ОСОБЕННОСТИ ПРОИЗВОДСТВА АВТОКЛАВНОГО ГАЗОБЕТОНА

Autoclaved aerated concrete is produced in large factories and comes to the construction site in the form of ready-made blocks. The cellular concrete production process consists of the following stages: water, cement, ground quartz sand, carefully crushed lime and gypsum are mixed in the mixer (not in all industries), and a gas-forming agent is added. The mixture rises in a warm, moist chamber. Due to autoclaving, immediately after its completion, aerated concrete has the appropriate strength, durability and frost resistance. Autoclave treatment of aerated concrete is performed not only to speed up the hardening process of the mixture. The main idea is that a new mineral, dobermorite, is formed in aerated concrete in an autoclave at a temperature of +180 ° C and a pressure of up to 14 bar. This increases the strength of the material and, most importantly, reduces shrinkage several times. Due to its characteristics, autoclaved concrete has many more applications. It can be used, for example, in reinforced structures — lintels, panels. Autoclave-hardened cellular concrete has reduced crack resistance and frost resistance. Autoclave treatment makes it possible to obtain products with sufficiently high strength in a shorter time with reduced binder consumption [1].

The main components of this material are cement, quartz sand, gas-forming agents, and the addition of gypsum and lime is also possible. Aluminum pastes and powders are used as specialized gassing agents.  The raw material is mixed with water, poured into a mold, and a reaction of water and a gas-forming agent occurs, leading to the release of hydrogen, which forms pores, and the mixture rises like dough. After gaining plastic strength, the array is cut into blocks, slabs and panels. The use of high-tech cutting equipment makes it possible to cut the resulting array with high precision into blocks and slabs.

After that, the products are subjected to steam hardening in an autoclave, where they acquire the necessary rigidity, or are dried under electric heating conditions. The gas formation process occurs due to a chemical reaction between calcium oxide hydrate and aluminum; the hydrogen released in this case causes the solution to swell, which, solidifying, retains a porous structure. When determining the composition of aerated concrete, it is necessary to ensure a given volume mass and its maximum strength with minimal consumption of a pore forming agent and binder. In this case, the structure of aerated concrete should be characterized by evenly distributed small pores of regular spherical shape. The bulk of aerated concrete and its porosity depend mainly on the flow rate of the pore-forming agent and the degree of use of its pore-forming ability. They are somewhat influenced by the temperature of the mixture and the amount of water used to seal the mixture, i.e. the water-solid ratio in /T. An increase in V/T increases the fluidity of the mixture, and therefore improves the conditions for the formation of a porous structure, if sufficient plastic strength of the mixture is ensured by the end of the gas formation process [2].

The strength of aerated concrete also depends on the nature of its porosity, the size and structure of the pores, and the strength of the pore shells. With an increase in V/T to the optimal value, which provides the best conditions for the formation of the structure of the mixture, the strength of aerated concrete increases. The strength of the shells, in turn, depends on the optimal ratio of the main binder and the silica component, In / T, as well as the conditions of heat and moisture treatment. It follows from this that the use of mixtures with a minimum value of V/T, provided that a high-quality structure is formed (for example, by vibration welding), makes it possible to obtain aerated concrete of higher strength.

For the manufacture of aerated concrete, Portland cement grades 300, 400, 500 are used, which meets the requirements of GOST 970-61, 31108-2016. The production of aerated concrete imposes special requirements on Portland cement regarding the alkalinity of the cement dough – the pH of the dough should not be lower than12.      In case of insufficient alkalinity of the solution, lime or alkali in the form of caustic soda (NaOH) should be additionally introduced into the aerated concrete mass. When used as the main binder of lime, special attention is paid to a significant amount of active calcium (CaO) and magnesium (MdO) oxides. The total activity of lime should not be less than 75%, the amount of MdO should not exceed 1.5%. Lime can be used in production.  The lime must be evenly burnt.  River or mountain quartz sand, ash from thermal power plants, marshalite and other materials are used as a silica component in the production of aerated concrete. Quartz sand for the manufacture of aerated concrete and gas silicate must be clean, free of clay and organic substances, with a SiO2 content of at least 80%. The presence of clay slows down the hardening of aerated concrete and reduces its strength. Organic impurities have a detrimental effect on the course of the gas release reaction; the swelling of aerated concrete in the presence of organic impurities worsens. Fly ash can be used in the production of gas-reinforced concrete with a SiO2 content of more than 55%. Fly ash should contain a small amount of sulfur compounds, unburned coal particles and calcium carbonates [3].

The most important technological feature of obtaining high-quality aerated concrete products with maximum porosity and sufficient strength is the creation of optimal conditions for two simultaneous processes of gas extraction and gas retention. It is necessary to ensure a correspondence between the rate of the gas release reaction and the rate of increase in the structural viscosity of the cement paste or mortar. In this case, the gas release should be completed as completely as possible by the beginning of setting of the cement-water system. The course of the gas formation process is determined by a large number of different factors. The type, quantity and properties of the gas-forming agent, the alkalinity and temperature of the medium, etc. have the greatest influence on the speed of this process. Aerated concrete is manufactured using wet or dry methods. The wet method is more economically feasible, in which the silica component or its mixture with lime is ground in the presence of water to produce sludge. In the dry method, the grinding and mixing of the components are carried out in ball mills in dry form. The sand is ground in ball mills. To carry out wet grinding, heated water is introduced into the mill.