic acid
CaSO4 + SiO2 CaSiO3 + SO2 + 0.5 O2
Start of decomposition at 1 000 °C, end at 1 250 °C
Dissociation of anhydrite in the presence of iron oxide
CaSO4 + Fe2O3 CaO*Fe2O3 + SO2 + 0.5 O2
Start of decomposition at 1 100 °C, end at 1 250 °C
Providing these temperatures apply, the reduction of the sulphate content and the volume increase caused by the expansion process are not prevented and can be utilized for the gypsum separation with the simultaneous production of expanded granulates.
3 Characterization of the starting materials
The following starting materials were supplied by the gypsum and the clay brick and tile industries:
A model wall built with vertically perforated clay bricks and plastered with gypsum plaster (»Fig 1)
One pallet of clay bricks filled with rockwool and one pallet of clay bricks filled with perlite (»Fig 2), to which crushed gypsum moulds were added.
The clay bricks filled with rock wool consisted of 80.0 mass% clay brick and 20.0 mass% rock wool. The bricks filled with perlite consisted of 88.2 mass% brick and 11.8 mass% perlite. Different gypsum components of 5.0 and 15.0 mass% in the form of crushed gypsum moulds and gypsum plasters were added to the filled clay bricks supplied.
For the preparation of the starting mixes, the supplied materials were crushed in a jaw crusher. Then the silicon carbide SiC used as expansion agent was added. The mixes were ground in a ball mill and homogenized in the process. After grinding, in the case of model wall, 98 vol% of the starting material is in the particle sizes < 63 µm. In the case of the clay bricks, the content < 63 µm was 90 vol% for the clay bricks filled with rock wool and 85 vol% for the clay bricks filled with perlite.
The chemical compositions of the starting materials (»Table 1) differ in respect of their Al2O3 and CaO content. The Al2O3 content is somewhat higher in the clay bricks filled with perlite, the CaO content somewhat lower. The amount of gypsum added is reflected in the increase in the SO3 content.
A first assessment of the suitability of the starting materials for the production of expanded granulates comes from the ternary diagram for SiO2 – fluxing agent FA – Al2O3. The materials used here lie completely inside or slightly outside the areas typical of expanded clays (»Fig 3). With the addition of gypsum, a slight increase in the CaO content results. The composition shifts slightly in the direction of the “fluxing agent corner”.
To be able to use the thermal process of lightweight granulate production for desulphation, the gypsum plaster dewatered to anhydrite must be decomposed in a temperature range that is not above the expansion range caused by release of the expansion gas. That was verified with the help of differential scanning and hot stage microscopy analyses. In the hot stage microscopy analyses, a cylindrical specimen is heated in a tube kiln and the area change of its shadow determined as a function of the temperature with the help of a camera and plotted by the device software. The temperatures at which characteristic shapes taken from slag research occur  are output, as are the entire measurement data on the area changes as a function of the specimen temperatures. The results of the differential scanning analysis and the hot stage microscopy are shown by way of example for the model wall built with vertically perforated clay bricks in »Fig 4 and »Fig 5. According to these, decomposition of the CaSO4, which originates from the gypsum plaster, takes place in the temperature range from 975 to 1 100 °C (»Fig 4). At a spherical temperature of 1 186 °C, the cylindrical green granulates take a bead-like form (»Fig 5 top). The increase in area reaches a first maximum at 1 147 °C. It is completed at 1 226 °C (Fig. 5 bottom). Accordingly, the above-mentioned condition is fulfilled.
4 Production of expanded granulates in the IAB pilot plant
4.1 Process steps and aggregates used
In the first process step, the dried and coarsely comminuted starting materials were ground in batches of 200 kg each in a ball mill with a grinding chamber volume of 500 litres (»Fig 6). Then the green granulates were prepared on a granulating pan with a diameter of 1.0 m (»Fig 7). After being redried, these granulates were fired in a natural-gas-heated rotary kiln (»Fig 8). The rotary kiln has an inside diameter of 0.60 m and is lined with a 0.15-m-thick refractory mortar layer. On account of the structural conditions, its length is limited to 6 m. The inclination can be varied in steps 0.5° / 1° / 2° / 3° and the speed of rotation between 0.3 and 3.0 revolutions per min. The kiln can be operated in a temperature range from 500 to 1 500 °C. It is equipped with numerous measurement devices for temperature, pressure and exhaust gas composition. The flue gas is cleaned in a bag filter with the addition of slaked lime.
