Biochar as carbon sequestration material combines with sewage sludge incineration ash to prepare lightweight concrete

[Display omitted] •Sewage sludge incineration ash was used to prepare lightweight concrete with foam.•Incorporating biochar into lightweight concrete for long-term carbon sequestration.•Biochar significantly enhanced the mechanical performance of these concretes.•Biochar reduced Cr (VI) to Cr (III) and reinforced the immobilization of all metals.•Biochar of high pyrolysis temperature displayed higher effect in the above benefits. Sewage sludge incineration ash (SSIA) is a solid waste containing heavy metal pollutants, which could be recovered as construction material via cement solidification. Biochar, as a carbon sequestration material, has properties of water absorption, metals immobilization, electron donor, etc. This study produced lightweight concrete (LC) with the ratio of SSIA to Portland cement being 1:1 (w/w), and incorporated peanut shell biochar produced at 500℃ and 700℃ pyrolysis (BC500, BC700) with addition rates of 1–5 %. Both BC500 and BC700 increased the mechanical properties of the LC at their optimal addition rates, i.e., 5 % BC500 and 3 % BC700 respectively, achieved 10.8 MPa and 11.1 MPa compressive strength, 2.1 MPa and 2.8 MPa flexural strength, being much higher than the control (6.2 MPa compressive strength and 1.35 MPa flexural strength). Biochar also significantly enhanced the water resistance of LC by 56.3 % (BC500) and 60.4 % (BC700). Instrumental analysis (TGA, XRD, SEM-EDS and heat flow) revealed that the alkaline and porous biochar promoted and prolonged the hydration reactions with forming more cementitious materials such as calcium silicate hydrate and ettringite. Biochar simultaneously strengthened the metals immobilization in LC including Cr, Cu, Zn, Mn, As and Cd, especially it acted as electron donor to reduce Cr(VI) into Cr(III) with less toxicity and mobility. Based on leaching test, it was confirmed that converting 1-ton biomass into biochar and incorporating into SSIA-based FC could achieve 541 and 980 kg CO2eq carbon sequestration for BC500 and BC700 respectively.