Storage, Ageing and Processing of Pulverised Fuel Ash for the Manufacture of Autoclaved Aerated Concrete

Autoclaved aerated concrete (AAC) is a lightweight masonry product, which has been manufactured in the UK since the 1950s, using pulverised fuel ash (PFA), also known as fly ash, as a major raw material. PFA is a by-product of the generation of electricity in coal-fired power stations. With the move away from coal-fired power generation, PFA production in the UK has declined rapidly in the last decade, and is expected to cease entirely by late 2024. However, there is a vast potential resource of over one hundred million tonnes of PFA available in landfill, stockpile, or lagoon storage.

This study examined the ageing of fresh ash from three UK power stations, placed in stockpile and lagoon storage scenarios, for periods of up to four years. When ash was placed in storage and exposed to water and atmospheric carbon dioxide, it underwent physical changes, becoming coarser and agglomerated. Chemical changes also took place, with mobilisation of soluble species including calcium and sulphate ions, leading to the formation and deposition of new mineral phases including gypsum and calcite. Such changes were more rapid and pronounced when ash was stored in stockpile conditions compared to lagoon conditions, due to cyclic wetting and drying and greater exposure to the atmosphere in the stockpile. The bulk composition of the ashes was unaltered by storage and ageing.

Autoclaved aerated concrete made with stored PFA was found to have a reduced compressive strength, reduced mix stability and a poor cosmetic appearance compared to that made with fresh ash from the same source. This was attributed to the observed agglomeration of ash during storage. Ball milling or high shear mixing of stored ash modified its particle size distribution, so it was comparable to that of fresh dry ash, and the properties of AAC made with it were markedly improved. Depending on the degree of processing, AAC strengths comparable to or greater than those made with fresh ash could be achieved. This was closely related to an increase in fines content (<10µm) and specific surface area (SSA) of the processed ash.

The mechanisms which underlie the changes in strength of AAC with ageing and/or processing of ash are unclear. The air pore structure of the AAC, and the distribution and interconnection of reaction products formed during autoclaving (tobermorite and other calcium silicate hydrates) within the AAC skeletal matrix appear to be important influences. There was no apparent change in the total quantity of tobermorite formed. Ash agglomerates were disruptive to the continuity of the skeletal matrix and detrimental to strength.

The practical outcomes of this research, particularly the identification of effective ash processing methods and a deeper understanding of the required particle size distribution of processed ash, will enable the utilisation of stored PFA for the continued manufacture of ash based AAC products in the future. Further work is required to understand the fundamental mechanisms which influence the strength differences observed in AAC made with fresh, stored, and processed ashes.