Synergistic effects of silica fume and slag on the microstructure and mechanical behaviors of ultra-high-strength and high-ductility cementitious composites

Abstract

This work evaluated the synergistic effects of silica fume (SF) and ground granulated blast furnace slag (GGBS) binders on the microstructure and mechanical behavior of Ultra-High-Strength and High-Ductility Cementitious Composites (UHS-HDCC). The potential mechanisms of the synergistic effects of GGBS and SF on the fresh flow, mechanical properties, and microstructure of UHS-HDCC were investigated. The results indicated that the use of GGBS instead of SF could effectively improve the flowability of fresh mortar and UHS-HDCC mixtures. The hydration process of cement and the heat of hydration were delayed and reduced when the content of GGBS was below 25 %. Moreover, reducing the SF while increasing the GGBS reduced the compressive strength of the composites but had little effect on the flexural strength. The compressive strength of all specimens remained above 120 MPa. All specimens achieved strain-hardening behavior after the initial cracking of the matrix in uniaxial tension and four-point bending, and had a variety of cracking characteristics. Furthermore, the range of ultimate tensile strain capacity and mid-span deflection were 3.26 %–5.79 % and 16.73 mm–33.05 mm, respectively. The S10G30 specimen reached a maximum tensile strain energy of 414.6 kJ/m3. As the level of GGBS replacing SF increased, the number of cracks in the tensile region or the compressive bending surface of UHS-HDCC specimens increased, while the crack width and crack spacing decreased. The relatively smooth sidewalls of the fiber tunnels and fiber exit points meant that matrix fragments attached to the fiber surfaces were caused by matrix fragments generated throughout the matrix cracking process. Besides, the interfacial frictional stress and slip hardening coefficient acquired from the single fiber pullout test reduced with increasing levels of GGBS replacing SF.