The Technology of Autoclaved Aerated Concrete … Technology of Autoclaved Aerated Concrete Blocks...

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The Technology of Autoclaved Aerated Concrete Blocks & Its Greener Side

Autoclaved Aerated Concrete (AAC) Blocks also known as autoclaved cellular concrete (ACC) or autoclaved light-weight concrete (ALC), was invented in the mid-1920s by the Swedish architect and inventor Johan Axel Eriksson. It is a lightweight, precast building material that simultaneously provides structure, insulation, and fire and mold resistance. AAC products include AAC blocks, AAC U Blocks, AAC wall panels, AAC floor and roof panels, and AAC lintels. It has become one of the most used building materials in Europe and is rapidly growing in many other countries around the world. AAC is a lightweight, load-bearing, high insulating, durable building product, which is produced in a wide range of sizes and strengths. AAC offers incredible opportunities

to increase building quality and at the same time reduce costs at the construction site. AAC is produced out of a mix of quartz sand and/or pulverized fly ash (PFA), lime, cement, gypsum/anhydrite, water and aluminium and is hardened by steam-curing in autoclaves. As a result of its excellent properties, AAC is used in many building constructions, for example in residential homes, commercial and industrial buildings, schools, hospitals, hotels and many other applica-tions. The construction material AAC contains 60% to 85% air by volume. The solid material part is a crystalline binder, which is called Tobermorite by mineralogists. Besides the binding phase Tobermorite contains grains of quartz and in minor amounts some other minerals. The chemical compo-

Sonjoy Deb, B.Tech, Civil Associate Editor

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sition of Tobermorite shows silicium dioxide, calcium oxide and water. The silicium dioxide is obtained from silica sand, fly ash (PFA), or crushed quartzite. It is possible to obtain silicium dioxide as a by-product from other processes, e.g. foundry sand. The calcium oxide is obtained from quick lime, hydrated lime and cement. Gypsum/Anhydrite is added in small quantities as a catalyst and for optimizing the proper-ties of AAC. Aluminium powder / paste is used as expanding agent. Refer Figure 1 for AAC blocks and applications.

namely roof/floor panels and lintels. Good structural proper-ties of the newly created AAC material soon spread all over Western Europe, with more than 6 plants in Sweden alone.

AAC Production Types

There are two types of autoclaved aerated concrete (AAC) production methods which are chemical and mechanical processes. In the chemical process, some metallic com-pounds would be added to react and generate tremendous amount of air bubbles in concrete texture while in mechani-cal process expansive foaming agent is normally employed. In general, AAC could be prepared in a high pressure auto-clave under conditions of temperature and pressure higher than 180oC and 12 bar, respectively. Approximately the po-rosity is 80% of the volume of the processed cement, result-ing in its very light weight. Additionally, AAC has excellent properties of acoustic insulation, fire resistance and allergy-free while it tends to suffer edge damage or breakage if it is subject to abrasion or collision.

The Chemistry & Micro Structure Behind AAC

All in all, the combination of cement, lime, gypsum (an-hydrite), finely ground sand and most importantly aluminium powder causes the mixture to expand considerably. From the beginning to the end the simplified chemical reactions look as follows:

1 CaO+H2O Ca(OH)2 + 65,2 KJ/mol 2 3Ca(OH)2 + 2Al + 6H2O Ca3(Al(OH)6)2 + 3H2 3 6SiO2 + 5Ca(OH)2 5CaO • 6SiO2 • 5H2O

This is the final Tobermorite or Hydrated Calcium Silicate C5S6H5. Refer Figure 2 for model of Tobermorite structure consisting of protonated silicate ions and water.

Figure 1: Typical AAC applications

History of AAC

In 1880 a German researcher Michaelis was granted a patent on his steam curing processes. Czech Hoffman suc-cessfully tested and patented in 1889 the method of “aerat-ing” the concrete by carbon dioxide. Americans Aylsworth and Dyer used aluminium powder and calcium hydroxide to attain porous cementitious mixture for which they also re-ceived a patent in 1914. Swede Axel Eriksson made a serious next step towards developing modern AAC when in 1920 he patented the methods of making aerated mix of limestone and ground slate (a so-called “lime formula”). The real breakthrough in the masonry industry came in 1923 when same architect Axel Eriksson discovered that this moist foamed mass can easily handle pressurized steam curing process, also known as autoclaving. While applying for a patent two crucial conclusions were drawn: 1. the material hardened fast thanks to the autoclaving process 2. shrink-age was almost absent after steam curing compared to the normal air curing. Additionally, later it was also discovered that alternative materials, such as pulverized ash, could be used instead of lime/cement, allowing to economize on ex-pensive raw material binder.

