Design of a concrete mixture to be placed by the wet-mix shotcrete method is essentially the same as normal cast-in-place concrete mix design. The major differences with shotcrete mixtures are:
- The maximum coarse aggregate size is generally limited to about 3/8 in. (9.5 mm);
- They use a fairly low water-cementitious material ratio (w/cm) and slump to allow shooting on vertical surfaces without sloughing;
- The potential to use an accelerator that can be added at the nozzle; and
- The pumpability is an important workability characteristic.
Since you desire a high-early-strength mixture, using fly ash as a supplemental cementitious material (SCM) wouldn’t be recommended because it slows set and strength gain at early ages. Microsilica may be beneficial for early strength gain. Consideration should be given to using accelerator added at the nozzle. There is some guidance on concrete mixture design in ACI 506R-05, “Guide to Shotcrete”; however, because local materials (aggregates, cements, SCMs) can vary significantly, you should consult with an engineer or concrete testing laboratory familiar with shotcrete to produce and test a mixture design to meet your requirements.