Where is autoclaved aerated concrete used?

The world's largest aerated concrete plant is in Hungary

In 1985, the then state-owned plant was built near Gyöngyös, around 90 km northeast of the Hungarian capital Budapest, in an industrial park. The technology, including the machines and systems, came from the company Hebel. The location was favorable: right next door is the second largest power plant in the country - a lignite power plant. It supplied the fly ash, which was initially used for the production of aerated concrete from the summer of 1986. However, in order to be able to produce higher quality wall building materials, the switch to sand took place in 1990. In 1992, the plant was sold to the Ytong company as part of the privatization process, and two years later it was rebuilt as part of a major investment to manufacture the Ytong core range. As a result, the plant was able to produce in four shifts and produce around 430,000 m³ of aerated concrete per year.

In order to be able to meet the growing demand for aerated concrete in neighboring Romania, the capacity of the plant was expanded by a further 100,000 m³. The necessary renovation work was carried out in January 2005 as part of a major repair in just 24 days. The implementation of this project was made possible by the installation of a new cutting system. In contrast to most of the aerated concrete plants in the Gyöngyös / Halmajugra plant, it was not the capacity of the autoclaves that was decisive for the production bottleneck, but the efficiency of the cutting system. In addition, investments were made in a fully automatic oil and mold cleaning system. The result was a reduction in cycle times to less than 3 minutes per mold.

The plant in Halmajugra, which has been part of the Xella Group since 2003, is a superlative plant: around 14,000 m² are built on the more than 200,000 m² plant area, and 63,000 m² storage space is available for Ytong aerated concrete products. Mainly flat stones (smooth and NF + GT) in the bulk densities P1.6-0.3 are produced here; P2-0.4; P2-0.5 and P4-0.6 with lengths of 500 mm and 600 mm, heights of 200 mm and 250 mm and widths from 50 to 375 mm. Around 90% of the production corresponds to the stone strength class P2-0.5. Special products such as lintels, U-shells, corner perforated stones or ceiling parking stones are also manufactured.

The recipe

In addition to water, the aerated concrete at the Hungarian plant essentially consists of quartz sand - up to 73% in most products. In order to produce even lighter stones, the proportion will be increased further. Portland cement is used as a binding agent. Lime and an anhydrite in the form of gypsum are also required. The gypsum is obtained as FGD gypsum from the power plant next door, which also supplies the electricity. REA gypsum is a high quality gypsum that is used in the R.also gasentsulphurisationapower plant systems are built. In addition, aluminum is added to the raw mixture, which acts as a propellant. For this purpose, two aluminum pastes with different properties are mixed together and processed in a certain mixing ratio per m³ of aerated concrete.

The quality of the aerated concrete is largely dependent on the fine grain of the sand. Therefore, the quartz sand is ground before further processing. This is done in a ball mill that can process around 30 tons of sand per hour using the wet grinding method (Fig. 1). Another, smaller mill is also available as a reserve to cover peaks.

The fine sand sludge produced in the sand mill is then collected in an intermediate container. Before it is mixed with the other raw materials, each individual component goes through its own special mixer with an agitator (Fig. 2). The various individual components only flow together in a common container shortly before they are poured into the mold. The aluminum is added as the last component of the aerated concrete mixture. It is also important to monitor the temperature profile during the entire process.

The procedure

Within a few minutes, the molds are filled with the liquid aerated concrete mixture (Fig. 3). The molds, which have a capacity of around 6 m³, are filled about halfway so that there is enough space for the mixture to expand. As soon as the molds are filled, the material is compacted in order to remove unwanted air inclusions. After a short time you can see with the naked eye how the mixture expands and finally fills the shape to the brim within a short time. Due to the influence of aluminum, the typical pores in concrete have now developed. After reaching the final filling level, the mold moves into the heating chamber within the circulation process, where the material can pre-harden (Fig. 4). The entire process from processing the raw materials to mixing and filling the molds is controlled and monitored using special software (Fig. 5).

