In Malaysia, regarding the cement specifications on use of blended cement with 25% Fly-Ash, cement supplier sometimes will propose on use of Ground Granulated Blastfurnace Slag (GGBS) as an alternative to Fly-Ash based on the logistic reasons, location of factory, stock available & etc.
25% Fly-Ash specified in the project will fall under CEM IIB-V Cement i.e. Portland Fly-Ash Cement. This is of similar performance as a CEM IIIA Blastfurnace Cement as discussed earlier.
And as a rule of thumb and by virtue of cement chemistry, a 25% Fly-Ash = 50% GGBS for equivalent performance.
CEM IIB-V Portland-Fly Ash Cement and CEM IIIA Blastfurnace Cement complies with MS EN 197-1: 2007.
Related Topic: In term of low temperature, GGBS or Fly Ash better?