The Description of Foam Concrete and its Varieties

The method of producing cellular concrete by mixing pre-formed persistent mechanical air foam with a binder was first proposed by Bayer, a Danish engineer, back in 1911. But it is only by 1925 that the method of producing cellular concrete came to be widespread, first in Germany and then in other countries as well.

Foam concrete is a lightweight cellular concrete obtained due to curing of mortar consisting of cement, sand, water and foam. Foam provides the required air content and uniform distribution of air in the concrete. Foam is produced from foam concentrate (foaming agent). Various organic and non-organic compounds can be used as a foaming agent: those derived from natural protein, and synthetic ones obtained in course of detergent production.

Foam concrete is inexpensive, cost-effective, durable, eco-friendly, biologically resistant material, it is close to wood in terms of ecological compatibility, but is durable and not combustible. In some countries, foam concrete blocks are called ‘bioblocks,’ as only environmentally friendly natural components are used as raw materials. Foam concrete combines the advantages of stone and wood: firmness, light weight, processability, and does not need combinations with other construction materials. It can be plastered, painted with facade paints in any colour. The ability to obtain the required specific weight, the intended firmness, necessary thermal resistance, desired shape and volume make it an attractive material for manufacturing a wide range of construction goods. This product can be used both as a structural and thermal insulation material. In terms of durability, unlike mineral wool and polymeric foams whose properties tend to deteriorate with time, the firmness and thermal insulation characteristics of foam concrete keep improving as the time goes by.

Houses built with the use of foam concrete is characterized by greater comfort, as well as the following operational qualities:

- the walls of such houses ‘breathe’ and do not sweat;

- the walls retain warmth in winter and coolness in summer;

- absence of ‘cold bridges’;

- excellent sound insulation - 60 dB;

- saving on electric power needed for heating;

- ideal surface for any type of finishing;

- high fire resistance;

Multiple varieties of foam concrete are classified according to the following main features:

1. According to functional use foam concretes are divided into three groups: heat-insulating, heat-insulating/structural and structural.
2. According to the type of binder. In the technology of foam concrete production, cement and lime are mainly used as a binder, less often gypsum.
3. According to the type of silica component. Silica sand is the most widely used component, as well as fly ash from the combustion of brown and black coal, smelter slags and alumina production waste.
4. According to the method of curing, foam concrete is divided into non-autoclaved one that involves steaming, electric heating or other types of heating at normal pressure, and autoclaved concrete curing at a higher pressure and temperature.

 

Materials Used for Foam Concrete Production

Portland cement is used as a binder for cement cellular concrete. Ground quicklime is also used in the production of autoclaved foam concrete.

Silica component (ground quartz sand, fly ash from thermal power plants and ground granulated blast furnace slag) helps reduce binder consumption, concrete shrinkage and improve the quality of foam concrete. Quartz sand is usually ground down. Grinding increases the specific surface area of silica component and boosts its chemical activity.

Foamed concrete is made by mixing separately prepared mortar mix and air cell foam. The mortar mixture is composed of a binder, silica component and water.

Foam is prepared in foam generators or centrifugal pumps from an aqueous solution of foaming agents containing surfactants. Glue-resin, resin-sapoin, a luminosulpho-naphthenic, organic and synthetic foaming agents are used.

 

Physical and Mechanical Properties of Foam Concrete

Type of foam concrete	Foam concrete grade by mean density	Compressive strength class of foam concrete, not lower	Thermal conductivity coefficient of foam concrete in a dry state, λ W/(m  C0), not more	Mean compressive strength, MPa
Heat-insulating	D 400	B 0.75; B 1	0.1	0.95
	D 500	В 1; В 1.5	0.12	1.4
Heat-insulating/structural	D 600	B 1.5; B 2	0.14	2.1
	D 700	B 2; B 2.5	0.18	2.9
	D 800	B 2.5; B 3.5	0.21	3.4
	D 900	B 3.5; B 5	0.24	4.2
Structural	D 1000	B 5; B 7.5	0.29	6.3
	D 1100	B 7.5; B 10	0.34	7.8
	D 1200	B10; B12.5	0.38	9.7

Frost resistance grade, not less: F 25 – for external wall blocks F 15 – for internal wall blocks

Physical and mechanical properties of foam concrete depend on the uniformity of pore distribution, their nature (open, communicating or closed), type of binder, curing conditions and a number of other factors.

