It depends on which dataset you are using:

The Australasian LCI has predefined materials with different concrete strength and fly ash or blast furnace slag blends so you can choose the specific material by selecting concrete Mpa and % of cement replacement.
See more details in this example

BRE IMPACT LCI has predefined concrete materials with 30% fly ash content (PFA – pulverised fuel ash) and it also has the fly ash as a material which allows the user to customise the concrete mix and % of cement replacement. The basis to create a concrete template with fly ash in the mix is to add the standard concrete material (m3), subtract the amount of portland cement (kg) entered as a negative number and add the same amount as aggregate/fill GGBS (kg).
See more details in this example