Low-carbon cementitious grouts with straw biochar and ladle furnace slag: A micro- to macro-scale investigation

Combining agricultural and industrial-derived wastes offers a promising strategy to develop low-carbon cementitious materials. In this study, twelve cementitious grout formulations incorporating straw biochar (BC), fly ash (FA), and ladle furnace slag (LFS) were designed for application in grouted macadam pavements. The grouts were characterised in terms of fresh properties (flow spread, efflux time, and bleeding), hardened performance (mechanical properties, drying shrinkage, and water penetrability parameters), and microstructural features (hydration kinetics, XRD, TGA, and SEM-EDS). In addition, sustainability impacts were briefly assessed by considering only the material production stage, encompassing cost, embodied carbon, and energy. The results show that BC contents up to 15% can be effectively incorporated into ternary grout systems while meeting workability requirements, whereas higher BC dosages (25%) markedly increase viscosity and reduce flowability. BC-containing mixes exhibited comparable 3-day compressive strength ( f c ) to other blended systems under water curing, likely due to micro-filler effects, but reduced f c at longer ages because of clinker dilution and limited chemical reactivity. Nevertheless, mixes containing ≥ 70% Portland cement (PC) with 15% BC achieved an acceptable 28-day f c (≥40 MPa). Under air curing, all mixes showed strength reductions relative to water curing; however, BC-modified systems experienced lower strength losses (at least 20%) than BC-free mixes, likely benefiting the potential BC internal-curing effect. Flexural strength was maintained in BC-modified grouts, particularly when combined with LFS, even at reduced PC contents. Although high BC content generally increased water absorption and drying shrinkage due to its intrinsic porosity, FA partially improved dimensional stability. Simplified material-level sustainability analysis showed that ternary mixes with 15% BC plus 15% LFS reduced the normalized embodied carbon per flexural and compressive strength units by 66% and 40%, respectively, suggesting the suitability for semi-structural low-carbon applications.