Evaluation and characterization of wood bottom ash from circulating fluidized-bed boiler for use in cementitious materials

Abstract 50 Gt of sand and gravel are extracted annually for concrete production, creating environmental pressures and highlighting the need for sustainable alternative materials. This study evaluates wood bottom ash (WBA from circulating fluidized bed (CFB) boilers as fine aggregate replacement (0–100%) in mortar. WBA exhibited a finer but comparable particle size distribution to natural sand, with lower particle density (-7–10%) due to internal porosity induced by the combustion process. Mortars with increasing WBA content showed a compressive strength reduction of 10% at full replacement after 180 days (56.4 vs. 62.9 MPa for reference), while exhibiting higher relative late strength gain of + 42% from 28–180 days. This suggests filler effects rather than reactivity, as low reactivity was determined via the modified R 3 test method. The combustion process also caused surface roughness, which improved bonding in the cement paste, as well as internal cracks and pores in the WBA particles. Despite reduced workability and increased water absorption, all mixes (except 100% replacement at early ages) showed similar strength properties. The results demonstrate that CFB-derived WBA can partially replace natural sand, contributing to resource efficiency while maintaining mechanical performance.