Performance and Surface Quality Regulation of Architectural Mirror-Finished Mortar Incorporating Modified Municipal Solid Waste Incineration Fly Ash

To meet the combined requirements of architectural decorative components for high surface quality, mechanical performance, and water resistance, this study employed hydrophobic and water washing–carbonation treatments to functionalize municipal solid waste incineration fly ash and prepared architectural mirror-finished mortars using different formwork materials. The effects of these treatments on setting time, compressive strength, surface gloss, grayscale value, pull-off tensile strength, water absorption, and pore structure were systematically investigated, while XRD, TG, and NMR were used to elucidate the underlying mechanisms. The results showed that untreated fly ash slightly delayed hydration and reduced strength, whereas water washing–carbonation treatment improved the mechanical performance. The 28 d compressive strength of CWF32 reached 44.7 MPa, only approximately 1.1% lower than that of the reference group. Formwork type had a pronounced effect on the mirror-finished surface, with gloss generally following the order PET > ABS > Fe; CP32 exhibited the highest gloss value of 75 GU. Hydrophobic modification markedly reduced water absorption, with HWF32 exhibiting a final water absorption of only 3.64%, 35.8% lower than that of WF32. NMR results showed that the total porosities of HWF32 and CWF32 decreased to 4.70% and 5.45%, respectively. XRD and TG analyses indicated that hydrophobic modification mainly acted by reducing particle wettability and suppressing water migration, whereas water washing–carbonation treatment enhanced matrix densification by promoting CaCO3 formation together with micro-filling and heterogeneous nucleation effects. Overall, water washing–carbonation treatment provided greater comprehensive benefits in terms of mechanical performance and mirror-surface quality, whereas hydrophobic modification was more effective in suppressing water transport. From an architectural perspective, combining modified MSWI fly ash with low-roughness PET formwork can produce high-quality mirror-finished surfaces, reduce subsequent repair and finishing, and promote high-value solid-waste utilization for green decorative building materials.