Relationship between drying shrinkage and pore structure of concrete with MSWI bottom ash as sand replacement

To promote resource utilization and large-scale engineering application of municipal solid waste incineration (MSWI) bottom ash, this study systematically investigates the relationship between drying shrinkage and microstructure of concrete incorporating MSWI bottom ash as natural sand replacement, and evaluates the shrinkage-compensating efficacy of calcium sulfoaluminate-calcium oxide (CSA-CaO) expansive agent at conventional engineering dosages under varying bottom ash substitution levels. Macroscopic tests indicate incorporating MSWI bottom ash significantly prolongs the concrete’s drying shrinkage duration and amplifies ultimate shrinkage magnitude. Quantitative decoupling via Grey Relational Analysis (GRA) shows the volumetric fraction of fine mesopores (<20 nm) exhibits the highest correlation with macroscopic drying shrinkage (grey relational grade > 0.8), and pore refinement directly dictates the increased shrinkage magnitude and extended shrinkage period. Additionally, applying CSA-CaO expansive agent effectively restrains cementitious matrix drying shrinkage, with a maximum mitigation efficiency exceeding 48 %. However, at high bottom ash replacement levels, the expansive agent’s compensatory efficiency attenuates due to a weakened passive restraining effect caused by the skeleton’s reduced static elastic modulus. Microstructural analysis reveals that at a water-to-binder (w/b) ratio of 0.2 and 50 % sand replacement level, the porosity variation rate reaches 10.17 %. Later, due to MSWI bottom ash pozzolanic activity, secondary hydration reactions refine the internal pores. This refinement amplifies capillary tensile stresses during internal moisture evaporation, explaining the microscopic reasons for exacerbated late-age drying shrinkage in high-volume bottom ash concrete.