Foam concrete properties are interrelated with each other. Thus, the thermal conductivity coefficient (λ) in a dry state mainly depends on the average density value. The type of binder, curing conditions and other factors have an insignificant influence on λ value. It is explained by the fact that the material of the pore-forming walls consists of cement stone or, which is similar, hydrosilicate framework. Therefore, thermal conductivity of foamed concrete is mainly determined by the porosity value and, consequently, the average density value.

Increased porosity is achieved when pores have different sizes and are characterized by a nonspherical shape. Pore size is mainly determined by the viscosity of the slurry and foaming agent type.

The firmness of cellular concrete depends on its density, type and properties of the initial materials, heat treatment mode, humidity and other factors.

The structure of interpore partitions and binder type have a significant influence on frost resistance. Portland cement-based cellular concrete is characterized by a higher frost resistance than gas silicates.

A non-autoclave production scheme is common in the manufacture of cement-based foamed concrete. Refusal from autoclave treatment results in somewhat decreased firmness of cellular concrete and its crack resistance. When steaming aerated concrete, the number of communicating capillaries in it increases, which enhances water absorption and permeability, creates moisture and thermal gradients, which contributes to the emergence of internal stresses.

Methods of Production of Foam Concrete Mixtures

The method of preparation of moulding foam concrete masses depends on the technology and type of foaming agent used.

Preparation of foam concrete mixture, regardless of foaming method, is based on obtaining heterogeneous gas-liquid-solid system and can be arranged in several ways.

Classical scheme. The essence of the method consists in mixing foam with mortar mixture. Foaming agent concentrate and a part of water are dosed by volume, then they are mixed to obtain a working solution of foaming agent. The working solution of the foaming agent enters the foam generator to produce foam. The second part of water is dosed by volume, cement and sand are dosed by weight and made into mortar mixture. The foam from the foam generator and the mortar mixture are fed into the foam concrete mixer. The foam concrete mixture prepared in the foam concrete mixer is pumped to the site where it will be cast into moulds or a monolithic construction.

Dry mineralisation of foam. The essence of the method consists in mixing foam with dry cement and sand of natural humidity. Foaming agent concentrate and water are dosed by volume and mixed to obtain a working solution of the foaming agent. Foam is prepared from the working solution in the foam generator, which is fed into the foam concrete mixer. Then cement and sand are dosed by weight and fed into the foam concrete mixer. The foam concrete mixture prepared in the foam concrete mixer is pumped to the site where it will be cast into moulds or into a monolithic construction. The foam concrete mixture can be transported under the effect of pressure created in the foam concrete mixer by the compressor.

“Penobarotechnology”. The essence of the method consists in porization of the mixture of all raw components under overpressure. Foaming agent concentrate and water are dosed by volume, cement and sand - by weight (or else, a specially made dry mixture composed of dry foaming agent, cement and sand is dosed by weight). All components are fed into a foam concrete pressure mixer, while the compressor blows in the air, creating pressure inside. The foam concrete mixture obtained in the foam concrete pressure mixer is transported under pressure from the mixer to the site where it will be cast into moulds or into a monolithic construction.

Foam concrete is used in construction to insulate roofs and floors, fill in hollow spaces, make building blocks: when used in building envelopes in low-rise construction.

 

Monolithic Foam Concrete in Construction

In modern multi-storey construction, natural curing foam concrete poured on site can be used in various elements of construction. The use of monolithic foam concrete allows to achieve a great economic effect, as its cost considering the work, is lower than the cost of foam concrete blocks. In addition, it eliminates the costs of transportation, loading and unloading, breakage, lifting up to higher-level floors, laying, to say nothing about the use of additional insulation and ‘cold bridges’ in the joints. The use of foam concrete with a density of 250-300 kg/m³ allows to reduce the thickness of the wall while maintaining thermal performance.

