Answer: per second loading rate ÷ corresponding mortar specimen strength grade × 7.5.
E.g., the new 0.25 ÷ 2.5 × 7.5 = 0.75 (KN);
The old 0.5 ÷ 5 × 7.5 = 0.75 (KN)
With a steel bottom mold for the mortar specimen, the mortar specimen strength was significantly reduced, so the strength of the mortar measured by the steel bottom mold was multiplied by 1.35 as the strength value.
The loading rate was changed to o.25~1.5KN per second, so the estimated strength of M7.5 mortar was about 7.5÷1.35=5.56MPa.
Therefore, I suggest to take the rate of 0.55 per second for the mortar of M7.5.