Fly ash reactions in concrete

A suite of flyashes was assembled including representatives of ASTM Class F (low CaO) and Class C (high CaO) categories. These were chemically analyzed, particle size distributions were obtained, and X-ray diffraction studies carried out. The Class C flyashes had CaO contents of over 30% and subtantial Na2O and SO3 contents. In contrast the Class F flyashes had less than 4% CaO and low Na2O contents. X-ray diffraction disclosed substantial differences between the nature of the glass present in the two kinds of flyash, and in the suites of crystalline components present. The Class C flyashes contained cementitious compounds and alkali sulfates as well as more inert crystalline components. Compressive strengths of both mortars and concretes with 30% replacement of flyash for cement were equal to those of reference mortars or concretes in a few months with Class C flyashes, but strength development with Class F flyashes was much slower.