Things about foam concrete

Foam concrete, also known as lightweight concrete or foamed cement, is a new type of waste-free, environmentally friendly, energy-saving, low-cost, lightweight, high-strength, non-water-absorbing, and non-combustible material. Energy-saving building materials can be used as roof insulation, floor heating, ground cushions, interior partition walls and exterior wall insulation, etc.

Foam concrete usually uses mechanical or compressed air methods to prepare an aqueous solution of foam agent into foam, and then adds the foam to siliceous materials, calcium materials, A concrete containing a large number of closed pores inside a slurry composed of water and various additives, which is mixed, stirred, poured, molded, and cured.

1. Problems in practical application

(1) Low strength, large dry shrinkage and water absorption Disadvantages such as high efficiency are the main reasons that hinder the further development of foam concrete.

Ultra-light foam concrete with density (less than 300kg/m3) has good thermal insulation and sound insulation properties, but its compressive strength is extremely low and the surface is easy to crack, resulting in The water absorption rate of concrete increases, thereby affecting the quality of the project; foam concrete with higher strength generally has a higher density, and thus has lower thermal insulation and sound insulation properties. Therefore, a balance should be maintained between the strength and density of foam concrete, that is, maximizing strength while reducing density. This can be achieved by optimizing cementitious materials and ultra-light aggregates, and selecting high-quality foaming agents.

(2) Production equipment problems.

There is no unified standard model of mechanical equipment for domestic foam concrete production and construction, and the cement slurry ratio, foam mixing ratio and foam slurry wet density still depend on Manual and empirical preparation leads to problems such as inability to accurately measure and difficulty in controlling the density of the slurry. Our country should develop automated, large-scale and sophisticated production equipment as soon as possible.

(3) Compared with foreign countries, the overall scale of my country’s foam concrete products is small, the technical level is not high, and there are no leading products for mass applications. Therefore, technological innovation of foam concrete in prefabricated and cast-in-place applications should be strengthened, such as steel structure cast-in-place walls and frame structure foam concrete walls, and its application should be promoted in high-tech fields such as electromagnetic shielding, anti-explosion and sound absorption. .

2. Three major factors affecting strength

Like ordinary concrete, the strength of foam concrete is not A fixed value, different types of cementitious materials, cement dosage, concrete mix ratio, water-cement ratio, foam dosage (different volume and mass), foaming agent, curing system and the use of other admixtures all affect the foam. Strength of concrete. We will focus on the three major factors of water-cement ratio, mix ratio and selection of admixtures.

1. The influence of water-cement ratio

In the preparation of ordinary concrete, the strength of concrete varies with The molding water-cement ratio increases with the decrease, but the situation found in the foam concrete test is not the same.

1) When the fluidity of the slurry is mainly controlled by the amount of additives (that is, the slurry already has good fluidity), as the water-cement ratio As the intensity decreases, the intensity increases; on the contrary, the intensity decreases. Analysis reason: As the water-cement ratio decreases, the amount of free water in the concrete decreases, and the water absorption rate of foam concrete decreases, effectively increasing the strength of the concrete; but when the water-cement ratio continues to decrease, due to insufficient water demand for cement hydration, the Absorbing moisture in the foam causes the foam to burst, resulting in a reduction in the number of closed bubbles and a decrease in the uniformity of the concrete, resulting in a reduction in strength.

2) When the fluidity of the slurry is mainly controlled by the amount of water, as the water-cement ratio increases, the strength increases; conversely, the strength decreases .
Analysis of reasons:

Higher molding water-cement ratio ensures good fluidity of the slurry and ensures that foam is evenly introduced into the cement slurry Medium and evenly distributed, resulting in increased strength. On the contrary, the reduction of water-cement ratio and insufficient fluidity of slurry material will cause uneven distribution of bubbles, thereby reducing the strength of concrete.

Based on the above situation, the impact of the water-cement ratio of foam concrete on the strength is multi-faceted. It should be analyzed based on the specific situation to adjust the water-cement ratio so that The internal material structure of foam concrete is uniform and has many closed independent cells, which is an important way to improve the strength of foam concrete.

2. Influence of mix ratio

Cement can be used alone or at the same time when preparing foam concrete. Mixing materials other than cement (such as silica fume, slag, fly ash, etc.) are used, and fine aggregate (sand) may also be used at the same time.

The first situation: It is relatively simple to use cement alone. The higher the strength grade and the greater the dosage of cement used, the greater the strength of the prepared foam concrete. Therefore, when you want to prepare higher-strength foam concrete, you need to choose high-strength grade cement.

Second case: The introduction of mixed materials will lead to a significant reduction in the early strength of foam concrete, but will have little effect on the later strength. If appropriate strength activator is filled in, the early strength can be effectively slowed downreduce. In actual construction, it is most common to mix mixed materials and sand at the same time. Therefore, there is an appropriate range for the mix ratio of foam concrete, which needs to be determined based on experiments.

3. Influence of the dosage of admixtures

The admixtures used in foam concrete mainly include cement strength stimulation agents, water reducing agents and foaming agents.
Cement foaming agent

From the analysis of the preparation principle of foam concrete, the foaming agent is used to introduce a large number of pores to give it lightness and sound insulation that ordinary concrete does not have. , thermal insulation, high fluidity and other functions are the basis for the preparation of foam concrete; but from the perspective of structure and mechanics, it also causes the structural density to be greatly reduced, thereby reducing the strength of foam concrete. So, how to control the amount of foaming agent to achieve functions such as light weight, sound insulation, insulation and high fluidity, while ensuring that the concrete strength is not lower than the expected target is a new topic in concrete production scientific research.

There are many substances that can produce foam, but not all substances that can produce foam can be used as foaming agents. Only foaming agents with a large enough foaming ratio that the film will not be damaged when the foam and slurry are mixed, with sufficient stability and no harmful effects on the coagulation and hardening of the cementitious material are suitable for the production of foamed concrete.

The influence of cement foaming agent on the strength of foam concrete is mainly reflected in the size and uniformity of the foam (uniformity of size and distribution), stability of the foam, Foaming ability, etc. The foaming agent is required to have strong foaming ability, low density, small unit water carrying capacity, firm, fine and evenly distributed foam, and little side effects on concrete.
Cement strength stimulator

Cement strength stimulator is mainly used in cement mixture materials, which can reduce the early strength reduction of foam concrete. However, the use of activators tends to reduce the ultimate strength of foam concrete.
Water-reducing agent

Concrete water-reducing agent enables foam concrete to successfully complete the mixing of slurry and foam even at a low water-cement ratio. Preparation Produce foamed concrete with evenly distributed foam. Therefore, adding an appropriate amount of high-efficiency water-reducing admixture is one of the important means to prepare high-strength foam concrete. However, because water-reducing agents are relatively expensive and some water-reducing agents have opposite effects on certain properties of foaming agents, the type and amount of water-reducing agents must be determined by testing. </p