Gulf Construction Online - AAC concrete makes world cleaner place

Green Buildings

A miniature model of a structure made of AAC.

AAC concrete makes world cleaner place

The environmental benefits of AAC are apparent right from the manufacturing stage, says Hess AAC Systems, highlighting how it reduces raw material and energy consumption as well as carbon dioxide emission.

01 June 2010

AAC (autoclaved aerated concrete), although lightweight, is a solid building material with unique properties and advantages that are built in during the production process.

Its unique properties enable AAC – also known as PAC (precast aerated concrete) – to be used in various types of construction like houses, commercial and industrial buildings, schools, hospitals, hotels and many other applications, offering incredible opportunities to increase building quality.
AAC plants are preferably erected where the source of main raw material is located. Silica sand, a natural product, makes up the highest content of AAC – around 55 per cent, depending on the density. The energy consumed by this raw material is basically in the delivery of the material from the excavation site to the factory and then in its treatment for slurry preparation.
Lime and cement form around 35 per cent (depend on density) of AAC and the energy consumption levels are comparable with those by other building materials.
AAC derives an excellent energy balance from its inherent low density trait. If we compare the commonly used building materials, we can see a huge difference in the average densities (Figure 1).
Although for part of its production, AAC needs more energy when compared to other building materials due to the autoclave process, however, the total energy balance shows that the energy used in the production of the raw material as well.

AAC can be used for any part of the building.

For example, to produce cement and lime 4,400 MJ and 6,000 MJ respectively of primary energy is required. To reduce the consumption of primary energy in the building material, it is necessary to minimise the use of lime and cement.
Over the years, the density of AAC has been adjusted to reduce the consumption of raw material to a minimum, while maximising the corresponding strength and thermal insulation.
Aluminium paste or powder is also required in the manufacture of AAC. While it may be argued that production of aluminium is highly energy-intensive, the paste or powder used for AAC is recycled material – that is, waste material from the normal manufacturing process. Hence, aluminium accounts for just seven per cent of the primary energy used in AAC production.
For the production process of AAC itself, around 40 per cent of primary energy is needed. Of this, the biggest portion is used for the autoclave process. AAC needs to autoclave over six to eight hours under a pressure of 11 bar. To produce the required saturated steam, natural gas-operated boilers are generally used, with which the emission can be reduced to a minimum.

Zero waste
Water circulation for AAC production is possible as a closed cycle where there is no wastage of water. Condensate coming from the autoclave is used for pre-heating and as mixing water for production. Condensate can be neutralised by smoke gas from the boiler and be used for the sand milling process.
During a normal production cycle, some waste material is generated from net cutting or as a safety layer for transportation. When state-of-the-art technologies are used, no waste material will go to autoclave. In Hess technology, for example, no process-related waste is produced as all the cut-offs before autoclaving are mixed with water and reintroduced as active raw material into the production process. That avoids waste and saves the consumption of raw material as this technology allows for calculation of accurate material proportions.

Cut in fuel consumption
While in the Gulf it is not necessary to cover the products for storage and during transportation, it is common practice to use shrinking foil in Europe, where as well as providing weather protection, the foil bearing the company logo serves advertising purposes. Transporta-tion can be done on simple wooden pallets, the capacity of which depends on the packing system and the client’s requirements – ranging between 1.5 to 1.8 cu m.
One truck can carry around 20 tonnes. Figure 2 shows the volume (cu m) of blocks of various building material that can be carried in one trip. Figure 3 shows the number of trips required to a building site needing 1,000 cu m of building materials. This reveals the enormous savings in fuel consumption during transportation, while at the same time cutting down carbon dioxide (CO2) emissions. Figure 4 provides a comparison of primary energy consumption for the production of common building materials.

Energy-efficient structures
Turning to energy savings in actual use, AAC provides thermal insulation benefits to a building. Considering that about 80 per cent of energy is used for heating and/or cooling of a building, the structural design of a house must avoid thermal transfer as much as possible, particularly through walls, ceilings and roofs, which make up the largest surface area of a building. Hence, the thermal conductivity of building materials used is an important consideration. The heat transmission coefficient ‘U’ is measured in W per sq m K.
The thermal resistance R is calculated using the thickness of wall divided by thermal conductivity of used material. In the Middle East, the wall thickness is generally 200 mm and a 200-mm-thick AAC wall fulfils the requirement of common thermal regulations in the region. AAC can fulfil any thermal regulation by adjusting to a lower density, which is possible to achieve through the production recipe and by applying of different wall thickness.
To get the same U value with other building materials, the consumption of materials will increase dramatically as thicker walls are required. Taking perforated clay bricks as an example, to get a wall with the same heat transmission coefficient as AAC, the thickness has to be increased to 720 mm instead of 200 mm – an increase of 360 per cent in building material per sq m!
Thanks to the strength of AAC with a minimum compressive strength of 2.5 N/sq mm and density of 450 kg per cu m, it is possible to use it for wall-bearing systems. It is important to avoid any thermal bridges in the building. Efforts continue to enhance the quality of AAC without compromising its physical properties.
Many studies have shown that AAC is a true green product, starting from production to its use within a building.
All in all, AAC is a high-tech development that saves sources of raw material, avoids unnecessary use of energy and thus reducing CO2 emissions, which is a topic that is rightly on the agenda of any environmentally-conscious government, business or person.