Impact of freeze-thaw weathering on integrity, internal structure and particle release from micro- and nanostructured cement composites

Nowadays, both micro- and nanomaterials are processed in cement to improve application properties or reduce cement consumption in concrete. The present study deals with the impact of nanostructured SiO2, TiO2 and milled slags in cement composites on material properties, aging resilience and release behaviour into the environment. In this context, hydrated and hardened cement pastes were weathered artificially by means of the standardised capillary suction of de-icing solution and repeated freeze-thaw cycle test method. Mineralogical, physical, mechanical and chemical properties as well as hydration products were analysed by electron microscopy, mercury intrusion porosimetry, thermogravimetry as well as differential scanning calorimetry and ultrasonic propagation analysis. Release characterisation was performed concerning weathering-induced fragment release into the liquid phase by gravimetry and sanding-induced particle release into the air by gravimetry, differential electrical mobility analyses, time of flight spectrometry, condensation nuclei counting and electron microscopy. Freeze-thaw weathering led for all cement composites to considerable but different progressed deteriorations. The worst material performance was observed for pure cement and the cement composite with state of the art ground granulated slag. The cement composites with nanostructured SiO2 and activated ground granulated slag showed a higher durability and less deteriorations. These effects could be explained by structure density, porosimetry and the formed hydration products. Furthermore, it was shown that weathering-induced changes in the material properties lead to significant changes concerning sanding-induced particle release into air. In summary, this study investigated the consequences of (nano)-materials, which modulate the recrystallization during hardening of cement pastes to a solid, internally nanostructured material, on multi-structural release. In this context, critical stress scenarios, which are encountered during intended use, were simulated. The transforming synthesis differentiates this case study from almost all other release studies for nanomaterials, which typically assess polymer nanocomposites, where nanomaterials act as additives, pigments or fillers in an otherwise unchanged matrix.