From the experience in the preparation of the lightweight aggregates in the rotary kiln described and from the temperature and residence time measurements conducted parallel, it was established that dwell time in the firing zone is too short to fully exploit the potential for volume expansion (»Fig 9). Moreover, for this a second and sometimes a third kiln cycle was necessary. In the case of the granulates from the filled clay bricks, on account of damage to a burner lance and the associated change in the temperature profile, this procedure did not have the desired effect of an increase in the granulate volume or reduction of the density. To ascertain the actual expansion potential, an additional thermal treatment was realized in the muffle kiln.
4.2 Properties of the lightweight aggregates produced in the rotary kiln
In the pilot plant, from the model wall and the two brick-insulation material mixes with the different gypsum additions of 0, 5 and 15 mass%, around 200 kg expanded granulate per batch was prepared in each case (»Fig 10).
The sulphate contents of the granulates from the model wall fired in the rotary kiln decrease compared to the sulphate contents of the green granulates (»Table 2). Already after one firing in the rotary kiln, values below 0.7 mass% were achieved, which further decrease in the second firing. The type of brick filling did not have any influence. For lightweight and for recycled aggregates intended for use in lightweight concrete production, e.g. for masonry bricks, the content of acid-soluble sulphate should not exceed a value of 0.8 mass% , . This limit value is reliably complied with. The concentrations of elutriable sulphate of the lightweight aggregates prepared from the model wall, which were determined on a “LAGA” eluate  with a water-solids ratio of 10 to 1, were below 50 mg/l (»Table 3). Accordingly, use of the material in unbound road construction courses, which is another application for lightweight granulates is also possible without any restrictions.
The densities of the granulates prepared from the model wall without the addition of SiC range between 2.1 and 2.2 g/cm³. With the addition of SiC, a considerable decrease in density to 0.73 g/cm³ is achieved (»Table 4).
The densities of the lightweight aggregates from the filled bricks did decrease compared to the densities of the green granulates, did not, however, reach the expected values on account of the above-mentioned reasons. (»Table 5).
To test what expansion potential is still present after the rotary kiln firings, in addition to the tests in the pilot plant, thermal treatment was conducted in the muffle kiln (»Fig 11). Independent of the pretreatment of the granulates fed to the muffle kiln – untreated green granulates, rotary kiln granulates after one firing, rotary kiln granulates after two firings, densities well below 1 g/cm³ were reached. The bricks filled with rockwool exhibited a lower density than those with perlite filling. That agrees with the information obtained with the hot stage microscopy.
From the secondary raw materials that will in future come from “clay bricks filled with rockwool” or “clay bricks filled with perlite”, lightweight rock aggregates with low to very low densities can be produced.
Gypsum-containing masonry waste or clay bricks filled with insulation materials and covered with gypsum plaster can hardly be utilized at present and therefore have to be landfilled. An alternative is the utilization of these materials as a raw material for the production of lightweight aggregates in a thermal process. From bricks filled with rockwool and with perlite, to which 5 or 15 mass% gypsum was added, such lightweight aggregates could be produced in the pilot plant at IAB Weimar gGmbH. The sulphate content of these aggregates from the firing process was, even with a gypsum addition of 15 mass%, below the content of acid-soluble sulphate of 0.8 mass% that must be met by lightweight or recycled aggregates. With less than 50 mg/l, elutriable SO32- was practically no longer existent. The densities corresponded to those of commercial lightweight aggregates. With this and the knowledge available at the IAB, the next step – the erection and operation of an industrial-scale plant – moves into feasible reach.
The studies were conducted within the scope of the programme “FuE-Förderung gemeinnütziger externer Industrieforschungseinrichtungen – Innovationskompetenz” (R&D Support of Not-for-Profit External Industrial Institutions – Innovation Expertise) initiated by Germany’s Federal Ministry of Economic Affairs and Energy under the project sponsorship of EuroNorm Gesellschaft für Qualitätssicherung und Innovationsmanagement mbH.
The project was stewarded and financially supported by specialists from industry and associates of the clay brick and gypsum industry. The authors would like to express their thanks for their great interest as well as the critical and constructive discussions during the project meetings.