Effects of different nanomaterials on the early performance of ultra-high performance concrete (UHPC): C–S–H seeds and nano-silica

Abstract

The early hydration of UHPC can be retarded by the low water/binder ratio, the high polycarboxylate superplasticizers dosage, and the large supplementary cementitious materials (SCM) content. In this study, the effects of the prepared nanoscale C–S–H seeds and nano-silica (NS) on the early performance of UHPC were compared. The mechanism of nanomaterials influencing the early hydration of UHPC was further characterized by calorimetry, thermal analysis, nuclear magnetic resonance (NMR), and mercury intrusion porosimetry. The results showed that the 1-day compressive strength of UHPC samples with 0.3 wt% C–S–H seeds and NS increased by 457% and 25% respectively compared to the control sample. Compared to NS, C–S–H seeds were more effective in accelerating the early (before 3 d) hydration to produce hydrates and refine the pore structure of UHPC. The NMR results further implied that compared with the NS, the C–S–H seeds was 12% and 343% more efficient in accelerating the early hydration of cement and SCMs, respectively. However, the performance enhancement of nanomaterials was effective at 1 d and had little effect thereafter.