The Negative Effects of Fly Ash The quality of fly ash can have an impact on the strength and durability of cement concrete. Poor grade fly ash can increase the permeability of concrete and cause structural damage. It is recommended that you use class-f or better fly ash when preparing your own mortar or concrete. Class-F fly ash is clean, free of clinkers, and has a low iron content. Class-A fly ash is cleaner than class-F and has less plasticity than class-C fly ash. Class-B and class-C fly ashes have higher iron contents and lower calcium oxide ratios than class-F fly ash; thus they are more reactive with water and require careful monitoring during mixing and placement. Iron in concrete reduces its strength and increases its porosity. Concrete containing 10% or more fly ash by volume should not contain more than 1.7% by weight of total iron content. Concrete that contains excess iron may appear gray or brown instead of white. This color change is due to the presence of ferrous sulfate which forms when water reacts with the iron in the fly ash. Ferrous sulfate is very sensitive to light and will turn black if exposed to sunlight for several days. The concrete also may exhibit other undesirable properties such as high permeability or poor resistance to heat.