There are mainly five stages involved in the production of AAC block and panel units.
They include mixing of raw materials, casting, expansion or rising, wire cutting and autoclaving. For both AAC blocks and AAC panels, the manufacturing steps are same. The only difference is in the cutting step, where the steel cages are plugged into the mould in case of AAC panel instead of wire cutting in AAC blocks. The detailed study on the manufacturing of AAC has been carried out in the ACC block manufacturing industry (KD Infra, Guwahati, India). All the stages for producing AAC are depicted in Figure 1.2. The detailed manufacturing stages are described in the following subsections.
Figure 1.2 Block diagram representing the manufacturing stages of AAC block
1.5.1 Mixing of raw materials
Raw materials used for producing autoclaved aerated concrete are fine grade materials. Fly ash or sand, lime, cement, water and aluminum powder are main raw materials for producing AAC. A total of 3150 kg of mixture (4345 % fly ash or sand, 3638 % water, 1314 % cement, 67 % lime, and 0.0250.03 % aluminum powder by weight) is prepared for filling a single mold. Weighing of materials is carried out by an electronic scale controlled by programmable logic controller (PLC) system as shown in the Figure 1.3. The percentage of fly ash or silica sand used is the highest among other ingredients in aerated concrete mix, whereas the aluminum powder contributes the least. The variation of aluminum powder affects the final density of the AAC and hence, the strength of the final AAC products. After the addition of water to mix, the hydration starts with cement forming bond between fine aggregates and cement paste.
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Figure 1.3 Photographs of devices used for mixing process (a) PLC automatic system to control mix proportions and (b) machine for mixing raw materials
1.5.2 Pouring of final mix
After the preparation of the final mix with all its ingredients, the final mix slurry is poured into an empty oil-treated mold through up and down pouring device as shown in Figure 1.4. Presently, the mold used in the industry (KD Infra, Guwahati) is of size 4200 (length)×1200 (thickness)×600 (width) mm3. The internal surface of the mold is treated with waste lubricating oil (waste from machines) for avoiding any sticking of mix on the surface of mold. The volume of raw material mixes allowed to occupy the mold is fixed to 6065%
of mold volume. The remaining 3540% volume of mold is kept unfilled for the expansion process to save the material spillage.
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Figure 1.4 Photographs of mold and pouring device (a) position of mold and pouring device and (b) pouring of the slurry into the mold
1.5.3 Expansion of the mix slurry
After the pouring process is completed, the filled mold is transferred to the pre-curing room by a ferry cart for pre-curing under constant temperature. The pre-curing of about 45 h is done in order to get required hardness for wire cutting. The pre-curing also allows the cake for its expansion and gaining initial hardness. The aluminum powder reacts with calcium hydroxide, which is the product of reaction between cement and water. This reaction causes forming of microscopic air bubbles which results in increasing of paste-volume. The formed hydrogen, which is a lighter gas rises and is replaced by air which is a denser gas. As a result, the hydrogen gas escape out of the material leaving the small air bubble in it. The volume increase is dependent upon the amount of aluminum powder added. The volume increases with increase in the amount of aluminum powder. The reaction can be written as
Aluminum powder + Hydrated lime + Water = Tricalcium hydrate + Hydrogen (1) 2Al (s)+3Ca(OH)2+6H2O = 3CaO.Al2O.6H2O+3H2 (g) (2)
Lesser expansion will produce a higher strength (denser) material, while higher expansion will produce lower strength material (less dense). The whole expansion process takes about 45 h to complete depending upon the climate. Both the expansion and solidification process occur simultaneously. Figure 1.5 shows the expansion and solidification of slurry in the mold.
