A method of estimating the hydration reactivity of coal ash with high precision and determining the amount of milling water for obtaining a milling product in funicular form at the stage of milling; and a molding method being free from the problem of adhesion to a pressurization board at the stage of molding after the milling. When any collected coal ash exhibits a corrected basicity ((CaO+Fe.sub.2O.sub.3+MgO)/SiO.sub.2 (weight ratio)) of 0.1 or higher and a reactivity index (corrected basicity/(R.sub.2O/Al.sub.2O.sub.3) (weight ratio)) of 10 or higher, the coal ash as a raw ash is transferred to raw ash silo (2). Otherwise, the coal ash is transferred to waste ash silo (3). The raw ash within the raw ash silo (2) is poured in mill (8), and lime and gypsum are added and further milling water is charged thereto and milled. The amount of milling water is regulated so that with respect to agitator power per weight of material milled by the mill, the ratio of (average of power during 30 to 40 sec after completion of water charging)/(power at idling) ranges from 3 to 4, and so that with respect to agitator power per weight of material milled during 30 to 90 sec after completion of water charging, the ratio of (width of power fluctuation)/(average of power) ranges from 0.1 to 0.3.