Microstructural Development in Fly Ash Modified with Lime and Gypsum

Physicochemical and microstructural developments of a low-lime fly ash modified with 6 and 10% lime and 1% gypsum were studied through X-ray diffraction, differential thermal analysis, scanning electron micrograph, and energy-dispersive X-ray microanalysis tests to gain information on the fly ash-lime and fly ash-lime-gypsum interaction. Specimens were cured up to 10 months at 30 ± 1°C, which prevails commonly in the Tropics. X-ray diffraction test results showed the appearance of new peaks, some of which are not so prominent, indicating that the new phases formed may be in small quantity or in an amorphous state. Scanning electron micrographs of specimens modified with both lime and gypsum showed evidence of the development of a compact matrix at 3 months' curing and a densified compact network of pozzolanic reaction products of fly ash-lime-gypsum with the increase in curing period to 10 months. Further, this development was substantiated by improvement in strength and durability and reduction in permeability of the stabilized material. The modified fly ash with improved engineering characteristics has potential for applications in civil engineering construction.