Long-Term Properties of Ternary Cementitious Grout Systems Incorporating Wood-Based Biochar and Fly Ash

In the transition towards a circular economy, using waste and byproduct substances is gaining significant attention in construction materials for their potential to enhance sustainability. This study investigates the long-term properties of ternary grout systems incorporating wood-based biochar and coal fly ash as partial Portland cement (PC) replacements. Biochar, produced from wood biomass, was added in the 0–15% range, alongside a fixed 25% fly ash content. The research evaluated the workability of the grouts (via a mini-slump flow test) and mechanical properties (flexural and compressive strength) after 56- and 210 days of curing. Durability after exposure to 50 freeze-thaw cycles, and water absorption across five different mixes after 210 days of curing was also investigated. The results indicated that biochar additions up to 10% had a beneficial impact on flexural strength but led to a decline in compressive strength due to its high porosity. However, including 25% fly ash partially compensated for the loss in compressive strength due to pozzolanic reactivity, contributing to a denser matrix over time. Mixes containing biochar (up to 10%) demonstrated superior freeze-thaw resistance, exhibiting lower reductions in flexural strength after accelerated aging, compared to those with only fly ash. This synergy between biochar’s carbon sequestration potential and fly ash’s pozzolanic activity could maintain grout performance while reducing the environmental impact, owing to a reduction in PC usage by over 35%.