Eriksson’s success immediately attracted a much-needed commercial interest and in 1929 the first large scale manufacturing facility of these artificially-made crystalized stone blocks was launched in a factory “Yxhults Stenhuggeri Aktibolag“, Sweden under the name Yxhult. In 1940 the “Yx-hult” name was changed to YTONG as this name was easier to pronounce. In 1932 the factory Carlsro Kalkbruk Skovde started with AAC block production and the product acquired the brand name Durox. Important competitor arose in 1934 which started to manufacture AAC blocks under the brand name Siporit (as of 1937 a well-known Siporex). Siporex was also the first to introduce AAC reinforced elements in 1935,

Figure 2 : Model of Tobermorite structure consisting of protonated silicate ions and water

SEM investigation on the surface morphology of the two typical samples of light weight AAC and Non-AAC blocks, it could clearly be observed that the surface of Non-AAC sample consisted of large pores while the AAC sample ex-hibited smoother surface with much smaller porosity. Refer Figure 3, 4 and 5 for SEM micro graph, surface morphology and crystalline structure comparison of AAC and non AAC concrete.

Production process of AAC : The detailed production process for AAC blocks is mentioned below. Refer Figure 2 for the flow diagram of the manufacturing process.

Raw material preparation and mixing

A ball mill wet-grinds the quartz sand with water to a

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sand slurry. The sand slurry is stored in slurry tanks and pumped into the slurry weighing hopper in the mixer tower. The binders (lime, cement and anhydrite) are stored in silos. It is also possible to mill the anhydrite together with the sand in the ball mill. The aluminum powder or paste is prepared in a separate building where it is dispersed in water. All the components are accurately weighed, and are released into the mixer in a pre defined order. AAC can also be prepared with alternative raw materials, for example with fly ash.

Casting, rising/pre-curing and mould circulation

The mould consists of four fixed sides and one detach-

able platform. The inner mould surfaces are covered with demoulding release oil before casting. This oil is applied ei-ther manually or automatically. The mix is then poured into the moulds. A mould circulation system conveys the moulds to the rising area, where the cake pre-cures for 2-3 hours after which it is ready for cutting. Depending on the plant de-sign, the moulds are handled by a mould traverse or by the tilting-manipulator.

Reinforcement preparation

Depending on the required capacity, the reinforcement cages can be outsourced or welded on site. The cages can then be assembled as per mould and hung onto holding frames with cross bars and needles. A corrosion protection is then to be applied. Immediately after the mix poured into the mould, the reinforcement frame assembly is inserted. Before cutting of the cake the holding frames with needles are lifted, leaving the reinforcement in the cake.

Cutting

The cake can be cut by high precision cutting machines. Cutting is done by cutting knives and by pneumatically ten-sioned cutting wires. Refer Figure 2 below.

Figure 3: SEM micrographs of non-AAC and AAC specimens

Figure 4: Comparison of surface morphology of Non-AAC and AAC speci-mens

Figure 5: Comparison of crystalline structure of Non-AAC and AAC speci-mens

Figure 2 : Flow Diagram of the manufacturing process of AAC blocks

Figure 2: Cutting of AAC

Back-tilting and bed removal

In most tilt-cake systems the cake is autoclaved in the vertical position. After the cutting is completed, the cake is tilted back by 90° onto a cooking frame. No part of the mould or platform used for cutting should go into the autoclaves. After the cake has been tilted back into the horizontal ori-entation, the bottom/ bed waste will be removed before au-toclaving.

Frame and bogey circulation

The green cakes on the cooking frame are stacked three high onto autoclaving cars, referred to as bogeys. Autoclave buffer tracks in front of the autoclaves ensure that the cut-ting and packaging processes are less dependent on one an-other. An autoclave traverser is used for loading and unload-ing the autoclaves, ensuring that this process is performed in the shortest possible time, in order to optimise autoclav-ing capacity.

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Autoclaving

In the autoclaves the cakes are cured for 10 – 12 hours at a temperature of 190° C with saturated steam at a pressure of 12 bar. The fully automatic autoclave control system en-sures a safe and optimal autoclaving process, also allowing for steam transfer and energy reutilization in combination with the condensate system.

Unloading and packing

After autoclaving is completed, the cakes are destacked and unloaded from the cooking frames. Usually blocks will be delivered as packs on wooden pallets, strapped and/or covered in foil.

Process control and plant automation

The entire production process can be controlled by mod-ern available automation systems. User-friendly, multi-lin-gual operator interfaces with touchscreen monitors allow easy and understandable operation.

Green Aspect of AAC Blocks

AAC offers both material and performance aspects from a sustainability perspective. On the material side, it can contain recycled materials like fly ash and rebar, which may help contribute to credits in LEED or other green rat-ing systems. Further, it incorporates such a large quantity of air that it contains less raw material per volume than many other building products.

Depending on the location of manufacturing relative to the project site, AAC may also contribute to local materials credits in some green building rating systems.

