Sequential Ca(OH)2–silica fume Pretreatment of Municipal Solid Waste Incineration Bottom Ash for Use as Fine Aggregate in Mortar

• A new two-step Ca–SF pretreatment increased the density of BA to 2590 kg/m³. • Water absorption decreased by 53.8%, and the crushing index dropped by 33.8%. • With 10% treated BA, mortar strength reached 46.3 MPa (comp.) and 6.9 MPa (flex.). • Freeze-thaw mass loss after 100 cycles: 0.83%, comparable to natural sand mortar. To improve the performance of municipal solid waste incineration bottom ash (BA) as fine aggregate in mortar, a two-step Ca(OH) 2 –silica fume pretreatment method (Ca–SF) was developed. The pretreatment improved BA quality by reducing connected porosity and densifying the particle surface. After Ca–SF treatment, the apparent density increased from 2400 to 2590 kg/m 3 , while water absorption and crushing index decreased by 53.8% and 33.8%, respectively. Mortar containing 10% Ca–SF-treated BA showed a 30% increase in flowability, and its 90-day compressive and flexural strengths reached 46.3 MPa and 6.9 MPa, respectively, exceeding those of untreated BA mortar and natural sand mortar. After 100 freeze–thaw cycles, the C1 mortar retained a residual compressive strength of 43.1 MPa, with a strength loss of 6.71%, close to that of natural sand mortar. Heavy-metal leaching results showed that the regulated metals in Ca–SF-treated BA mortar were all below the corresponding limits. These findings indicate that Ca–SF pretreatment can enhance the mechanical and freeze–thaw performance of BA mortar while also providing preliminary evidence for environmental safety under the adopted leaching conditions.