Effect of adding solid waste silica fume as a cement paste replacement on the properties of fresh and hardened concrete

Due to the large-scale infrastructure construction and environmental protection pressure in China, the demanded cement is in short supply and the price is rising, and the overcapacity of silica fume has caused its price to fall, thus replacing cement paste by silica fume could achieve better economic and environmental benefits. In this study, silica fume (SF) with SiO2 content of 86.3% and high purity silica fume (HSF) with SiO2 content of 96% were used to replace cement paste in different proportions. The effects of adding SF or HSF on the workability of fresh concrete, the mechanical properties and microstructure of hardened concrete were investigated. The microstructures (i.e., porosity, pore size distribution and micromorphological characteristics) were analyzed by using low field nuclear magnetic resonance (LF NMR) and field emission scanning electron microscopy (FE-SEM). The results showed that the slump and air content of the concrete gradually decreased as the amount of SF or HSF increased. When the replacement amount of SF was increased from 0% to 10%, the cubic compressive strength and splitting-tensile strength of concrete were increased by 26.7% and 40.7%, while the cost per cubic meter of concrete was only increased by 1.9%. When the replacement amount of HSF was increased from 0% to 10%, the cubic compressive strength and splitting-tensile strength of concrete were increased by 44.7% and 57.4% respectively, while the cost per cubic meter of concrete was only increased by 5.3%. The total porosity of concrete gradually decreased as the amount of SF or HSF increased, with the proportion of macropores (>1000 nm) decreasing, and the proportion of mesopores (100–1000 nm) and capillary pores (0–100 nm) increasing, significantly refining the pore structure and densifying the microstructure of concrete.