Foam concrete is a kind of green environmental protection material with light weight, thermal insulation, sound insulation, high compressive strength, good durability, strong processability, convenient construction, good fireproof performance and no environmental pollution.
Stability
When the density of foam concrete mixed with cement slurry is consistent with the design density, and there is no leakage and segregation of the slurry, it can be considered to be a stable mixture.
Generally speaking, for concrete with a density of 400kg/m3, a stable mixture can be formed when other cementitious materials do not replace Portland cement, and when the density is less than 400kg/m3, stable foam concrete can also be obtained by using 5% calcium sulphoaluminate instead of cement.
Similarly, the stability of the foam concrete mixture can be improved by adding silica fume and fly ash.
Silica fume can promote foam to be more uniform and closed, which helps to improve the stability of foamed concrete.
Nano-silica and methylcellulose can make the life of the foam longer, but the addition of nano-silica will gradually reduce the size of the foam.
The addition of hydroxypropyl methylcellulose can increase the thickness of the foam film without changing the pore size.
After the foamed concrete slurry is mixed, the collapse of air foam in the mixture will affect the stability of the mixture, which can be prevented by properly increasing the water-solid ratio of the mixed slurry.
The buoyancy of the foam is proportional to the size of the foam; the lower the density, the greater the buoyancy.
When the buoyancy overcomes the surrounding foamed concrete, the foam will replace the surrounding solids and rise to the surface, causing the mixed slurry to be unstable.
Larger bubble sizes and smaller solids make the separation wall thinner, making it easier for the gas to migrate.
Generally, from higher internal pressure (smaller bubbles) to lower internal pressure (larger bubbles), the buoyancy of bubbles will further increase.
Compressive strength, elastic modulus, and flexural strength
Density is the main reason that affects the compressive strength of concrete.
Other factors affecting the compressive strength of concrete include water-cement ratio, type of foaming agent, curing conditions, types of additives, pore shape, size distribution, and so on.
The strength of foam concrete increases with the decrease of the water-cement ratio, which is mainly because the reduction of the water-cement ratio reduces the large-size air bubbles in concrete.
The addition of slag can improve the water-cement ratio of foam mortar so as to improve the quality of foam distribution in mortar.
The foamed concrete using protein-based foaming agents and synthetic foaming agents show higher compressive strength.
Fiber mainly improves the compressive strength of foam concrete by preventing the occurrence of micro-cracks.
The fibers commonly used in foam concrete are polyolefin fiber, polypropylene fiber, polyvinyl alcohol fiber, glass fiber, polyamide fiber, carbon fiber, and so on.
Although the strength of steel fiber is higher than that of other fibers, it is easy to settle in foam concrete because of its high density, which affects the distribution and quality of foam.
The type of cement also has an obvious influence on the compressive strength of foam concrete.
With different grades of cement, the compressive strength after solidification is also different. For example, the compressive strength of 52.5 cement is higher than that of 42.5 cement.
The compressive strength of foam concrete can be improved by using fly ash and micro-silicon instead of cement.
From the results of microscopic SEM and XRD analysis, it is found that the reaction of microsilicon added in foamed concrete with free calcium hydroxide (CH) in cement can produce more hydrated calcium silicate (C-S-H). In contrast, the strength and durability of hydrated calcium silicate (C-S-H) is higher than that of CH.
Thermal conductivity
It is of great significance to ensure the thermal insulation performance of the building structure before the design and construction of the building structure.
Foamed concrete has excellent thermal insulation performance because of its honeycomb microstructure and thermal conductivity is the key to determining the thermal insulation effect.
Its thermal conductivity is mainly affected by density, pore size and structure, aggregate type, fiber, and mineral admixture [13].
Adding 20% air to the concrete can increase the thermal resistance by 25%, reduce the dry density of the concrete by 100 kg/m3, and reduce the thermal conductivity by 0.04 W / (m ·K).
The foamed concrete with lower density shows higher porosity, so the thermal conductivity is lower.
At lower temperatures, the thermal conductivity increases slowly with the increase of density and the thermal insulation performance of concrete increases with the decrease of temperature.
Because there are a large number of voids in foamed concrete, the change of density from 700 to 1,400 kg/m3 corresponds to the thermal conductivity from 0.24 to 0.74 W / (m ·K), so the foamed concrete with low density has higher insulation capacity and can preserve internal heat.
The pore structure also affects the thermal conductivity of foamed concrete. Generally, the higher the porosity, the worse the thermal conductivity. However, due to the strengthening of the pore connection, it sometimes shows an increase in value.
The position and relative direction of pores have a great influence on the thermal conductivity of foamed concrete. If the pores are arranged at right angles to the heat flux, more heat will pass through the pores, showing greater thermal resistance.
If the first layer of pores is parallel to the direction of heat flow, it will produce less heat flow resistance.
The thermal conductivity of foam concrete modified by polypropylene fiber is lower than that of foam concrete modified by other fibers (such as glass fiber, jute fiber, steel fiber, oil palm fiber, and natural fiber).
The addition of fiber does not necessarily increase the thermal conductivity because the porosity increases, resulting in a decrease in density and thermal conductivity.
Although additives such as silica fume and fly ash greatly improve the mechanical properties of foamed concrete, the addition of silica fume will lead to a slight increase in thermal conductivity.
Sound insulation characteristic
Because the foam concrete has a hollow structure, it has a good absorption effect on the transmitted sound.
The sound absorption rate of foam concrete is higher than that of ordinary concrete.
After adding fly ash to foamed concrete, the absorption of low-frequency sound waves is not changed, but the absorption of high-frequency sound waves (800mm 1600 Hz) is increased.
Increasing the amount of foam to 5% to 10% can effectively improve the absorption efficiency of foam concrete to 600-1000 Hz frequency sound waves.
Fire resistance
The coarse aggregate existing in ordinary concrete will expand in a high-temperature environment and lead to concrete cracking.
Compared with ordinary concrete, foam concrete has no coarse aggregate, and its pores are wrapped in gas, so it has excellent fire resistance.
The fire resistance of foamed concrete with a density of 400 kg/m3 is three times that of a density of 100 kg/m3.
By comparing the high-temperature resistance test of ordinary concrete and foam concrete, it is found that when the surface temperature reaches 1 039 ℃ when the surface temperature reaches 1 039 ℃, the lowest temperature on the other side of the traditional concrete wall is obviously higher than that of foam concrete.
Foam concrete additive Supplier
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