The mechanical behaviour of ceramic refractories formulated in the MgO-CaO-SiO2- ZrO2 system is analysed in terms of the room temperature Young´s modulus, the modulus of rupture and the work of fracture at 25 and 1100°C. The materials have been designed taking into account the phase equilibrium relationships to obtain MgO-CaZrO3-Ca2SiO4-Ca3Mg(SiO4)2 or MgO-CaZrO3-Ca3Mg(SiO4)2-c-ZrO2 as final crystalline phases. Different relationships between the proportion and sizes of the fines and the aggregates have been explored. The microstructure of the materials has been characterised in terms of density, crystalline phases and phase distribution and morphology. A combination of X-Ray diffraction (XRD) analyses and Reflected Light Optical Microscopy (RLOM) has been used. The relationships between the obtained phases and microstructures and the grain size distributions of the used raw materials have been established. The relationships between the mineralogical composition and the obtained microstructure and the mechanical behaviour are discussed.