The strength development of a cement is not only defined by the composition but also by the grinding fineness, divided into classes (32.5 - 42.5 - 52.5).
This classification takes place based on a pressure test after 28 days. Within each strength class the performances define from an early stage the distinction between the ‘Normal’ (N) and ‘Rapid’ (R) version.
The mortar prisms are based on a standard ratio of cement, water and sand.
The strength indication for cement has only a limited effect on the strength of concrete or mortars to be achieved. This is because a type of concrete with among others a lower water-cement factor (the ratio of cement as opposed to water) than the type of mortar can be created, which is used for the strength classification of cement. The composition of the aggregate also has an effect on the final strength of concrete. For this reason, the strength class of cement cannot be traced back to a maximum strength class for concrete or mortar.
Class 32.5 is indicated for applications where a high initial strength is not required, at average ambient temperatures (10 - 15°C) and with constructions of standard thicknesses (< 50 cm).
Cements with strength class 42.5 are mostly used in the event that the required compressive strength of concrete at 28 days must exceed 30 N/mm² (i.e. higher than class C25/30 according to NBN B 15-001). These cements are also suitable for use at lower temperatures.
Class 52.5 is used for applications where an even higher initial strength than class 42.5 is required. E.g. for the rapid stripping of prefabricated elements.