Problems and solutions to foam concrete

In recent years, our country has paid more and more attention to building energy conservation. With the implementation of policies related to building energy conservation, wall material reform has made remarkable achievements, and energy-saving materials are very popular. my country started late in the development and application of porous concrete. In the early 1960s, autoclaved foam concrete products were successfully developed. However, insufficient research on production technology and equipment has hindered development. The main characteristics of foam concrete as well as current problems and solutions are discussed below.

1 Characteristics of foam concrete

Foam concrete was first obtained in the United States, Britain, the Netherlands and other European and American countries It has been widely used, and our country is paying more and more attention to building energy conservation. As the field of building construction integrates with international standards, lightweight building energy-saving materials are very popular. Foam concrete has been widely used in our country due to its good performance. . In recent years, both domestic and foreign countries have attached great importance to the research and development of foam concrete, making it more and more widely used in the construction field. The performance characteristics of foam concrete mainly include the following aspects:

1.1 Light weight and low density:

The density of foam concrete is generally 300-1200 kg/m3, which is about 30% lighter than conventional building materials. It can reduce the cost of structures and foundations and has good seismic performance.

1.2 Good thermal insulation

Foam concrete is a lightweight material containing many Independent, non-penetrating small holes, when the bulk density is 180-1400kg/m3, the thermal conductivity is generally between 0.047-0.355w/(m·k), which is a good inorganic thermal insulation material.

1.3 Good pumpability and strong waterproof ability

Since the incorporated foam is water film When mixed with cement (sand) slurry, part of the foam will burst and turn into water. Therefore, foam concrete is a material with a large water-cement ratio, generally above 0.6, with high fluidity and self-compacting properties. specialty.

1.4 A large amount of industrial waste can be utilized

Many industrial wastes such as fly ash, coal gangue and Steel slag has been proven to be effective as an admixture in the preparation of cement-based foam concrete. Its application is of great significance to building energy conservation and environmental protection. Therefore, it has been highly valued and widely used in the construction industry at home and abroad.

2 Existing problems and solutions

Due to the research results of foam concrete and its application technology, domestic and foreign Most of them are reflected in the form of patents. In the domestic production of foam concrete, there is a lack of complete sets of equipment, complete data and experience accumulation, and a lack of theoretical guidance for construction. Therefore, the superior properties of foam concrete are not fully reflected, and problems such as strength often occur. Problems such as low level, concrete cracking or water absorption have seriously affected its widespread use in the construction industry.

The performance of foam concrete is affected by many factors, such as the properties of cement, foaming agents and other auxiliary materials as raw materials, as well as the mix ratio of foam concrete.

2.1. The problem of low strength

When foam concrete has a large amount of foam, although the pores The efficiency is very high, but it still has relatively ideal usage intensity and can meet various needs. Its strength mainly comes from the gelling effect produced by the cementitious material itself. The gelling materials used generally require strong gelling effect, especially for products with high porosity. Therefore, it generally uses a large amount of high-grade cement and magnesium cement with a content of 80% to 85% as the cementing material. High-strength gypsum can also be used when the foam content is low. If the gelling effect of the cementitious material is not strong, the strength of the foam concrete cannot be guaranteed. Therefore, it has three technical characteristics in this area: large dosage of cementing material, generally greater than 50%; high gelling force; good early strength and fast setting.

The strength of foam concrete decreases with the increase in porosity caused by the introduction of foam. The compressive strength of some foam concrete projects is generally lower than 2.0mpa, but currently domestic Foam concrete technology is developing rapidly. Some companies in Beijing (such as Beijing Zhongke Zhucheng) have increased the strength of 500kg per cubic meter of foam concrete to 2.5mpa. The hardened surface cracks and absorbs large amounts of external moisture. The more foam introduced into foam concrete, the greater the porosity after hardening, the smaller the bulk density, and the more obvious its light weight, thermal insulation, sound insulation and other properties. However, the strength decreases greatly, so the characteristics of foam concrete are At the expense of reduced strength. It is necessary to balance its strength with its special properties, that is to say, to minimize the strength of foam concrete while reducing the bulk density.

According to previous studies by many workers [1,2], as the amount of fly ash increases, the compressive strength of concrete decreases significantly. Because as the amount of fly ash increases, the amount of cement continues to decrease, which will inevitably cause the strength of concrete to decrease. However, fly ash has potential activity. In an alkaline environment, as the age increases, high-content fly ash foam concrete will The compressive strength will be significantly increased. The compressive strength of foam concrete is mainly affected by density. Using a large amount of fly ash instead of cement (cement can reach 67Market competitiveness.

Harbin Architecture University[8] developed polystyrene foam concrete, which has excellent properties such as light weight and small thermal coefficient, and can be used for interior and exterior non-load-bearing walls. Material, can also be used for roof insulation materials.

vinarov[9] used cereal protein foaming agent to prepare lightweight foam concrete, which also achieved good results in sound insulation and thermal insulation properties.

2.4 Foam concrete pore structure issues

At present, the application of foam concrete is becoming more and more popular. However, there is still a lack of research on its mechanism, especially the relationship between the pore-forming materials, pore structure and macroscopic mechanical properties of foam concrete. This has limited the vigorous promotion of foam concrete and the further improvement of its performance. The submicroscopic structure of foam concrete can be observed through modern analytical methods, especially the pore structure and its distribution.

The pores must be closed, not connected. The shape of the pores should be close to spherical, and the deformation should not be too large. The pores should be uniform in size, and the size of the pores should not be If it is too large, the porosity should be compatible with the strength. Only when these conditions are met can high-quality foam concrete products be produced.

Niu Ningmin [4] et al. invented the sulfoaluminate cement foaming agent and manufactured foam, through which a large amount of air was introduced into the sulfoaluminate cement mortar. , mixed into foam concrete, the foam pores are basically closed circles. The air and water in this kind of foam concrete do not flow, so that there will be no shrinkage, collapse and deformation after pouring.

It further shows that the pore structure has a certain impact on the performance of foam concrete, and further research work is needed.

2.5 Water absorption problem

Foam concrete introduces a large number of bubbles during production, and the raw materials are powdered Mainly with fine particles, it shows high water absorption and shrinkage in performance. As the bulk density decreases and the amount of introduced bubbles increases, the saturated water absorption rate of foam concrete increases greatly, which can easily cause freeze-thaw phenomena and serious It affects the use effect of foam concrete and restricts its application fields. There are many technical measures to reduce the water absorption of foam concrete, such as adding hydrophobic agents and coating waterproof materials on the surface. In addition, the incorporated materials can also be exposed and the surface of the introduced porous materials can be "wrapped" to "isolate" the pores from the slurry, thereby reducing water absorption.

The saturated water absorption rate of foam concrete with a bulk density of 600kg/m3 developed by Li Juan et al. [10] reached 53% in 72 hours after adding 0.8% hydrophobic agent. The rate dropped to 13.7%; the saturated water absorption rate of foam concrete with a bulk density of 1000kg/m3 was 38.3%. After adding 0.8% hydrophobic agent, the water absorption rate dropped to 9.8%.

Xiong Chuansheng et al. [11] studied the effect of steel slag and fly ash as admixtures on the properties of cement-based foam concrete. The results show that replacing cement with high-quality fly ash can not only reduce the dry bulk density and thermal conductivity of foam concrete, but also improve the strength of low-foam concrete under the same conditions; steel slag powder has no effect on the dry bulk density of foam concrete. Large, but it will reduce the strength of foam concrete, and good results can be obtained when steel slag powder and fly ash are combined to replace cement; the amount of foam is the main factor affecting the dry bulk density, compressive strength and thermal conductivity of foam concrete. The amount of foam The increase will cause the density of foam concrete to decrease, the strength will also decrease, and the thermal conductivity will decrease; the thermal conductivity of foam concrete has an approximately exponential function relationship with the dry volume density.

Sun Haiyan et al. [12] studied different water-binder ratios, different fly ash contents, expanded perlite, foam agents, polycarboxylate water-reducing agents and polyurethane water-reducing agents. The influence of propylene fiber on the bulk density, compressive strength, water absorption, deformation, thermal engineering and pore structure of foam concrete. The research results show that when using animal protein foaming agent with good foam stability, it can be used at lower water glue. Under the circumstances of comparison, a foam concrete with integrated functions such as light weight, high strength, excellent thermal insulation, small drying shrinkage, and waterproof was developed.

Based on the above research, it can be seen that the main technical measures to prevent the shrinkage and cracking of foam concrete, which will cause large water absorption, are: selecting appropriate cement varieties and dosage; using low-temperature cement during molding Water-cement ratio; optimize maintenance and strengthen water retention measures.

2.6 Price Issue

Under the existing market economy conditions, price is an important issue that has attracted much attention factor. Reasonable prices are the prerequisite for mass production of foaming agents and widespread promotion of foam concrete. The price of traditional foaming agent is about 7,000 yuan/ton, but the foaming effect is average, especially the foam stability is poor. At present, protein-based foaming agents have good foaming and foam-stabilizing properties, but their production costs are high and their raw material sources are limited, so their applications are subject to certain restrictions. The price of a good foaming agent reaches 25,000 yuan/ton. Therefore, finding an economical foaming agent with good foaming effect is the main direction of current research.

3 Conclusion

In order to give full play to the advantages of foam concrete, we can only solve the existing problems Only by solving these problems can it fully play its important role in the national economy as a new green building material. On the basis of fully solving its strength, bulk density and water absorption, it is necessary to further theoretically understand the foaming mechanism based on the microstructure of foam concrete, find new, efficient and economical foaming agents, and further reduce costs.

��An important role in the national economy. On the basis of fully solving its strength, bulk density and water absorption, it is necessary to further theoretically understand the foaming mechanism based on the microstructure of foam concrete, find new, efficient and economical foaming agents, and further reduce costs.

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