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Autoclaved Aerated Concrete (AAC) is a kind of gas powder added to the ingredients of siliceous raw materials and calcium raw materials. It is stirred by water, cast, expanded, and pre-cut. Porous silicate material cured by high pressure steam. Its bulk density is 300-800 kg/m3, which is 1/3 of red brick; the thermal conductivity is 0.09~0.22 W/(m ̇K), which is only l/4~1/5 of red brick. Because of its light weight, heat preservation, heat insulation, sound absorption, fire prevention, and processability, it is widely used in industrial and civil buildings, and has become a new building material with energy saving and environmental protection. In particular, autoclaved aerated concrete can be used in a large amount of solid waste as a siliceous raw material, which is highly concerned with the development trend of energy conservation, environmental protection and recycling economy in today's society.

With the rapid development of industrialization and urbanization in China, on the one hand, a large amount of solid waste is discharged and stored, occupying valuable land, polluting the environment and endangering human health: on the other hand, the demand for building materials products in urban and rural construction development has increased sharply, and resources and environment are constrained. The contradiction has become increasingly prominent. In 2011, the National Development and Reform Commission issued the "Twelfth Five-Year" Wall Materials Innovation Guidance Opinions, clearly pointed out the key work of wall material innovation: "implementing new waste wall material demonstration projects, promoting the integration of bulk solid waste Through the construction of demonstration bases, through technology research and development, support enterprises to use construction waste, municipal sludge, tailings, phosphogypsum and other solid waste to produce new wall materials, improve the comprehensive utilization efficiency. National wall reform policy proposed to 2015, towns The new building will implement no less than 65% of the building energy efficiency standards, and 95% of the new urban buildings will meet the mandatory standards for building energy conservation.

Undoubtedly, protecting the environment and conserving resources has become a basic national policy. Building energy conservation is the trend of the times. The research and development and application of green environmentally-friendly building materials have become the key to realizing building energy conservation. This paper reviews the research progress of using autoclave to prepare autoclaved aerated concrete wall materials, focusing on the characteristics of fly ash, quartz tailings and tailings slag and its materialization technology methods, and the problems in current research. And the future development direction will be discussed.

1 aerated concrete of different materials

Essentially, autoclaved aerated concrete is made up of aluminum powder in the slurry and fixed in the hardening process to form a pore structure; CaO and SiO2 react under hydrothermal conditions to form hydrated calcium silicate (mainly CSH(I) gel, water garnet and tob mullite, combined with unreacted material particles to form the overall strength of the concrete. Usually, CaO is supplied from lime, cement, etc., and SiO 2 is supplied from a siliceous material. Research on the preparation of aerated concrete using silicon-rich solid waste has been focused on fly ash, quartz tailings and tailings slag.

1.1 Fly ash aerated concrete

As early as 1958, China began to study steamed fly ash aerated concrete. After long-term exploration and practice, more mature preparation techniques have been formed. In general, fly ash with a SiO 2 content of ≥ 40% and a particle size of 180 mesh (80 μm square sieve sieve residue ≤ 25%) can be used for the production of autoclaved aerated concrete products.

At present, the fly ash aerated concrete produced and applied in large quantities is mainly B05-B07 grade products, which has a large dry bulk density (>500 kg/m3), low compressive strength (≥2.5 MPa), and high thermal conductivity. (≥0.14W(m ̇K)), and there are problems such as poor freeze-thaw resistance, low carbonization ability, and easy cracking of the wall. Therefore, recent research has focused on the development of aerated concrete products with improved performance and high strength and low bulk density. Wang Ligang's research group tried to use a fine fly ash (5~15 um) to make aerated concrete, and found that ultra-fine ash has obvious reinforcing effect on aerated concrete. However, due to the small particle size of the ultrafine ash, the large specific surface area, and the enhanced particle filling effect, the porosity of the product is decreased and the bulk density is increased; and because the activity is higher than that of the original ash, the slurry is thickened quickly and the casting stability is deteriorated. Du Chuanwei et al. explored the improvement of pore structure by adding physical foam and coagulant, and increasing the strength of the product by increasing the amount of cement. The results show that since the added foam is easily destroyed during the agitating pulping process, the improvement of the pore structure and the strength of the product is not significant. M. Serhat Baspinar E et al. studied the effect of cement on the properties of the product and found that reducing the amount of cement can increase the compressive strength of the product. The reason is that when the cement content is high, a large amount of ettringite phase is formed in the early hydration reaction; although the ettringite phase is beneficial for increasing the early strength of the green body, it causes cracking of the product and reduces its durability. They inhibited the formation of ettringite phase by adding silicon powder, which improved the hydration reaction conditions to some extent and improved the strength of the product, but greatly increased the bulk density of the product. Zheng Huailin developed a B03 grade fly ash aerated concrete insulation block by adding various admixtures and precisely controlling the process parameters. However, its absolute compressive strength is only O.89 MPa (the national standard is 1.0 MPa), and the practical application value is not large.

In summary, there has been no breakthrough in the research of fly ash aerated concrete, especially the basic problems such as the reaction mechanism of the starting materials and the relationship between the phase, structure and properties of the products.

1.2 Quartz tailings aerated concrete

Quartz tailings is a large amount of waste produced after pulverization and screening in the production process of glass. It is mainly crystalline quartz with a content of SiO2 of over 90%. It can be used as a siliceous raw material for aerated concrete after treatment. This not only broadens the range of raw materials for the preparation of aerated concrete, but also solves the pollution problems caused by it.

