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

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Abstract:The inductive method is a robust and simple non-destructive test to assess the content and the distribution of the fibres in SFRC. Despite the advantages in comparison with other methods, further studies are still needed to define the accuracy, the theoretical basis and the equations for the conversion of the inductance into fibre content and distribution. In fact, although the test provides an indirect estimation of the fibre distribution, currently no equation exists for the assessment of the orientation number, which is a valuable parameter for the design of structures. The objective of the present paper is to address this issue. Initially, the theoretical basis for the calculation of the fibre content is provided. Then, alternative equations are deducted for the fibre contribution and for the orientation number. Different experimental programs and finite element numerical simulations are conducted to evaluate the accuracy of the method and to validate the proposals. The results indicate that the equations currently used may lead to errors of up to 24%. Instead, the formulation proposed here show errors far below 2.6%, allowing the prediction of the orientation number in all directions with a high accuracy. This opens up a new field of application for the test and represents an advance towards the characterization and the quality control of SFRC. represents an advance towards the characterization and the quality control of SFRC. 27 Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation