Fly ash or pond ash for AAC blocks
I get to interact with many entrepreneurs who want to set up AAC block factory. One of the most frequently asked questions is whether they should use fly ash or pond ash as basic raw material. They are interested in using pond ash as cost difference between fly ash and pond ash is huge. But before I move ahead with the topic, let me give you a brief idea about what pond ash and fly ash are and differences and similarities between them. I have also listed implications and challenges involved with using pond ash for AAC blocks as a separate post.
Thermal power plants use Electrostatic Precipitators (ESP) for arresting ash from flue gas generated after combustion of coal. Usually there is a series of ESPs in a thermal power plant. Ash coming out from those at the beginning of this series has larger particle size ( in microns) and higher amount of Loss of Ignition (LoI – non-burnt coal particles). Since quantity of this peculiar ash is much higher compared to that coming out at the end of ESP series, storing this ash becomes a problem. In order to store and transport it, this ash is mixed with water and sent away to ponds. Hence the name ‘Pond Ash’. It is also called ‘Dyke’. Ash that comes out of ESPs near the end of the series is very fine, has lower LoI and is called as ‘Fly Ash’ or ‘Pulverized Fuel Ash’ (PFA). This ash can’t be stored in open and needs to be stored in structures called ‘Silo’. Fly ash is usually consumed by cement plants in order to produce Portland Pozzolana Cement (PPC) by blending it with Ordinary Portland Cement (OPC). Although there is difference between pond ash and fly ash in terms of particle size and LoI, common factor is Silica (SiO2) content. SiO2 level is almost similar in both kinds of ash generated from same power plant.
Coming to the commercial part, there is a huge disparity between cost of pond ash and fly ash. E.g. Most thermal power plants provide pond ash for free while they charge anywhere around Rs. 200-600/ton for fly ash. After adding transportation component, onsite price for pond ash comes to around Rs. 300-500/ton while that of fly ash would range between Rs. 500-1100/ton. Due to this huge difference it is not hard to identify what would be the raw material of choice for most AAC block factories.
While pond ash provides low cost alternative to fly ash, there are various issues related with using pond ash for producing AAC blocks. Primary challenge is bigger particle size and high LoI. These factors play a major role affecting rising (foaming) of green cake and hence quality of entire product. It takes some time to master recipes using pond ash. In fact some of the biggest and well-known players in AAC block market are using pond ash for their entire production. So my suggestion is to begin with fly ash and once you have established your product in market with product quality and marketing, then proceed to pond ash. You can start in incremental batches of 9-12 m3 each and check if end product meets the quality standards or not. Once required quality standards are achieved, port entire production to pond ash. By working in this manner you’ll have enough stability to cater to your market without affecting your product quality and supply of finished goods. For a detailed understanding of using pond ash for AAC blocks manufacturing, you can refer to – AAC Blocks Manufacturing Using Pond Ash – Implications And Challenges .