Since the most reactive part of the fly ash is its amorphous part, the main chemical composition of the amorphous part was calculated by subtracting the crystalline component, as measured by QXRD (quantitative X-ray diffraction i.e.Rietveld), from the bulk composition, as measured by XRF. This is achieved by converting the crystalline phase compositions to equivalent oxide composition to make them comparable to XRF results. Ross and van Riessen have published one good paper in Fuel on this topic. I think you could find it in this site. However, please keep in mind that some silica and alumina could also be refractory, though, they are amorphous as state. But the the content of amorphous and refractory aluminosilicate is lower than that the amorphous and reactive aluminosilicate. So, you may rely on the content of amorphous aluminosilicate as reactive aluminosilicate. 

Thanks for the answer, Jadambaa. What do you think about the method in which you make a calibration curve by adding amorphous content to samples and determine the ratio of the area of amorphous halo to the whole area? Then you can measure the unknown sample amorphous halo to total area ratio and see the amorphous content.

Actually, I think that is a quite complicated method. Because, I think  amorphous halo of the pure aluminosilicate is a bit different position than amorphous halo of the aluminosilicate with inclusion of other cations such as Ca, Mg, Fe or K in the XRD pattern. As you know fly ashes may contain to many inclusions in their amorphous part.

You can use this method

SiO2amorphous = SiO2 (with  XRF) - SIO2 crystalline ( with XRD)