Basically, the whole technology is reduced to the use of removable or fixed formwork and pouring in monolithic non-autoclaved foam concrete. Using bricks, building stone, porous concrete slabs, as well as any sheet moisture-resistant material and attaching them to a framework of wood or light metal structures is a proven method of making a permanent formwork, widely used in the construction of villas. Notably, in case of using permanent formwork made of sheet material, pouring foam concrete results in a ready-made multi-layer wall that does not require finishing.

1 - laying in permanent formwork (brickwork outside, brickwork inside, plasterboard or any sheet material, and foam concrete is poured between them);

2 - laying in removable formwork.

 

The process of erecting multi-storey houses provides for three options for construction of self-supporting wall envelopes when building monolithic and frame houses.

The first option: foam concrete is poured through technological holes in the floors between brick walls that are mounted on the floors.

The second option: foam concrete is poured between the inner and outer brick walls, laid from the foundation close to the floor slabs. In this option, the floor slabs are covered with bricks, which contributes to an aesthetic appearance of the building.

The third option: a wall is built from the foundation at some distance from the floor slab and attached to the floor slabs by means of reinforcement. The second wall is erected along the edge of the floor, and the monolithic foam concrete is poured into the resulting partition.

In case of all three options, foam concrete with a density of 250-400 kg/m³ is used. The third option is the most preferable, as it helps increase the useful area of the premises.

When building mansards one can do without rehousing or closing down the company’s operation. In this case, foam concrete with a density of 220-250 kg/m³ is poured between cement boards attached to the rafters. There is an experience of such work, when foam concrete is fed to a height of up to 30 metres.

Monolithic foam concrete is also used for making road pavement bases, for slope reinforcement, plugging oil and gas wells up to 3000 m deep, for setting sound-absorbing motorway screens, etc.

Using monolithic foam concrete is a cost-effective way to solve the problem of insulation in old and reconstructed housing stock: foam concrete mass is poured into a permanent formwork, which is set to the wall to be insulated (inside or outside) at a distance equaling the required insulation thickness.

 

Foam Concrete Floors

One of the most labour-intensive operations in construction is the arrangement of levelling cement-sand screeds. The use of foam concrete screeds with a density of 800 – 1200 kg/m³ greatly facilitates the work and improves the characteristics of thermal conductivity and weight. In this case, the loads are reduced by 30-40% and sound insulation is enhanced due to porous structure of foam concrete.

Floor pouring can be done by different methods. Depending on the technology, the productivity is 3 to 15 m3/hour, foam concrete is fed by hoses either horizontally, to the distance of up to 60 metres, or vertically up to 30 metres in height. The thickness of the foam concrete layer for the floor base is 30-50 mm. A layer up to 100 mm thick can be applied. The minimum thickness of foam concrete layer when laying it onto floor slabs is 30 mm.

The greatest efficiency is achieved by using a combined floor construction option, when foam concrete with a density of 300-500 kg/m³ is used for the insulating bottom layer, and concrete screed or foam concrete with a density of 600-1200 kg/m³ is used as the top layer (in this case, no concrete screed is needed). The required thickness of foam concrete layers must be calculated separately in each individual case.

Another option is to use foam concrete of the same density.

Thus, foam concrete with a density of 800 kg/m³ can be used when reconstructing both residential and industrial buildings, which allows to simultaneously insulate the floors of flats and industrial premises and to level them, i.e. to make a screed which is similar to a mortar screed.

By applying monolithic foam concrete when pouring thick floor screed (100-200 mm), it is possible to obtain technological and economic effect that compares favourably with reinforced cement-sand or concrete screed, thus attaining a three-time reduction of the load on bearing structures, as well as additional thermal insulation.