Wang Yang et al. controlled the fineness of quartz tailings to the remaining material of 0.08 mm sieve as the main raw material for preparing aerated blocks, studied the influence of the amount of calcium materials (lime, cement) on the properties of the products, and found that the calcium materials were added. If the amount is too small, the amount of hydration product formed is insufficient, the residual SiO 2 is too much, and the product strength is low; the amount of calcium material added is too large, and Ca is excessive, and an alkali hydrate which is low in strength is easily formed in an alkaline environment. Through experiments, when the amount of calcium material added was 39% (cement 6%, lime 33%), a sample with a dry bulk density of 523 kg/m3 was prepared, and its compressive strength reached 3.9 MPa, and the drying shrinkage (fast method) was 0.7. Mm/m. Wang Changlong et al. studied the effect of the fineness of quartz tailings on slurry stability and product properties. The results show that the tailings are too thick, the fluidity of the slurry is poor, and the slurry is easy to precipitate after pouring, which leads to the collapse of the slurry. The fineness of the tailings can increase the specific surface area and increase the reactivity; but if the particles are too fine, the slurry is thick. It is fast, the gas is blocked, and it is easy to produce helium, which causes the product to crack, the porosity decreases, and the bulk density increases. Through the experiment, the quartz tail sand was ground to a specific surface area of ​​321.57 / kg, and the dry weight of the sample was 562 kg/m3, and the compressive strength was 4.34 MPa. Due to the poor grindability of quartz tailings, the crystal structure is dense, and the hydration reaction activity is low, resulting in low effective utilization of tailings.

1.3 Tailings aerated concrete

With the continuous development and utilization of mineral resources in the society, the solid waste of mines has increased year by year, and a large number of silicon-bearing tailings are piled up like mountains, such as iron tailings, gold tailings, phosphorus tailings, copper tailings, tungsten tailings, lead and zinc. Tailings and so on. At present, the treatment of tailings is mainly to recycle and use its valuable components, but this does not reduce the stock of tailings, and the problems of occupying land and polluting the environment cannot be cured. The use of silicon-rich tailings to prepare new building materials has become a viable way to eliminate tailings.

Zheng Wenxin used a chemical method to pre-activate the gold tailings to stimulate the activity and then used it as a siliceous material to prepare a B06 grade aerated concrete material. Wang Changlong et al. finely ground the low-silicon iron tailings to 7.3% of the 200 mesh sieve, and then added some high-silicon raw materials to prepare aerated blocks with a bulk density of 637 kg/m3 and a compressive strength of 4.31 MPa. Qian Jiawei used Bronze tailings instead of 32% silica sand to prepare B06-grade aerated blocks. Huang et al. attempted to prepare autoclaved aerated concrete using copper tailings and water-quenched blast furnace slag. It is believed that the water-hardened slag has a high active CaO content, which can meet the demand of calcium without adding quicklime, thereby reducing the cost of raw materials. However, due to its high MgO content and extremely slow digestion, the hydration reaction continues during the hardening or autoclaving of the green body, which easily causes volume expansion and destruction.

Since most of the tailings have coarser particles and complex mineral composition, SiO2 is produced in a small amount in the form of a quartz, and is mainly present in various mineral phases in the form of a compound, so that the reactivity is poor. To improve its reactivity, the currently effective method is grinding or chemical activation. This will undoubtedly increase the difficulty and cost of its utilization and limit the large-scale production of tailings aerated concrete. Therefore, research on low-cost activation methods for tailings needs to be strengthened.

2 Problems and prospects

In the aerated concrete industry, there is a bottleneck that is difficult to break through, that is, the fineness of raw materials can not be lower than 180 mesh, which makes it difficult to apply a large amount of siliceous waste slag micropowder. Therefore, how to make full use of the ultrafine powder without the need of fine grinding, large specific surface area, high reactivity, and the preparation of lightweight thermal insulation wall materials with excellent performance has become an urgent problem to be solved.

The research on the correlation between raw materials and products needs to be strengthened, especially according to the resources of solid waste, adapting to local conditions, developing new preparation techniques and methods, and improving the comprehensive utilization level of industrial waste. In the selection of siliceous raw materials for aerated concrete, the content of SiO2 has always been the standard. In fact, the properties of the article depend on the formation of hydrated calcium silicate, and the effective release of SiO 2 in the feed and chemical reaction with CaO is critical. Therefore, it is necessary to strengthen basic research on the structure and physical and chemical properties of raw materials, and develop active excitation technology for the different occurrence states of SiO2 to maximize the utilization of low-silicon solid waste resources.

With the acceleration of industrialization and urbanization in the “Twelfth Five-Year Plan” period, the generation and storage of bulk solid waste occupies a large amount of land, and the problem of polluting the environment is becoming more and more serious. At the same time, with the rapid development of urban and rural construction and the continuous improvement of modern building energy-saving requirements, it is urgent to develop a new type of air-filling that is ultra-lightweight, high-strength, good in durability, excellent in thermal insulation performance, and able to adapt to the needs of the external wall self-insulation integration project. Concrete wall material. Therefore, the development and application of energy-saving building materials using solid waste will become a key area.

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