Yuekai Xie
The utilisation of recycled aggregates and agricultural residues provides an environmentally friendly pathway for concrete production. This paper investigated the mechanical properties under the ambient temperature and after cyclic heating and cooling of low-carbon to carbon-negative geopolymer concrete with the incorporation of coconut shell biochar and recycled concrete aggregates. The compressive, flexural, and split tensile strength of the geopolymer concrete were 79.6, 8.29, and 5.56 MPa under the ambient temperature, respectively. Those strengths were improved by 7.5% (85.6 MPa), 2.7% (8.51 MPa), and 2.6% (5.71 MPa) with 5% biochar addition and further increased to 87.2, 8.77, and 5.86 MPa with replacement of 25% recycled concrete aggregates. The enhanced production of CSH and CASH gels associated with biochar inclusion was verified by X-ray diffraction and thermogravimetric analysis. After 40 heating-cooling cycles at 200 and 400 °C, the residual compressive strength and strength retention ratio of the geopolymer concrete with appropriate quantity of biochar and recycled concrete aggregates exhibited improved residual strength and retention ratios. The carbon footprint of the geopolymer concrete with 5%, 10%, and 15% biochar varied from 41.8 to 53.9 kg CO 2 -eq/m 3 , −12.4 to −1.7 kg CO 2 -eq/m 3 , and − 64.2 to −55.0 kg CO 2 -eq/m 3 , with significant reduction compared to the control group (112 CO 2 -eq/m 3 ). The above results indicated the potential application of biochar and recycled concrete for low-carbon and carbon-negative geopolymer concrete with facilitated mechanical performance and durability.