Compressive strength and microstructural properties of dry-mixed geopolymer pastes synthesized from GGBS and sewage sludge ash

Mixed solids (precursors) containing equal portions of ground granulated blast-furnace slag (GGBS) and sewage sludge ash (SSA) by weight were activated by mixed alkaline solutions that was comprised of sodium hydroxide and sodium silicate in prescribed proportions to prepare geopolymer pastes in a dry-mixed form. Tests were carried out to provide data on compressive strength and microstructural properties of the geopolymer pastes synthesized from different mass ratios of alkali to the mixed solids, i.e., Na2O content, and different molar ratios of SiO2 to Na2O in the mixed alkaline solution (i.e. different moduli). The results showed that the maximum compressive strength of 32.8 MPa at 28 days could be achieved at the optimal Na2O content of 4.0% and a modulus of 0.95. Analyses of the microstructures of the geopolymer pastes by XRD, QXRD, FTIR, SEM and EDX revealed that SSA participated in the geopolymerization process and the quartz and hematite crystals in it were largely dissolved in the reaction. The microanalyses also revealed that the main reaction product of the optimal geopolymer mixture was a C-(N)-A-S-H type gel with Fe substitutions. The drying shrinkage of all specimens were less than 0.06% at the age of 14 days. Testing for metal(loid)s showed that SSA could be used with GGBS as precursor materials for geopolymerization with no leaching problems. This work provides a potential option to recycle SSA and builds up a theoretical basis for proportioning design in utilizing GGBS and SSA in dry-mixed geopolymers.