Nano-mechanical behavior of a green ultra-high performance concrete

• The UHPC contained high volumes of fly ash and used river sand as aggregate.
• The nano-mechanical behavior of the UHPC was investigated by nanoindentation.
• The hydration products of the UHPC are dominated by high-stiffness hydrate phases.
• The mechanical bond at the interfacial zone is strong and efficient.
• High-stiffness phases such as mullite and hematite in fly ash were detected.

As a revolutionary construction material, ultra-high performance concrete (UHPC) has been extensively studied in the last two decades and one research interest has been focused on preparing UHPC with low-priced raw materials or industrial by-products. In this study, a green UHPC which used high volumes of fly ash and river sand as part of raw materials was prepared, and the objective is to investigate the nano-mechanical behavior of the UHPC using nanoindentation. The results show that the hydration products, which account for about half of the paste by volume, are mainly high-stiffness hydrate phases, and significant quantities of unreacted cement and fly ash have higher mechanical properties than the hydration products and can function as micro-aggregates to strengthen the UHPC paste. Moreover, the mechanical properties of the paste near aggregate or fiber surfaces are similar to those of the bulk paste, which indicates that the UHPC has a strong and efficient bond at the interfacial zone. The experimental findings at the nano-scale could help to understand the macro-performance of the green UHPC.