Hydrothermal performance of pulverised fuel ash and the manufacture of autoclaved aerated concrete
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.