| Coded variables (Xi) | Factors (Ui) | Experimental Domain | |
| Low level (−1) | High level (+1) | ||
| X1 | U1: Temperature (˚C) | 35 | 80 |
| X2 | U2: Acid type | H2SO4 | HCl |
| X3 | U3: Acid normality (N) | 1 | 5 |
| X4 | U4: Activation duration (H) | 3 | 10 |
| X5 | U5: Clay granulometry (µm) | 2 | 50 |
| X6 | U6: Mass ratio | 3.5 | 7.5 |
| X7 | U7: Clay nature | montmorillonite | kaolinite |