Composite specimens for tensile testing were made from various amounts of Kevlar fibers in Portland cement. Testing revealed about 50% improvement in the Elastic Modulus and yield strength over the unreinforced cement. The maximum amount of fibers, which was possible to add without destroying the continuity of the cement matrix, was about 2%. Specimens containing less than about 0.15% of fibers failed in a brittle manner like a single phase material. Specimens containing more than that amount of fibers displayed multiple fractures of the matrix before the final failure. The ultimate tensile strength started to improve at the critical volume of fibers equal to 0.3% and not as predicted theoretically at 0.75 % . The most dramatic improvement in the properties of cement was in the Energy to Fracture. This increased almost two orders of magnitude with 1.96% of fibers by volume.

The global consumption of sand by the concrete industry has increased significantly over the years. Natural sand has become a desired commodity in numerous regions of the world. To protect both the existing resources of natural sand and utilize waste materials, drastic actions are urgently needed. The production of cement mortars, which are solely based on fine aggregate, is responsible for the large consumption of natural sand. In the described research program, we proposed to substitute 50 % of the natural sand in mortars with fly ash and slag. The strength properties and durability characteristics of the new mortars were tested. It was proved that mortars with fly ash and slag can be used for specific applications in civil engineering. Areas for future research associated with the mortars in question were also pointed out.