Effect of hexadecyltrimethoxysilane modified micro silica particles on workability and compressive strength of ultra-high-performance concrete

This paper presents the results of investigation on the rheological properties, workability, hydration, pore structure and compressive strength of UHPC with silane modified micro silica particles. Hexadecyltrimethoxysilane (HTMS) was adopted due to the steric hindrance and water expelling effect of its long carbon chain. Fourier transform infrared spectrometer and thermogravimetry test were used to characterize the adsorption of HTMS. Water adsorption on surface of micro silica particles was measured to reveal the water expelling effect of HTMS. Workability and compressive strength of UHPC were test. Rheological properties, hydration heat, nonevaporable water and pore structure were measured to comprehend the effect of HTMS modified micro silica particles on workability and compressive strength of UHPC. The results showed that the long chain decreased the adsorption capacity of HTMS and thus reduced the water expelling effect. The steric hindrance acted as the main effect to decrease the viscosity and yield stress and promote the workability of fresh UHPC. Adsorption of HTMS hindered the effect of micro silica particles as crystal nucleus for cement hydration, which mitigated the workability loss and induced the decrease of compressive strength at 1 day. However, the acceleration effect of HTMS on cement hydration and diminished hindering effect on pozzolanic reaction at 3 and 28 days facilitated the precipitation of products, which fined the pores and mitigated the negative effect of air-entraining effect of HTMS on compressive strength of UHPC.