Effect of the addition of ultrafine cement and short fiber reinforcement on shrinkage, rheological and mechanical properties of Portland cement pastes

Abstract The packing density of a powder can be improved by adding a fine powder to a coarse one. This conventional technique, frequently used in ceramic production, also can be applied to optimise the properties of cementitious binders, especially for the production of high performance concrete. In this paper the effect of mixing ultrafine cement and normal grain sized Portland cement is studied. The rheological properties of the fresh paste are influenced positively. An important dispersing effect is observed, decreasing yield value and plastic viscosity. This permits mixing of very low w / c -ratio cement pastes with low porosity and high strengths, applying conventional mixing procedures. Due to the low amount of water, that is available in the narrow pores, the hydration of the cement is not complete. At the same time, permeability is strongly reduced, leading to a lack of water with ongoing hydration. Self-desiccation especially at early age is the consequence. Shrinkage and as a consequence crack formation may be observed. In a special experimental setup early shrinkage (from nearly time zero after mixing) was monitored continuously. Fresh pastes of different mix proportions were put in a cone and the length change was measured by a laser system. Additionally, the shrinkage of hardened pastes was measured until 90 days by conventional technique. The influence of different surrounding climates was studied. Such dense materials generally are very brittle. The use of fibers increases the ductility significantly and leads to a further improvement of the shrinkage and strength properties. The excellent rheological properties of the cement matrix containing ultrafine cement also allows a conventional mixing of composites with a high fiber content. The effect of different amount and type (PP, carbon) of fibers on the shrinkage at very early age and the influence of different curing conditions at early age on the mechanical properties was studied.