Phase evolutions of cementitious materials with very low water/binder ratios

Abstract

High performance and ultra-high performance concrete have been increasingly used as building materials in modern constructions, and their superior properties are derived from a tailored microstructure. In this study, Rietveld quantitative X-ray diffraction analysis was adopted to investigate phase evolutions of four cementitious pastes with varied addition of fly ash or silica fume at very low water to binder ratios. The results indicate that the phase changes concerning the contents of the major clinker phases mainly happen within the first 7 days. The content of portlandite (CH) in pure cement paste increases monotonically while due to pozzolanic reaction, it decreases in blended pastes between 7 and 28 days. At early age, addition of silica fume accelerates the hydration of cement, while introduction of fly ash shows significant retardation especially at 1 day. The degree of hydration (DOH) of cement is mainly determined by the effective water to cement ratio of the paste, and the incorporation of fly ash which show little reaction initially results in a higher DOH of cement. With steam curing, the content of CH significantly decreases in the blended pastes, which indicates a promoted pozzolanic reaction.