AAC Blocks Manufacturing Process
Step 3 – Casting, Rising and Pre-curing
AAC blocks manufacturing process involves casting, rising and pre-curing. Once desired mix of raw materials is ready, it poured is in moulds. This process has different names. It is called casting, pouring or moulding. For current discussion, we’ll call it casting.
After thorough mixing, slurry containing fly ash (or sand), lime powder, cement, gypsum and Aluminium is poured in moulds. Moulds can be of various sizes depending upon installed capacity. Standard moulds for 160-500 m3/day are 4.2m x 1.2m x 0.65m in size. Some Autoclaved Aerated Concrete (AAC) blocks plant might have moulds of 3m, 4.8m or 6m.
Before casting, moulds are coated with a thin layer of oil. This is done in order to ensure that green-cake does not stick to moulds. While slurry is mixed and poured into greased moulds, Aluminium reacts with Calcium Hydroxide and water to form Hydrogen. Millions of tiny Hydrogen bubbles are released due to this reaction. This leads to formation of tiny unconnected cells causing slurry mix to expand. Such expansion may be twice its original volume. This process is very similar to rising of idli or dhokla dough. It must be noted that bubbles generated during AAC blocks manufacturing process are unconnected and closed bubbles unlike open bubbles of CLC blocks. Bubble size is usually 2-5mm. These cells are the reason behind light weight and insulating properties of AAC blocks. Once rising process is over, green-cake is allowed to settle and cure for some time. This ensures cutting strength required for wire cutting.
Usually rising and pre-curing process takes around 60-240 minutes. Rising is dependent on raw material mix and weather conditions. It should be noted that weather is a major factor affecting rising process. As reaction is affected by weather conditions, it is recommended to maintain constant temperature in pre-curing area. Due to this, pre-curing is also referred as ‘heating room pre-curing’. This can be achieved by deploying radiating pipes for indirect heating. It should be ensured that green-cake is not subjected to vibrations during pre-curing or else it might develop cracks.
At end of pre-curing process, green-cake is hard enough to be wire cut as per requirements. Pre-curing is not a complicated process, but it should be monitored constantly. Operators must monitor the slurry change during rising. Constant feedback must be provided to dosing, mixing and casting operators. Pre-form defects (cracking, sinking, etc) mainly occur during the process.
AAC blocks manufacturing process (…contd…)
AAC Blocks Manufacturing Process: Step 3 – Casting, Rising and Pre-curing