The need for “ecological” and “green” materials is increasing because of the strong demand to reduce the speed of climate change and decrease carbon footprint of human activities in nature. In the cement production process high amount of CO2 is obtained as by-product. In the present research cement in a concrete mix is partially replaced by oil shale ash (OSA). OSA is a by-product itself, so utilisation of it also positively affects the environment. OSA particles have different size, and fabricating of such “ecological” concrete composite material with the concrete matrix reinforced by OSA particles is obtained. Similar situation is obtained during reinforcement of epoxy resin by OSA particles. Obtaining particle reinforced composite, the problem of precise prediction of the composite material averaged elastic properties appeared. The particles have different size, shape and are chaotically distributed in the concrete volume. Elastic properties averaging methods – theoretical models, including the rule of mixtures Voigt method, rule of mixtures Reuss method and Halpin–Tsai method, were used. Results were compared with experimental data. For that, samples (prisms 40x10x10cm) of concretes having cement replaced by 0%, 10%, 15%, 20%, 25%, 30%, 35% of OSA were fabricated and tested by the four-point bending test. The results that were obtained showed that the rule of mixtures Voigt and the rule of mixtures Reuss method as well as Halpin-Tsai methods generally slightly underestimate the value of elastic modulus for concrete-OSA, whereas the Mori-Tanaka and Halpin-Tsai methods provided closer predictions.