Application of an industrialized ultrafine composite powder in cement-based materials: Hydration characteristics, microstructure, and corrosion resistance

To reduce the reliance on cement clinker, employing ultrafine powders as supplementary cementitious materials (SCMs) is gradually gaining importance. This study investigated the effect of an ultrafine composite powder (UCP) having a specific surface area of approximately 730 m2/kg, produced by industrial ball milling ordinary blast furnace slag and fly ash at a 1:1 ratio, on cement hydration and its durability. The ultrafine composite powder was characterized by X-ray fluorescence spectrometer (XRF), X-Ray diffraction (XRD), particle size analyzer, and scanning electron microscope (SEM). The effects of ultrafine composite powder on the macroscopic properties of cement-based materials were assessed through flowability tests, setting time tests, and mechanical properties tests. Then, the hydration heat release, hydration products and microstructures of the pastes were characterized using isothermal calorimetry, XRD, thermogravimetric (TG), SEM and mercury intrusion porosimetry (MIP). Finally, the effects of ultrafine composite powder on the durability of cement-based materials were evaluated by electrical flux, chloride diffusion coefficient, and mechanical properties tests. The results showed that ultrafine composite powder enhanced the flowability of cement paste under the synergistic effect of superplasticizer. However, the setting time was prolonged due to the dilution of cement by ultrafine composite powder. The ultrafine composite powder significantly improved the early mechanical properties of the mortar, and the mechanical properties after 28-days was equivalent to that of the control. The ultrafine composite powder could significantly decrease the cumulative heat release of cement paste. Due to the high SO3 content, an additional exothermic peak representing ettringite formation was observed. As the hydration time extends, the hydration products in the paste changed, especially the transformation from ettringite to kuzite, and the formation of hemicarbonate and monocarbonate. As the ultrafine composite powder content increases, the proportion of gel micro-pores increased, whereas the proportion of middle and large capillary pores decreased. Consequently, the total pores in the paste also decreased. The addition of ultrafine composite powder enhanced the durability of the cement-based materials, making it an ideal choice for use in coastal engineering projects.