MICROSTRUCTURAL DEVELOPMENT IN HIGH-VOLUME FLY-ASH CEMENT SYSTEM

The writers present the results of research findings on the microstructural development in a high-volume fly-ash (HVFA) cement system containing 60% fly ash by weight of binder. Although the one-day strength of HVFA mortar is low compared to plain cement mortar, from three-day onward the strength starts to increase. Corresponding to the development of microstructure, the strength increases significantly at later ages and reaches 78% of the control specimen at 180 days. The literature survey reveals that hydration of fly-ash aluminate phases produces hydrates similar to those in cement. To reveal the fly-ash hydration process more vividly, a special type of cement containing very low C(3)A and alkalies was used with a class F fly ash. In order to precisely determine the role of high-volume replacement in a cementitious system, a comparative study of plain cement and HVFA paste was carried out. The mechanism of fly-ash hydration in this system is discussed and a correlation between its microstructure/strength development established. Better dispersion of cement grains, smaller Ca(OH)(2) crystals, and formation of more gel phases largely account for the strengthening effect. The effect of fly-ash replacement on neat water-to-cement ratio, initial pore content, Ca(OH)(2) crystallization, and dissolution of fly-ash glass phase are discussed. Comparison of experimental results determined by x-ray diffraction (XRDA) analysis, strength measurement, and thermogravimetry/differential thermal analysis (TG/DTA) is also presented.