After around 5 hours, the aerated concrete mixture is hard enough to be stripped and cut. This so-called cake is checked for the correct degree of hardness (Fig. 6) and then first detached from the bottom and the front sides of the mold (Fig. 7). Hanging between the two sides of the pan, the cake is now lifted to the milling station. The profile (tongue and groove) of the later stones is milled in on the underside (Fig. 8). Then the raw material is placed on the cutting machine. At the same time, the cake is now on a hardening grid, which removes the side parts of the formwork that have remained so far as a carrying aid. The molded parts are reassembled, cleaned fully automatically and wetted with release agent (Fig. 9). Thus, the mold has completed a complete cycle and can be filled again. A total of 83 forms go through this cycle.

The aerated concrete cake is then processed further on the cutting machine. In the first step, vertical cuts are made. Oscillating steel wires cut through the cake from bottom to top and thus determine the thickness of the stones (Fig. 10). In the second step, the stone height and plate length are determined by horizontal cutting. In addition, the uneven surface is severed and the side panels are cut off. They fall into the cutting ditch onto a conveyor belt (Fig. 11). The residues are mixed with water, transported to a sludge tank and recycled. The back sludge is completely introduced into new aerated concrete mixes (residue-free production). At the next station, the remaining layer on top of the cake is removed. It is lifted off by means of a vacuum system before tongue and groove are milled into the top of the cake (Fig. 12). Recessed grips can also be milled in here for easier handling of the stones on the construction site.

The cakes or stones now have their final shape and are placed in the autoclave to harden (Fig. 13). In three floors (Fig. 14) and with a total of six curing trolleys - a total of 18 cakes per autoclave - they are driven into one of the nine autoclaves, all of which can be reached via (train) tracks. At peak times, up to 3 hardening processes can be carried out daily, meaning that 405 molds can be autoclaved. After the 50 m long and 100 m³ capacity autoclave has been closed, it is first evacuated at -0.6 bar before the interior is heated and the aerated concrete blocks harden in saturated steam at 12 bar pressure. These conditions are maintained in the autoclave for between 6 and 8 hours. Including the up and down of the pressure ratios, a cycle takes 8 to 10 hours until the products harden completely. Energy can be saved by using an overflow method: the excess pressure in one autoclave can be quickly diverted to another autoclave.

After autoclaving, an unloading crane transports the stone blocks onto a tilting table, which tilts the blocks by 90 ° and brings the stones into a stacked position for final packaging (Fig. 15). Now the blocks are divided up according to the usual packaging unit, subjected to a visual inspection, placed on a wooden pallet, shrink-wrapped in the typical yellow Ytong film and provided with the product label (Fig. 16).

The short way from the autoclave to the packaging of the finished product is, by the way, one of the many places where it becomes clear what really matters in an aerated concrete plant - namely the know-how. In some other plants it is necessary to separate stones that are sticking together after autoclaving. This not only requires additional work (e.g. with cutting pliers), but also leads to more breakage due to damage. "We have no problems gluing stones together," says plant manager Gábor Glück. "We can prevent that through our experience with the recipes". This is also made possible by the in-house test laboratory. In addition to compressive strength tests, which are carried out on test cubes from each individual shape, research work is also carried out here. The plant is supported by the technology and research company of the Xella Group at the Brück and Emstal locations in Brandenburg. The basics of many of the desired product properties, such as high thermal insulation with low weight, are created here and optimally implemented in the Hungarian plant. It is therefore no wonder that the plant achieved an excellent fifth place in the global comparison of “Steinwerke 2010 Quality Benchmarking”, in which quality standards such as strength or bulk density are checked.

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In 2007 and 2008, the Hungarian plant reached its highest production levels to date. 565,000 m³ of aerated concrete were produced. With the construction industry currently declining, capacities have been adapted to current requirements.