The objective of this study is to evaluate the early age hydration mechanisms of accelerated CEM I pastes. Liquid phase analysis, conductimetry, isothermal calorimetry, in situ XRD and SEM were performed on cement pastes produced with equivalent doses of an alkaline accelerator (sodium aluminate solution) and of an alkali-free accelerator (aluminum sulfate solution). Results showed that the addition of these chemicals first changes the ionic equilibria of the medium by the consumption of Ca2+ and SO42- ions from the liquid phase. The alkali-free accelerator contributes to the rapid formation of ettringite, as well as to a faster rate of alite dissolution and hydration. The aluminate accelerator leads to AFt and AFm formation, rapidly depleting gypsum and filling up the space, inhibiting further alite hydration. According to the results, variations in cement hydration mechanisms caused by accelerators with different chemical.compositions are explained.Peer ReviewedPostprint (author's final draft

The objective of this work is to parametrize the early age hydration behavior of accelerated cement pastes based on the chemical properties of cement and accelerators. Eight cements, three alkali-free and one alkaline accelerators were evaluated. Isothermal calorimetry, in situ XRD and SEM imaging were performed to characterize kinetics and mechanisms of hydration and the microstructure development. The reactivity of all accelerators is directly proportional to their aluminum and sulfate concentrations and to the amount and solubility of the setting regulator contained in cement. Alite hydration is enhanced if a proper C3A/SO3 ratio (between 0.67 and 0.90) remains after accelerator addition and if limestone filler is employed, because undersulfated C3A reactions are avoided. Combinations of compatible materials are recommended to enhance the performance of the matrix and to prevent an undesirable hydration behavior and its consequences in mechanical strength development.Peer ReviewedPostprint (author's final draft

This works present an experimental study addressing the workability of fiber reinforced concrete (FRC). Only conventional measurements, such as slump and Ve-Be test, were used to evaluate the effect of the fiber content in the mixture. The fiber content shows to be of great influence in the concrete mobility reducing the slump and increasing exponentially the Ve-Be time. The fibers act as a barrier to coarse aggregates movement reducing the materials mobility. So, increasing the aggregate size or the fiber aspect ratio the flowability of the material will be reduced. In order to reduce that effect, an increase in the mortar content and/or in the initial slump could provide better workability conditions. The results showed that the Ve-Be test is not applicable to concrete with plastic consistency. On the other hand, slump test could not be adequate for stiff concretes that will be consolidated by intense vibration.