Experiences of processing fly ashes recovered from United Kingdom stockpiles and lagoons, their characteristics and potential end uses

Engineers and specifiers have a responsibility to protect the environment from which raw materials for construction products are drawn. Perhaps the most pressing of these, no matter individual viewpoints, is minimising embodied CO₂, given the generally agreed contribution it makes to climate change. Furthermore, there needs to be less waste and a reduced dependency on primary aggregates. For this reason worldwide, there is a renewed desire to exploit fly ash resources 'locked up' in long-term storage, which in the United Kingdom alone equates to more than 100 million tonnes. This paper summarises a two-year UK/US research project that investigated the effectiveness of recovering and processing lagoon and stockpile fly ashes from UK long-term storage sites and their complete utilisation in a range of applications. The main aim was to reduce size and/or compositional heterogeneity of the raw feed by splitting and, thereby, concentrating the material into size ranges, which enable utilisation in a wide range of added value applications. These ashes were produced under low emission conditions and presented significant challenges to processing. These were addressed by combining primary separation, column and mechanical froth flotation and lamella hydraulic classification. The characteristics of the processed ash fractions are described, including material yield details. The performance of these in a number of matched end uses are reported, including as a cement/sand component in normal, precast and foamed concrete, highway construction and clay brick manufacture. The calorific value of separated carbon particles was measured to investigate their potential as a co-fuel for reburning or in Portland cement manufacture.