What is 21 days strength of cement concrete?

concrete (cement) cures with age, well at least up to a point.

think of breaking up the concrete slab that was cast yesterday evening, versus a concrete slab that is a year old.

the optimum strenght of concrete (maximum strenght it is assumed to reach) is statistically reached after 28 days, if proper and adequate curing took place, and normal working / exposure conditions and control of quality took place. Curing is basically protection against severe exposures as draft winds that may cause cracking, and keeping the surface moist over the 7 days, by covering it with DPC plastic or regular sprinkling with water.

after only 7 days of curing, the 28 day strenght can be accurately predicted to determine if it will pass the requirements.

after 7 days (I have also seen 24hrs samples) a cube sample is crushed to determine the 7 day crushing strength. this sample was then cast at the same time as the structure being assessed.

the 7 day strenght is measured in Mega Pascals (Mpa) or equivalent units Kilo Newton per square millimeter (kN/mm2) and really depends on the application the concrete is used for:

walkways will probably be 10Mpa

single storey building footings and floor slabs, say 15Mpa

floor slabs where vehicles may drive could be even 35Mpa.

Suspended slabs, like first floor slabs etc, say 40Mpa.