Significantly inherent product characteristics lead to AAC being an important product of energy efficient constructions in world, hence the reason why the use of AAC continuously increased during the past decades. Monolithic AAC masonry and AAC masonry with marginal additional insulating layers are among the common exterior wall solutions in energy ef-ficient constructions. A single solid leaf of AAC masonry or element can be used to meet all the requirements for the house wall design. This provides a cost optimal solution resulting in an overall structure that is robust and durable, with over 80 years of experience. Despite low heat conductiv-ity AAC disposes a relatively high density. That means that high room temperatures in summer – as nowadays reported of highly insulated houses - can be reduced with AAC con-structions. As a consequence constructions with AAC are able to avoid or/and reduce the energy demand for cooling. Therefore, especially for hot summer regions, AAC can pro-vide a strong contribution to meet the goals of CO2 reduc-tion even under these special conditions. There are three established ways to construct a well insulated masonry for a nearly zero energy building, (a) Monolithic wall construction, (b) Monolithic wall construction with the combination of in-sulating material & (c) Cavity wall with insulation. The green advantages can be grouped under the following buckets.

Linear Thermal bridges

AAC is an isotropic building material showing the same

characteristics in any spatial direction assuring low energy losses at corners and joints (thermal bridges) of buildings. In buildings with low envelop U values, thermal bridging causes a higher amount of thermal transmission. AAC pro-vides a solution to this design issue by providing solutions developed to reduce the heat loss, including construction products such as AAC lintels and slabs, which further re-duce thermal losses.

Air tightness

Air tightness also becomes a key factor in the overall heat loss balance. AAC can be used to great advantage, since it is intrinsically airtight; constructions do not need ad-ditives such as foils or other artificially produced materials that are used in order to guarantee air tightness. In addition, the indoor air climate is healthy with no mould growth and with a good humidity control. Measurements prove that low infiltration rates can be obtained from on site testing. This explains why solid constructions can compare with light-weight frame constructions in all cases. To the contrary: the AAC capacity to keep the moisture out of the room leads to a comfortable and healthy internal environment.

Thermal mass

Constructions from AAC lead to a reduction of overheat-ing by its thermal mass and ability to retain heat during the hot periods and release during the cooling period. Good stor-age properties lead to a balanced room climate and offer an essential precondition for the comfort of inhabitants. AAC has both low thermal conductivity and an inherent heat storage capacity. In summer the room temperature in AAC buildings on average is 3-5°C lower than in lightweight constructions. The cooling load in AAC buildings basically to be covered by electricity is solely reduced caused by the good combination between low thermal conductivity and high storage capacity by 10-15 %.

Thermal insulation

Building components made of AAC provide low energy losses over the building envelope due to low heat conductiv-ity. It has the best performance of any solid load bearing ma-terial, which results in a single material capable of meeting many design functions.

Other Advatdages of AAC

- Large variety of sizes (Refer Figure below)

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- Extremely lightweight- High compressive strength- High dimensional accuracy - Great acoustic insulation- High fire resistance- Termite resistance- Increased Work Speed (Refer Figure below)

Conclusion

AAC combines insulation and structural capability in one material for walls, floors, and roofs. Its light weight/cellu-lar properties make it easy to cut, shave, and shape, accept nails and screws readily, and allow it to be routed to create chases for electrical conduits and small-diameter plumbing runs. This gives it design and construction flexibility, and the ability to make easy adjustments in the field. But the materi-al does have some limitations. It is not as widely available as most concrete products, though it can be shipped anywhere. If it has to be shipped, its light weight is advantageous. Be-cause it is lower strength than most concrete products or systems, in load-bearing applications, it must typically be reinforced. It also requires a protective finish since the ma-terial is porous and would deteriorate if left exposed. AAC block buildings can be used for producing net zero energy buildings. Research has established that building with an appropriate share of renewable energy source and an exter-nal wall structure erected from AAC masonry units or panels with a heat transfer coefficient of U = 0.20 –0.25 W/(m²K) can achieve a zero or even plus primary energy need.

Reference

1. www.hess-aac.com

2. http://www.cement.org/homes/ch_bs_autoclaved.asp

3. http://www.aeratedconcreteblock.com/

4. A Study On Material Properties Of Autoclaved Aerated Concrete (Aac) And Its Complementary Wall Elements: Their Compatibility In Contemporary And Historical Wall Sections, A Thesis Submitted To The Graduate School Of Natural And Applied Sciences Of Middle East Technical University By Simge Andolsun, September, 2006

5. h t tp : / /www.a ircrete-europe.com/ images/download/W.M.%20van%20Boggelen%20-%20History%20of%20Autoclaved%20Aerated%20Concrete.pdf

6. http://infosys.korea.ac.kr/PDF/JIEC/IE13/IE13-7-1103.pdf

7. http://www.ktu.lt/lt/mokslas/zurnalai/medz/medz0-99/14%20construction...(pp.356-362).pdf

8. http://www.eaaca.org/index.php/news/175-nearly-zero-energy-buildings-built-with-aac

9. https://extension.ucdavis.edu/unit/green_building_and_sustain-ability/pdf/resources/auto_aerated_concrete.pdf w

GREEN BUILDING MATERIAL: AAC