Hi Ivan -
1) Durigrid is a high modulus ceramic (alumina) that increases the modulus of the mix. Replacement with sand will produce a lower modulus casting than when using durigrid E sand ~ 70GPa E alumina ~ 300GPa
2) Fibers are added to Portland cement to bridge the inevitable drying cracking that occurs with Portland. There are concrete grouts based on calcium sulphoaluminate that potentially do not crack, these are the preferred cements for machine parts
BluCem-CSA-Cem-LR-R2.pdf (bluey.com.au)
3) Large machine parts are very low stressed and the fibers serve no purpose unless you are trying to improve the modulus. My tests have been disappointing in trying to improve the modulus using steel fibres. So I would not use them as they complicate the pour. If your concerned about sagging then the design is totally inadequate.
4) Buildings require failure modes that are safe ie the structure needs to be able to deform and absorb energy yet stay together vs suddenly falling down. Thats why ductility is required. Again not needed in machine designs. They are very low stressed and will not "fail" like a building may fall down.
5) I'm sure there are cement grouts that can be cast to very thick sections in europe. Try Sika products they have suitable grouts and are cheaper then Tegno products. I use one from the local hardware its E=56GPa and can be cast to 500mm by itself. The thickness can usually be increased by adding more aggregate. See attched data sheet.
Keep at it. Peter