It will be based on steel slag physical properties and gradation.... many publicatiovs are available in this regard....

https://ascelibrary.org/doi/10.1061/%28ASCE%29MT.1943-5533.0000760

Utilization of Steel Slag Powder as a Combined Admixture with Ground Granulated Blast-Furnace Slag in Cement Based Materials

This article seems have some good references regarding your query.

In my opinion mortar with different mixture could be experimentally investigated by compresive strength, UTV test and tensile strength in addition to the workability of the fresh mortar

Would it be safe to perhaps consult some technical papers first on using varying levels of steel slag to what ever your experiment is ..

you can see

https://ascelibrary.org/doi/10.1061/%28ASCE%29MT.1943-5533.0000760

I would like to thank all RG Colleagues for the answer this question, I wish you all the success.

I think 20% steel slag instead of cement is the optimum