Autoclaved aerated concrete blocks (Building wall)
Location: Izmir, Turkey.
Partners involved: AKG Gazbeton
General description: The objective of the Case Study 2 is to manufacture Autoclaved aerated concrete (AAC) blocks at industrial scale, with standard dimensions in order to build a wall (360 units).
Autoclaved aerated concrete (AAC) wall blocks with standard dimensions (Length: 60 cm, width: 25 cm, thickness: 15 cm) were produced in G2/350 class by using ceramic waste, electric arc furnace (EAF) slag and ladle furnace slag (LF) as secondary raw materials supplied by regional providers (Çanakkale Seramik and Ekinciler Demir&Çelik, out the consortium) in order to ensure the replicability.
Supply chain:Read more
AAC is manufactured in six steps:
1) Preparation of raw materials.
The supply of SRM used in AAC wall blocks was made by the waste of local suppliers. The number of waste materials required for the industrial-scale production was 275.1 kg of ceramic wall tile waste, 27.6 kg of EAF slag and 192.6 kg of LF slag. The ceramic waste was already ground by the supplier, EAF and LF slags were ground in a ball mill to obtain the particles with the size of 90 µm.
The industrial production plan was prepared in such a way that three moulds of AAC slurry containing each of the three types of SRMs were moulded. One mould of the standard AAC was moulded at the beginning of the moulding of AAC slurry containing the first of SRM. At the end of the moulding of AAC slurry containing the last of SRM and between every three-moulds of AAC containing each of SRMs. In total, four moulds of standard AAC slurry were moulded with the AAC slurry containing each of the SRMs. So, 275.1 kg of ceramic wall tile waste, 27.6 kg of EAF slag and 192.6 kg of LF slag were required for the three-moulds of AAC slurry and in total, nine-moulds of AAC were moulded in the industrial-scale production. To prepare the mould of AAC slurry, firstly quartzite, limestone and SRM were added into the mixing tank, in which the adequate amount of water was added and everything was mixed to obtain the mineral-based sand slurry. The required amount of SRM was fed into the mixing tank manually while the other raw meals were fed into the mixing tank via the dosage unit concurrently since there was not an additional dosage unit for the SRM.
The recycled AAC slurry prepared by mixing water and recycled AAC particles in a separate tank was added into the mixing tank and was mixed with the sand slurry in the mixing tank. Finally, cement, lime and foaming agent were fed into the mixing tank respectively and all of the raw meals were mixed to obtain the final AAC slurry to be moulded into the metal moulds used for the industrial-scale production of AAC product.
In the pre-curing process, the AAC slurry prepared in the mixing tank was subjected to specific conditions, such as a temperature of 55°C and the relative humidity of 80%. The pre-curing of the AAC slurry in the metal moulds continued until the AAC slurry was transferred into the green cake form, also named as semi-product of AAC. Aluminium suspension, used as the foaming agent, was prepared by mixing the aluminium powder and water in a separate vessel. The aluminium powder reacted with lime (calcium hydroxide) resulting in the formation of hydrogen gas that bubbled up out of the AAC slurry causing the increase of 4 times of the original volume of the AAC slurry. During the pre-curing process, the AAC slurry was transferred into the green cake form that can be removed from the mould and movable by the lifting crane. The green cake removed from the mould was delivered to the cutting unit to be sized into the required size.
Before the cutting process, the surfaces of the green cake, except the bottom surface, were removed from the mould and smoothened by the steel wires. The production wastes obtained from the smoothening process were delivered to the recycled AAC tank to reuse them in the industrial production of the AAC. The volume of the green cake decreased from 6 m3 to 5.4 m3 after the smoothening process. The green cake containing each of the SRMs removed from the mould was cut perpendicular and horizontal respectively by moving the several steel wires of the cutting unit to obtain the AAC blocks with the size of 60 cm X 25 cm X 15 cm required for the demonstration site.
5) Autoclave curing.
An autoclave is a large, steam heated and pressurized equipment in which the autoclaved aerated concrete products are cured with high temperature and high pressure after the cutting process. Autoclave curing is one of the most important process for AAC manufacturing in terms of the mechanical properties of the final product. the green cakes were transferred to the autoclaves and cured at 12 bar steam pressure for 5 hours with a temperature of approximately 190°C. The bonds within the tobermorite structure that are formed in the pre-curing process get stronger with the effect of the high temperature and high pressure, and this provides an increase of the mechanical strength of the AAC product.
Each mould is 5.4 m3 and totally 16.2 m3 ceramic waste used AAC, 16.2 m3 EAF slag used AAC, 16.2 m3 LF slag used AAC, and 21.6 m3 of standard AAC products cured in the autoclave at 12 bar pressures for 5 hours with the total time of 9 hours for a complete process from the beginning to the end.
The AAC products that came out of the autoclave were transferred into the packaging unit. The AAC blocks containing each of the SRMs were put on wooden pallets for packaging with colourful plastic film on which the label printed for the project was stuck.
7) On-site Installation.
During the on-site installation, two outer walls (360 units of AAC wall blocks used for 54 m2 wall) constructed and filmed in a real building located in Kemalpaşa/Izmir. For on-site installation, totally 6 pallets ( 2 pallets for each SRM and each pallet contains 60 unit AAC) AAC products transferred to the demo site.