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Figure 1.5 Photographs of mold (a) mold partially filled with slurry just after pouring and (b) mold fully filled with slurry after the expansion or pre-curing process
1.5.4 Wire cutting of cake
A solidified soft green cake of size equivalent to the mold size of 4200 (length)×1200 (width)×600 (height) mm3 is obtained after the final expansion process. The mold, with green cake in it, is transported to the locating position under tilting hoister by friction wheels, which take the mold to the cutting line. The tilting hoister turns the mold by 90 degrees and puts on the cart for demolding. After the demolding process, the large green cake is transferred to the wire cutting line. The wire used in this cutting process is highly tensioned by air pressure with help of pneumatic cylinder. The size of wire used is of 12 mm in diameter. A group of wires in both horizontal and vertical directions are adjusted according to the variable desired dimension of AAC block.
The size of AAC block generally produced in the industry are 600×250×200 mm3, 600×200×200 mm3, 600×150×200 mm3, 600×100×200 mm3 and 600×75×200 mm3 corresponding to length, thickness and width dimensions. The variable sizes of the AAC block produced are as per the need for customer requirements. The wire cutting operations are carried out in two separate stages, horizontal cutting and then vertical cutting. The first cutting cart carries the large green mold to the horizontal cutting machine. The head of the large green cake is cut along the length direction (4200 mm). During the horizontal cutting, the large green cake is passed through the arrangement of 5 wires fitted in horizontal cutting machine. After the horizontal cutting, the large green cake gets divided into 6 slices of
dimension 4200×200×600 mm3. After the horizontal cutting, the exchange device lifts the green cake and put on the second cutting cart for vertical cutting. During the vertical wire cutting, the cutting machine frame swings back and forth like a saw. Finally, the vertical cutting perpendicular to the length direction of large green cake are done after passing these stacked 6 slices through a vertical wire cutting machine. In the vertical cutting, a 4200 mm length side large cake is divided into 43 parts of different sizes. The AAC block of different thickness is obtained through the arrangement of 42 wires adjusted at a different distance in the vertical cutting machine.
Finally, a stack of 258 (43×6) AAC blocks of same length and width but of variable thickness (600×250×200 mm3, 600×200×200 mm3, 600×150×200 mm3, 600×100×200 mm3 and 600×75×200 mm3) are obtained after the wire cutting. The total number of final AAC block obtained from a single mold may vary with desired dimension of individual AAC block and hence, accordingly the arrangements of wires are adjusted in the cutting machine. After the wire cutting, the stack of 258 AAC blocks is send to the autoclave for heat treatment or hardening. Figure 1.6 shows the wire cutting machines for cutting in horizontal and vertical directions.
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Figure 1.6 Photographs of wire cutting machine (a) wire cutting of large green cake in horizontal direction and (b) wire cutting of the large green cake in vertical direction
1.5.5 Autoclaving or hydrothermal treatment
After the final cutting operation, the stack of AAC block is transferred for heating or steam curing in an autoclaved chamber. This large steam-heated vessel is in fact a large pressure cooker by which the autoclaved aerated concrete is cured to gain strength.
Autoclaving of AAC green cake is carried out with a steam at a high temperature ranging
from 190210 ºC and high pressure ranging from 1.01.3 MPa. The whole autoclaving process completes in 1618 h. During the autoclaving or hydrothermal treatment, the formation of tobermorite (5CaO.6SiO2.5H2O) occurs (Bisceglie et al. 2014). The formation of tobermorite, during the cooking period is affected by the presence of aluminum powder.
The mechanical properties of final AAC are influenced by the formation of tobermorite. After the autoclaving, the cured or hardened stack of AAC blocks is pulled out from the autoclave chamber with the help of ferry cart. The final product is kept outside for cooling so as to attain the room temperature. Finally, the finished AAC blocks are taken to the storage yard with the help of fork lift. All the stages such as mixing of raw materials, pouring, expansion, wire cutting and autoclaving are carried out in a single industry with the help of mechanized transfer line and material handling equipment. Figure 1.7 represents the autoclave chamber for steam curing of the green cake.
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Figure 1.7 Photographs of hydrothermal treatment of green cake: (a) autoclave chamber and (b) final AAC block after autoclaving