Novel cold-cured epoxy-silica hybrids as potential adhesives for concrete/masonry rehabilitation: environmental aging

The effects of environmental aging at various levels of humidity were studied on the experimental formulations of epoxy-silica hybrids, specifically designed for curing at ambient temperature to be employed as adhesives for concrete and/or masonry structures. The addition of small amounts of ammonium molybdate to the amine hardener was investigated using thick section specimens (4.5 mm thick) capable of retaining substantial amounts of ethanol either originally added to the resin mixture or subsequently formed by the sol-gel hydrolysis and condensation reactions. The changes taking place during aging were evaluated in terms of variations in glass transition temperature (Tg), water absorption and mechanical properties. It has been found that during environmental aging in moist atmosphere the siloxane domains of the epoxy-silica hybrids undergo additional sol-gel reactions, which bring about a further increase in Tg and substantial increases in Young's modulus (Eflex) and flexural strength (?max flex). The observed effects were found to be enhanced by the addition of small amounts of ammonium molybdate to the formulation. Furthermore, the moisture in the atmosphere was found to assist the extraction of the residual ethanol in the specimens.