Effect of silica fume and fly ash on hydration and microstructure evolution of cement based composites at low water–binder ratios

• We prepare four kinds of ultra-high performance cement based composites (UHPCC).
• We investigate the effects of SF and FA on the hydration process and microstructure of UHPCC.
• We investigate the hydration process, hydration products and pore structures of UHPCCs.

The hydration process of ultra-high performance cementitious composite (UHPCC) is distinctly different from common concrete owing to its low water–binder ratio (w/b). The effects of silica fume and fly ash on the hydration process and microstructure were investigated via different characterization methods in this work. The results indicated that the hydration process could be accelerated at the beginning by addition of silica fume, and fly ash, on the other hand, retarded hydration mainly in the dormant and acceleration periods. When they were incorporated together in cement, the result differed significantly from cement hydration at higher w/b (>0.35). As a result of Ca(OH)2 consumption by pozzolanic reaction, the porosity reduced (<4%) and the interface strengthened as the curing time increased. Nanoindentation results revealed that most hydration products of UHPCC were UHD C–S–H and unhydrated cement. The unhydrated cement particles were surrounded by UHD C–S–H, implying that the UHPCC had excellent mechanical performance and durability.