Pulverised fuel ash (PFA) is a reactive silica source used in the
manufacture of autoclaved aerated concrete (AAC). Experiments studied the
hydrothermal reactions of PFA samples from two UK power stations with
calcium hydroxide at 457 K, for periods up to 21 h. These conditions are
comparable to those used in the manufacture of AAC.
The process is characterised by the rapid consumption of ash particles.
Associated with this is the solubilisation of large amounts of silica, alumina and
alkalis. The formation of a semi-crystalline calcium silicate hydrate and a
hydrogarnet phase occurs during the early stages of autoclaving. The
hydrogarnet phase persists under the conditions studied, but conversion of the
calcium silicate hydrate into tobermorite occurs with prolonged autoclaving.
Differences in the hydrothermal performance of the two PFA samples are
evident, which cannot be explained by the bulk elemental composition.
Ash fractions obtained from a centrifugal air classifier have different
reactivities during autoclaving and can result in specimens with different
compressive strengths. Quantitative x-ray diffractometry showed that high
levels of aluminosilicate glass are associated with the fine ash fractions, whereas
most quartz, haematite and magnetite is associated with the coarse fractions.
Significant differences exist in the mineralogical analyses of the two sets of ash
fractions obtained from the bulk ash samples. The coarse ash fractions have the
most varied morphology and composition.
Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.