Effect of silica fume and fly ash on pore structures of blended pastes at low water to binder ratios

Cementitious materials with very low water to binder (w/b) ratios such as ultra-high-performance concrete (UHPC) have been increasingly used, and their superior performance is largely dependent on the improved pore structures. The benefits of incorporating supplementary cementitious materials (SCMs) to prepare these materials have been widely recognised, whereas there is a lack of research to reveal the effect of SCMs on the pore structure. The aim of this study is to investigate the influence of silica fume and fly ash on the pore structures of blended pastes at such low w/b ratios. Two curing regimes, standard moisture curing and steam curing, were adopted and the pore structures were determined by mercury intrusion porosimetry. The results show that the addition of silica fume refined the pore structure regardless of the curing method and age. The addition of fly ash decreased the porosity after 7 d, whereas the critical pore size was almost unaffected by fly ash under standard curing. Compared to 90 d standard curing, steam curing contributed to a decreased porosity for all the pastes and a reduced critical pore size for the reference and fly ash paste. Moreover, the introduction of both SCMs resulted in a similar pore structure to that of the silica fume paste.