Development of alkali activated cementitious material from sewage sludge ash: Two-part and one-part geopolymer

Abstract

In order to realize the resource utilization of sewage sludge, geopolymer was prepared from alkali-activated precursors that consist of sewage sludge ash (SSA) and ground granulated blast-furnace slag (GGBS). A traditional two-part geopolymer was prepared by adding alkali activator solutions, consisting of Na2SiO3 solutions and NaOH particles, to precursors consisting of equal portions of SSA and GGBS mixtures by weight. A one-part geopolymer was also prepared by just adding water to a mixture of SSA, GGBS and Na2SiO3-anhydrous solid alkali activators. Compressive tests were conducted on the two-part geopolymer with varying modulus and Na2O content, as well as the one-part geopolymer with varying Na2O content. The results showed that the compressive strength firstly increased and then decreased with the modulus of the alkali activators or Na2O content, and the compressive strength of one-part geopolymer was about 20% lower than the two-part geopolymer with the same modulus and Na2O content. Reaction process, mineralogy and microstructure of the two-part and one-part geopolymer were characterized and compared by the Isothermal calorimetry, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM) and Mercury intrusion porosimetry (MIP). The results indicated that SSA participated in the geopolymerization process, and quartz and other crystals were largely transformed into amorphous phase. Moreover, there was a higher degree and rate of geopolymerization reaction in the two-part geopolymer with a dense and compact matrix, while there was about 20% higher porosity in the one-part geopolymer. The toxicity characteristic leaching procedure (TCLP) revealed that concentrations of leached heavy metals from both types of geopolymer were well below the limits in Chinese specification. The cost index of the one-part geopolymer was about 15% higher than the two-part geopolymer, but the carbon emission index was 24% lower, indicating that it was a cleaner cementitious material.