Measurements and weight:
Cement bricks : 210x100x70mm / ±2,2 kg
Genuine clay bricks : 222x106x73mm / ±3,1 kg
Therefore you will use 17% more Cement bricks than Clay bricks to build the same area.
Clay bricks gives you ± 40% more mass than Cement bricks.
Conclusion:
You need 17% more Cement bricks than Clay bricks to build the same square meterage thus: