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The behavior of cement mortar incorporating fly ash varies with chemical and physical characteristics of fly ash. Of many parameters affecting the compressive strength of fly ash mortar, five were investigated in this study. They are the particle size, the particle size distribution, fineness of fly ash, the CaO content and the Fe2O3 content. Fly ashes from two different sources were used in this study. One comes from the Public Service Electric and Gas (PSE & G)'s electrical generation station in Hudson County and the other is from the PSE & G plant in Mercer County. Each was separated into seven different particle size ranges. The results of the tests indicated that there are direct relationships between the particle size, Blaine fineness, mean diameter of fly ash and the compressive strength. Fe2O3 content of fly ash is not found to have a notable effect on the mortar strength. CaO content of fly ash varying from 2.47% to 6.76% also has no significant effect on the strength of mortar. Additionally, the effect of chromium on cement mortar and concrete was studied. Trivalent and haxavalent [sic] chromium were used in the experiment. The compressive strength of mortar and concrete incorporating Cr(III), as well as mortar and concrete incorporating Cr(VI) were tested up to 180 days according to ASTM C-109. Leaching tests were conducted with different pH extractants. The results of the tests showed that the leaching characteristic of Cr(III) mortar and Cr(VI) mortar are different. The influence of Cr(III), and Cr(VI) on compressive strength of mortar and concrete is also varied. The results of leaching tests indicated that cement is very good for immobilizing Cr(III) under field condition, unless the pH is extremely low. However, Cr(VI) can be leached from the mortar at early ages. The compressive strength of Cr(III) mortar and concrete is higher than that of the conventional mortar and concrete at all ages. But the strength of Cr(VI) mortar and concrete is lower than that of the conventional mortar and concrete. A major finding was the discovery of significant amount of soluble chromium in Mercer fly ashes. The results show that the majority of the chromium in Mercer fly ash is concentrated in the small particles, those in the 0-10 micron range.