AAC blocks are made from portland cement, silica rich material like fly ash or sand, lime, water and aluminum powder as an expansion agent. The production process of AAC blocks is very carefully monitored to insure a controlled consistency in the proportions and timing of all ingredients that are used in each batch. The materials are first mixed into a slurry and then poured into large moulds where the expansion agent reacts with the alkalis in the cement and lime to produce millions of small hydrogen gas bubbles. The mix expands and rises almost 2 times its initial volume. The hydrogen evaporates and the 'cake' sets up and hardens into a stable closed cell matrix which can then be precision wire-cut into blocks or panels. The green aerated concrete is then steam cured in a pressurized autoclave for about 12-14 hours where upon it undergoes a second chemical reaction and transforms into the mineral Tobermorite or calcium silicate. The finished materials are palletized and delivered by truck directly to the job site. Very first step of AAC blocks manufacturing process is raw material preparation. List of raw materials and relevant details are mentioned below.
Raw Material Preparation of AAC blocks :
- For manufacturing Autoclaved Aerated Concrete blocks Key ingredient is silica rich material like fly ash or sand. Most of the AAC companies in India use fly ash to manufacture AAC blocks. Fly ash is mixed with water to form fly ash slurry. Slurry thus formed is mixed with other ingredients like lime powder, cement, gypsum and aluminium powder in quantities consistent with the recipe. Alternately sand can also be used to manufacture AAC blocks. A 'wet' ball mill finely grinds sand with water converting it into sand slurry. Sand slurry is mixed with other ingredients just like fly ash slurry.
- Lime powder is required for AAC production which is obtained either by directly purchasing it in powder form or by crushing limestone to fine powder at AAC factory. Purchasing lime powder might be little costly so many manufacturers opt for it rather than investing in lime crushing equipment like ball mill, jaw crusher, bucket elevators, etc. Lime powder is stored in silos fabricated from mild steel (MS) or built using brick and mortar depending of individual preferences.
- Portland Cement is required for manufacturing AAC blocks. Cement supplied by 'mini plants' is not recommended due to drastic variations in quality over different batches. Cement is usually stored in silos. Some factories might plan their captive cement processing units as such an unit can produce cement as well as process lime.
- Gypsum is easily available in the market and is used in powder form. It is stored in silos.
- Aluminium powder or paste is easily available from various manufacturers. As very small quantity of Aluminium powder or paste is required to be added to the mixture, it is usually weighed manually and added to the mixing unit.
Dosing and Mixing :
Process of dosing and mixing defines the quality of final products which is used to form the correct mix to produce AAC blocks. Fly ash/sand slurry is pumped into a separate container. Once the desired weight is poured in, pumping is stopped. Similarly lime powder, cement and gypsum are poured into individual containers using screw conveyors. Once required amount of each ingredient is filled into their individual containers control system releases all ingredients into mixing drum. Mixing drum is like a giant bowl with a stirrer rotating inside to ensure proper mixing of ingredients. Steam might also be fed to the unit to maintain temperature in range of 40-42℃. A smaller bowl type structure used for feeding Aluminium powder is also attached as a part of mixing unit. Once the mixture has been churned for set time, it is ready to be poured into molds using dosing unit. Dosing unit releases this mixture as per set quantities into molds for foaming. Dosing and mixing process is carried out continuously because if there is a long gap between charging and discharging of ingredients, residual mixture might start hardening and choke up the entire unit.