Before I begin this post about using pond ash for manufacturing aac blocks, I would like to thank you all. I am immensely grateful to all my visitors. But I would like to make a special mention of those with strong association with AAC blocks manufacturing sector. Not only do they visit my website, but also enrich my knowledge by sharing interesting information. Their experiences as their valuable inputs and help in growth of AAC manufacturing in emerging markets like India.
Shared below is a composite post contributed by Mr. Marcel Habers from Hess Group. He has discussed the issue of using pond ash for manufacturing AAC blocks and challenges involved. Information provided below is provided by him and I have just spruced it up:
In my opinion, if you want produce high quality AAC blocks I do not recommend to use pond ash.
Experiments and factory experience has demonstrated that ash recovered from these stockpiles is generally unsuitable for AAC manufacture.
Problems arise because of the chemical and physical changes that occur to ash particles exposed to water for period of months to several years. Weathering process has profound effect on the Pulverized Fuel Ash (PFA). This severely limits use of pond ash for manufacturing AAC blocks. Weather effect and compaction leads to physical agglomeration (chunk formation) of ash particles.
Exposure to water and atmospheric CO2 leads to the formation of compounds such as Calcite, Gypsum and Ettringite. Prolonged weathering can lead to some dissolution of the Aluminosilicate glass, which constitutes many of the ash particles. This leads to deposition of amorphous clay-like material on the surface.
Formation of various compounds as a result of chemical reactions bound discrete ash particles into agglomerates. An impermeable layer is formed around ash particles. This layer impedes dissolution of calcium silicate hydrates, which bind the intercellular matrix together, depending on the availability of silicate ions. The practical consequence of this reduction in the hydrothermal reactivity of the PFA is leads to lower compressive strength of Autoclaved Aerated Concrete products.
Agglomeration of the ash gives lumps and gritty particles, making handling of the raw material troublesome. It is difficult to disperse the PFA effectively and results in low quality of slurry mix. An unsatisfactory cellular structure forms during initial setting and this leads to unstable mixes, which are prone to slumping or collapse.
Q. If we take this “pond ash” from thermal power plant before it is mixed with water, will it be a good substitute for fly ash for manufacturing AAC blocks?
If you want to use the bottom ash first you have to grind it with a ball mill. Bottom ash contains a lot of slag and contaminants. The quality of bottom ash is never consistent. If you want to run a successful AAC plant, one of the important factors is a consistent quality of the raw materials.