The most basic reason for using silver sand while congealing the great flux expert armor was to increase the movement of flux energy, which would allow the cultivator to use the flux energy in their body at a quick speed.
The effect of gold granules, however, was allowing the flux energy to accumulate quicker and be more compressed. This increased the amount of flux energy with which the person could erupt in their attack.
As for the sacred steel?
This material was used for the great flux expert armor to increase its physical defense. Sacred steel possessed high density and usually couldn't be penetrated. After being purified by the flux flame, it would become much more lightweight, which turned it into an extremely useful resource which couldn't be replaced.
