The effect of nano-silica and silica fume on the sodium carbonate-activated slag system containing air pollution control residues

Abstract

The utilization of municipal solid waste incineration residues in alkali-activated granulated ground blast furnace slag (GGBFS) has garnered substantial interest for its potential in sustainable solid waste management and achieving a low-carbon footprint. However, incorporating these residues often leads to the deterioration of mechanical properties. This study revealed the role of silica fume (SF) and nano-silica (NS) derived from olivine within a sodium carbonate-activated GGBFS system incorporating air pollution control (APC) residues. The dosage of silica additives and APC residues ranges from 0 − 6 wt% and 0–15 wt%, respectively. The mechanical properties, reaction kinetics, phase composition, microstructure and carbonation resistance of the blended binder were investigated. Results indicated that SF slightly improved the early compressive strength with the formation of C-(A)-S-H gel (Ca/Si = 1.47, Al/Si = 0.23), hemicarboaluminate and hydrotalcite; reactive NS retarded the activation of GGBFS and inhibited the formation of hemicarboaluminate and hydrotalcite, while promoting the formation of C-A-S-H gel (Ca/Si = 1.01, Al/Si = 0.23), resulting in an impressive 80.3 % enhancement in compressive strength. Notably, NS-modified samples exhibited decreased carbonation resistance due to increased porosity and C-(A)-S-H gels that are vulnerable to carbonation. Conversely, 2 % SF addition decreased the diffusion rate of CO2, and APC residues improved the carbonation resistance by facilitating the formation of C-(A)-S-H gel with a higher Ca/Si ratio. This study provided an alternative management practice for APC residues with favorable early strength development and offered new insights into using silica additives to enhance waste-combined alkali-activated materials.