This work presents the results of the Weibull modulus of Portland clinker reinforced with several oxides like alumina, magnesia and silica. The Weibull modulus is a measure of the reliability of materials and it is used as an estimation of the probability of failure. The composite materials (3, 6 and 9% of oxides, by weight) were manufactured through Powder Technology processes including mixing in a ball mill, cold isostatic pressing and sintering at 1400°C in air. Sintering density for all the materials has been evaluated. Bending strength (three points) was the key property for measuring the Weibull modulus using at least 30 tests in all cases. Four different failure estimators were studied. These composite materials present a high Weibull modulus with good correlation coefficients. Some results are above typical values of the conventional and advanced ceramics. Microstructural analysis was carried out to explain the reliable behaviour of these composite materials.