On Buildings, what is the design strength of cement grout that are used to fill the cells of concrete hollow blocks?

Usually.5MPa but it is better to check project specification.

Standard mixing is 1 bag cement 2 bucket of gravel 3 bucket of sand

10mpa to 15 mpa for concrete strenght

3000 psi grout strength of Hollow blocks after minimum 28 days  

base on the standard it should be class AA 1 cement 11/2 sand and 3 gravel or agrregates.

ALL YOU NEED IS 1:3 MIXTURE AND BE SURE THE CEMENT GROUT IS WELL MIX TO AVOID VACUUM.