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Minimum Compressive strength of first class brick is 10.5 MPa
Not enough.
The compressive strength of 53 grade ordinary portland cement is 53 N/mm2 at 28 days.
The compressive strength of natural rubber is 30MPa.
The compressive strength of 7 days shall be about 70% of the 28-days compressive strength.
If your compressive strength at 28-days is 25 MPa, then the 7-days compressive strength expected to be (0.70X25MPa)=17.5 MPa
Regards,
Sami