Use of biomass ash from different sources and processes in cement

Abstract

Substitution of cement clinker with suitable excess materials from other processes is an effective way to reduce CO₂ emissions of cement production. Moreover, specific properties of the resulting mortar or concrete can be designed with different clinker replacement materials and their mixing ratios. In this study, bottom and fly ashes from six biomass power plants with different power scales and various flue-gas treatment strategies were admixed to mortars, whose properties (influence of water requirement and final compressive strength) were then assessed in the laboratory by following industrial standard procedures. Results reveal that fly ash from a cyclone of a medium-scale combustor burning Miscanthus straw amended with 2 wt % Ca(OH)₂ (to prevent slagging during combustion) turned out to be well suited as a clinker replacement material, even boosting final compressive strength of the mortar. Wood-chip bottom ashes and fly ash from a cyclone exhibited acceptable results, whereas fly ash from electrostatic precipitators (ESPs) and baghouse filters lowered final compressive strength of the mortar when admixed. The scale of the power plant is positively correlated with final compressive strength in the case of ESPs. Cenospheres, typical fly ash particles present in cyclone ash, seem to have a positive effect on water-to-binder ratio and final compressive strength. In contrast, potassium salts, which are most abundant in ash from ESPs and baghouse filters, appear to have a negative influence on these properties. Grinding of the biomass ashes to a typical Portland cement fineness had a positive effect on mortar quality. All fly ashes had high contents of Cd, and baghouse filter ash contained As in amounts about four times the Swiss limit value for cement of 30 ppm; only bottom ash and cyclone ash from Miscanthus exhibited concentrations below respective limit values for all critical trace elements. To assess the immobilization potential of contaminating elements during the cement hardening process, blended mortars were crushed and subjected to multistep leaching, followed by subsequent analysis of the leachates by atomic absorption spectroscopy. Immobilization of Cd by the mortar was particularly effective. Our results indicate that fly ash from wood-chip combustion is most suitable as an amendment to cement when it was trapped by a cyclone rather than by the ESPs or baghouse filters.

Keywords:

• cement
• biomass
• ash
• crm
• leaching
• recycling

Acknowledgements

The authors would like to thank the laboratory staff at the Institute of Earth and Environmental Sciences in Freiburg, especially Isolde Schmidt and Sigrid Hirth-Walther for help in chemical analysis and leaching experiments. Furthermore we would like to thank operators of the combustion facilities for their kind cooperation. Holcim (Schweiz) AG is acknowledged for providing lab, staff and material for cement tests.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Note

Notes

[1] In our experiments, we substituted the cement rather than the clinker itself by CRM candidates. Portland cement consists of ground clinker (about 90 wt %), calcium sulphate and a small amount of other materials.

Additional information

Funding

This work would not have been possible without financial support from the EU Interreg IV 746 Program (Oberrhein, project C35 BIOCOMBUST; www.biocombust.eu). This work was further supported in part by P30-ES13508 awarded by the National Institute of Environmental Health Sciences (NIEHS). The findings are not the official opinions of NIEHS or NIH.