Mechanical properties and key intrinsic factors of sprayed ultra-high performance concrete (SUHPC) with alkali-free accelerator

Abstract

• Sprayed thickness and hydration mechanism of SUHPC with accelerator were analyzed. • Mechanical properties of SUHPC and poured UHPC were compared quantitatively. • Pores caused by instantaneous reaction and spraying are well investigated. • Reaction degree of silica fume with accelerator was characterized quantitatively. • Al-products and C S H gel of SUHPC and poured UHPC were compared and analyzed. A sprayed ultra-high performance concrete (SUHPC) with suitable sprayed thickness due to the addition of an alkali-free Al 2 (SO 4 ) 3 accelerator based on the designed UHPC has been developed in this research, which can be used as repair material for old concrete. The results reveal that the field sprayed thickness of SUHPC could be up to 45 mm due to the instantaneous reaction caused by the Al 2 (SO 4 ) 3 accelerator, and the accelerator dosage was only 2 % by weight of binder. Meanwhile, the compressive strength of SUHPC was 113.6 MPa, decreased 20.8 % compared with UHPC at cured for 28 days, the continuous hydration of cement and gradually activated pozzolanic of silica fume both are the guarantee of strength increase slowly in later hydration stage. Instantaneous reaction and spraying process both causes air bubbles will be easily trapped into fresh SUHPC and lately transfer to macro-pores in hardened SUHPC matrix. Due to the presence of the Al 2 (SO 4 ) 3 accelerator the hydration reactions of SUHPC dramatically differ from those of UHPC, such as the synthetic C S H doping by Al (from accelerator) increases the coefficient of Al[4]/Si and mean chain length may directly weaken matrix strength.