Feedstock influence on mechanical properties and CO2 mineralization potential of biochar amended cemented soft clay

• Biochar reduces CO 2 emissions in clay stabilization by boosting sequestration. • Wood based biochars improve strength and CaCO 3 mineralization in cemented clay. • Accelerated CO 2 curing lowers strength but still meets Finnish design requirements. • Net CO 2 emissions drop significantly in slag-cement binders with biochar addition. Soft clay stabilization using lime-cement binders is one of the major sources of CO 2 emission in the Nordics. The current study explores the feasibility of different biochar types (waste wood (WWB), tree shavings (TB) and sewage sludge (SB)) as a partial binder replacement for stabilization soft sensitive clay through natural and accelerated CO 2 curing (ACC). A set of stabilized clay samples at 100 kg/m 3 (per cubic meter of clay) of binder usage was systematically studied in the laboratory wherein biochar at 10 % and 20 % replaced an alternative slag cement-based binder by weight. The developed cementitious composite’s physio-chemical properties and mechanical properties were measured at the 7th and 28th day after stabilization. While TB exhibited similar compressive strength compared to control samples (420–450 kPa), WWB improved strength at 10 % replacement levels by about 13 %. Under ACC, all samples exhibited consistent reduced strength due to reduced pH and lesser mobilization of hydrated products compared to the control samples. Nonetheless, the achieved strength in all the samples satisfied the limits for backfill materials (Finnish transport agency), and all the samples (except the carbonated 20 % SB samples satisfied the design limit provided by the Finnish guidelines for dry deep soil mixing (DDSM) design. While TB and WWB biochar amended binder boosted CaCO 3 , mineralization by 128 % and 118 %, respectively, SB amended binder led to lower increase in CaCO 3 (70 %) although it diminished strength at both replacement levels. Nevertheless, the net CO 2 emission for the complete DDSM process of all three studied biochar amended binders were smaller than those in literature. CO 2 sequestration potential of original cement binder (42.75 kg CO 2 eq./t) increased from around 50 kg CO 2 eq./t to 110 kg CO 2 eq./t when biochar was added.