Cement substitution by sludge-biomass gasification residue: Synergy with silica fume

The increasing volumes of sewage sludge and other wastes of biological origin force the use of downdraft gasifiers for waste utilization; however, thermal processes produce residues that need to be treated in an environmentally acceptable way. The object of the study is to evaluate the possible application of residue, derived from industrial sewage sludge-biomass gasification plants, with silica fume as a partial binder substitute in cement based materials. At total cement replacement levels of 10 and 12%, the dosages of gasification residue and silica fume varied in the range of 2-6% and 6-10% by cement mass, respectively. The impact of synergy between silica fume and gasification residue on the hydration and microstructure development of cement paste was analysed using SEM-EDS, XRD, TGA and N2 sorption methods. The synergy of wastes was found to change the morphology and assemblage of hydration products and to accelerate the consumption of portlandite. N2 physisorption revealed that the synergy of both substitutes had a significant impact on the distribution of pores in the mesoporosity range up to 30 nm, representing small microcapillary pores. The synergistic impact of the substitutes on selected properties of cement mortars was determined after 7, 28 and 56 days. Compared with the reference, all blended mortars showed 5-40% higher mechanical strength and 25-50% lower water ingress during the entire hydration period. The compressive strength increased with the reduction in the residue-to-silica fume (R/SF) ratio, while the flexural strength and sorptivity were less dependent on the ratio. The results revealed that the total cement substitution level may be increased up to 12 %, within 4-6 % covered by gasification residue, without losing the mechanical strength and consequently, to reduce the resources of Portland cement and silica fume.