Effect of silica fume on the efflorescence, strength, and micro-properties of one-part geopolymer incorporating sewage sludge ash

Abstract

The application of sewage sludge ash (SSA) in alkali activated cementitious material, i.e., geopolymer, was one of the effective measures to dispose of and recycle the sewage sludge. However, the geopolymer efflorescence could be aggravated due to elevated alkali content, especially for one-part geopolymer, where the dissolution reaction of solid alkali activator was less sufficient. This study focused on the efflorescence of GGBS-SSA based one-part geopolymer, as compared to two-part geopolymer, and the effect of silica fume on the efflorescence, strength and micro-properties of the one-part geopolymer. The results showed that the extent of efflorescence crystallization and the concentration of leached Na+ ions in the one-part geopolymer were more severe than the two-part geopolymer, due to the lower degree of geopolymerization reaction and the heterogeneous dense microstructure of the one-part geopolymer. After adding silica fume to the one-part geopolymer, the extent of efflorescence crystallization and the concentration of leached Na+ ions reduced significantly, and the compressive strength increased from 41.6 MPa to 57 MPa when the silica fume content was 10 %, but then decreased slightly. Adding the silica fume reduced the release rate of reaction heat, which helped to increase the solubility of solid alkali activator and improve the geopolymerization reaction, thus significantly reducing the content of mobile alkali ions and enhancing the compressive strength. Moreover, the silica fume with tiny spherical particle had better ball effect and micro-aggregate filling effect, which refined the pore size and reduced the connectivity of pore network, significantly reducing the diffusion rate and leaching of mobile alkali ions.