Microstructural Development in High‐Volume Fly‐Ash Cement System

The writers present the results of research findings on the micro‐structural 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 $\mathrm{Ca}(\mathrm{OH}{)}_2$ 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, $\mathrm{Ca}(\mathrm{OH}{)}_2$ 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.