The influenc.e of Belgian PFA (Powder Fly Ashes) substitution (1 0 to 80 %) on the hydration of OPC (Ordinary Portland Cement) was studied . · Scanning Electron Microscopy (SEM) and Energy Dispersive Analysis of X-Rays (EDAX) were used in t his investigation. The hydration time varies from three days to two years. The release of alkalis from pure PFA has been investigat ed by Atomic Absorption Spectrometry. The SEM data concerning OPC + PFA mixtures, at different hydration periods, demonstra te the pozzolanic activity of PFA and the deep heterogeneity of the pozzolanic reaction. After one year of hydration, some PFA particles seem to be intact, whereas others are completely reacted. The authors propose an original mechanism for the understanding of the attack of PFA particles by Ca(OH)2 generated by cement hydration. The presence of a new well crystallized compound (hexagonal shape) in the long time hydrated fly ash cement allows us to propose some original explanation with regard to the beneficial effect of fly ash substitution i n preventing harmful alkali aggregate react ion.
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