You can do that by addition of MgSO4 (Gypsum)

Thanks Dr. Maraqah, but I think Gypsum acts as an accelerator, not retarder? and I need to retard setting.

Dear  Nabil 

I want to ask if you tried it or not because it is used in cement preparation as a retarder

My understanding of Portland cement - based materials (which may not apply to the material in question...) is that adding sugar to the mix will delay or stop setting (I have heard of the surface of freshly poured / levelled concrete being sprayed with a sugar solution so that the surface remains soft for texturing or mechanical embossing when the underlying concrete has hardened..