Fly ash concrete final report - a study of the reaction products using x-ray diffraction and SEM (82-01-2)

Abstract

The setting and strength gaining process of P.C. concrete containing Class c high calcium fly ash were related to the reaction products which were identified using X-ray diffraction and scanning electron microscopy ( SEM). Four fly ash concrete mixes ( 20, 30, 40 and 50 per cent replacement of cement by fly ash) and similar paste mixes were compared to control mixes for cure periods up to one year. Setting time and early compressive strength were adversely affected by the addition of fly ash. Beyond one week all of the fly ash concrete mixes gained strength at a faster rate than the corresponding control mixes. X-ray diffraction studies suggest that the retardation mechanism may be associated with the high levels of ettringite formed early in the hydration process and its conversion to monosulfoaluminate. A decrease in the level of calcium hydroxide, typical of pozzolanic activity, was not in evidence. SEM micrographs of fly ash spheres in concrete in various stages of hydration reveal an intricate crystal framework and a simple heat of hydration test is presented which helps explain strength gains observed.