A study was made of the Weibull modulus of white clinker portland reinforced with alumina and silica. The Weibull distribution was introduced as a way of measuring the reliability of materials through the slope or shape parameter, known as Weibull modulus, and it is used as a reference and estimation of the probability of failure. The manufacturing process of composite materials (3, 6 and 9% Al2O3 and SiO2 by weight) includes mixing in a ball mill, cold isostatic pressing in wet bag at 180 MPa and sintering at 1400 degrees C in air. Bending strength was used as the key property for measuring the Weibull modulus using more than 20 tests in all cases. The correlation coefficients obtained in all the estimations for the studied materials are above 95%. These ceramic matrix composite (CMCs) materials present a high Weibull modulus (in some materials about 26) and better behaviour than plain white clinker portland. Most results are above typical values of the conventional and advanced ceramics (between 5 and 15). Microstructural analysis was carried out to explain the reliable behaviour of these materials, a behaviour that could make them very interesting for structural applications.