Mixtures of silicon and aluminum oxides to optimize the performance of nanoporous thin films in concrete

This study explored the effect of two combinations of silicon and aluminum oxides, nanosilica–nanoboehmite and nanosilica–gibbsite, on the hydration reaction of cement and the porosity of the interfacial transition zone (ITZ). The influence of sols on the cement hydration reaction was investigated using isothermal calorimetry while their effect on the porosity of the aggregate–paste interface was validated using scanning electron microscopy. The nanosilica–nanoboehmite mixtures were found to accelerate the hydration reaction to a higher degree than the individual components, nanosilica and nanoboehmite. Further, the effect was also found to be dependent on the stoichiometry of the mixture of nanoparticles. The nanosilica–gibbsite combinations not only accelerated the reaction but also increased the cumulative heat of hydration. In this case, the enhancement is attributed to the seeding effect of the gibbsite particles, being more prominent at the smaller particle sizes. Lastly, when these materials were applied as nanoporous thin films on the aggregates, all sol mixtures not only helped to decrease the overall porosity but also contributed to refinement of the porosity in the cement paste adjacent to the aggregate. These effects were observed up to 250 μm away from the surface of the aggregate thus not restricted to the typical length of the interfacial transition zone in concrete (40–50 μm).