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

High-performance and ultra-high-performance concrete have been increasingly used as building materials in modern construction, and their superior properties 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/binder ratios. The results indicate that the phase changes concerning the contents of the major clinker phases mainly happen within the first 7 d. 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 d. At early age, the addition of silica fume accelerates the hydration of cement, while the introduction of fly ash shows significant retardation especially at 1 d. The degree of hydration (DOH) of cement is mainly determined by the effective water/cement ratio of the paste, and the incorporation of fly ash, while initially producing little reaction, 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.