Mix Design of Foam Concrete and Use of Additives

What is foam concrete？

Foam concrete is a non-load-bearing concrete material, foam concrete density is 300 ~ 1600kg / m3, which is equivalent to about 1/5 ~ 1/8 of ordinary cement concrete, which can reduce the overall load of the building and reduce the cost of the structure and foundation. It's good against earthquakes. In addition to ensuring a specific application strength of foam concrete with different densities, studies on properties such as water absorption, shrinkage, and thermal conductivity are also different from traditional concrete. At the same time, foam concrete can be said to be the cheapest functional concrete with fire resistance, heat insulation, lightweight, and seismic performance. Although foam concrete is not as good as other composite functional materials in many applications when it is used as a thermal insulation material, its price is The absolute advantage that leads to the broad application prospects of foam concrete. It can also be seen from the industry fluctuations of foamed concrete insulation boards that it is not feasible to pursue ultra-lightweight concrete alone.

Foam concretemix design

Foam concrete is prepared by adding uniformly prepared and stable foam to the pre-prepared slurry and forming and curing the cement-based porous material. Therefore, general foam concrete is made by physical foaming. Physical foaming refers to the use of physical methods to generate foam. The main components of foam concrete include gelling materials, foaming agents, admixtures, and additives.

(1) Cementing material

As a cement-based porous material, foam concrete is still its primary cementitious material. But what is different from traditional concrete is that foam is added to the cement matrix. For the foam itself, because of the high surface energy of the gas-liquid interface, it is a thermodynamically unstable system, so the collapse of the foam is inevitable. At the same time, in the preparation of foamed concrete, to ensure that the foam and the slurry can be well mixed, the water-cement ratio of the foamed concrete is generally high. Therefore, the cementitious materials used in foamed concrete are also different from traditional concrete. To prevent the foam from cracking, merging, and collapse, it is usually necessary to accelerate the hardening in the preparation of foamed concrete. Therefore, rapid hardening cement and cement accelerator are often used to increase the hardening speed of foamed concrete slurry and ensure that the mud hardens during the foam stabilization time. The use of fast-hard special cement to prepare foamed concrete is too costly. At present, the most commonly used is to add a coagulant to ordinary portland cement as the primary cementitious material of foamed concrete.

(2) Foam concrete admixture

In the preparation of foamed concrete, in addition to the primary cementitious material Portland cement, a certain amount of admixture is often added. The addition of the admixture can not only play a role in the utilization of substantial waste resources, but also merely Foam concrete prepared from ordinary portland cement, due to the addition of foam will cause incomplete hydration of the cement particles, and the performance of the prepared foam concrete is reduced. Adding individual mineral concrete admixtures can affect the performance of foam concrete. The admixtures commonly used in foam concrete are generally divided into inert admixtures and active admixtures according to their activities. The inert admixtures usually include limestone powder and fine sand. The active fillers include fly ash and ore.

Dr. Jones used two different types of fly ash to conduct laboratory studies on the feasibility of using foam concrete as a structural material. Aggregate, the research results show that the use of fly ash to replace part of the cement produced foam concrete at a density of 1400 kg / m3 to obtain a foam concrete strength of more than 25MPa. Besides, the addition of fly ash can significantly improve the shrinkage of foamed concrete. Among them, fine fly ash can adjust the pore structure of foamed concrete, while coarse fly ash can substantially reduce the contraction of foamed concrete products.

(3) Foaming agent for concrete

Foamed concrete is a porous material, and a foaming agent generates its internal pores. The quality of the foaming agent is directly related to the quality of foamed concrete. The quality of foam produced by the foaming agent is measured by indicators such as toughness, foaming multiple, and bleeding volume. The toughness of the foam is the property that the foam does not break in the air within a specified time. It is often determined by the subsidence distance of the foam column in unit time; the foaming factor is the multiple of the foam volume more significant than the amount of the foaming agent solution; Refers to the amount of foaming agent solution produced after foam collapse. The foaming agent used in the foamed concrete industry is different from other sectors. It not only requires the foaming agent to have a high foaming factor and a long stabilization time, but also the strength of the primary gelling material and The initial setting time does not have a negative effect, so the foaming concrete foaming agent needs to have excellent compatibility with the medium used.

At the same time, there are currently two methods for preparing foam: a. The configured foaming liquid is subjected to high-speed physical stirring to generate foam; b. The prepared foaming liquid is foamed using a foam generator. In industrial production, a lot of foam needs to be prepared, so a foam generator for foam concrete is usually used to make the foam. In experimental research, a high-speed stirring method is often used to prepare the foam.

Foam concrete additives

Due to the large porosity and poor pore structure of foam concrete, foamed concrete has problems such as high water absorption, natural cracking, collapse, and low compressive strength. It is often necessary to add additives with specific functions to improve. Commonly used adjuvants include bulking agents that compensate for shrinkage, water-repellent agents that reduce penetration, and early strength agents and fiber-reinforced materials that will enhance physical and mechanical properties. By adding inorganic early strength agents CaCl2, Na2SO4, and triethanolamine, the strength of foam concrete can be significantly improved. Fiber can effectively inhibit the dry shrinkage of foam concrete, especially the early dry shrinkage, and the effect is pronounced. And as the fiber content increases, the contraction of the cement substrate decreases, and the ability to compensate for dry shrinkage increases. When the dry density is the same, the fiber has a very obvious strengthening and toughening effect on the foamed concrete. When the volume ratio of the glass fiber is 1.6%, the bending strength of the foamed concrete is increased by 143% and the toughness is increased by 8.1 times.

How to make foam concrete

The foam concrete production process is basically to measure, stir and mix cement, admixture, aggregate, composite admixture, and water separately, add the prepared foam solution, and continue to stir evenly. Injection molding, low amplitude vibration compaction, Finished products are obtained by curing, demoulding, and curing. When ordinary Portland cement is used, the length of mixing time has a significant effect on the foam concrete. Tests show that with the extension of hydration time, the strength of foamed concrete is increasing, the density is decreasing, the softening coefficient is increasing, and the rate of stomatal change is not significant.

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