Biochar-slag cements: valorising by-products from wastederived energy systems

Two major areas of current scientific/engineering research are deployment of sustainable low-carbon energy sources/materials and minimising/recycling waste.

Sewage treatment sludge (STS) is a potential low-carbon energy source. However, this material has a high water content and requires extensive pre-treatment before (modest) energy recovery in conventional systems. The recent EU-funded PYROCHAR project developed a prototype energy system for the pyrolytic conversion of STS to biochar. Critical to establishing future economic viability of this technology is the valorisation of the resulting biochar, which was demonstrated by Teesside’s (Ennis) work in PYROCHAR to be porous, moderately high carbon (and ‘carbon negative’ in terms of atmospheric CO2 budgeting), suitable for adsorption of heavy metal ions, and of alkali pH. A promising valorisation strategy for materials with these properties is as an alkali activator for pozzolanic slags as cement alternatives.

Concretes are traditionally Portland cement-based, whose energy intensive manufacture generates ~10% of global CO2 emissions. There is a drive towards developing low-carbon cements which recycle industrial waste [2]. Pozzolanic heavy metal-bearing slags from the steel industry possess ideal cementitious properties, which are activated using an alkali (NaOH). Recent life cycle analysis (LCA) of commercial alkali activators (AA) indicate that the carbon savings are low due to the large energy requirements for NaOH manufacture, high cost and CO2 emissions. Hence, low-energy waste-cements require investigation.