Hydration and Molecular Dynamics Simulation of Carbonated and Hydrophobically Modified Municipal Solid Waste Incineration Fly Ash Mortars

To improve the safe resource utilization of municipal solid waste incineration (MSWI) fly ash in cement-based materials, MSWI fly ash was treated by water washing–carbonation and hydrophobic modification and subsequently incorporated into sulfoaluminate cement mortar. The effects of modified fly ash on hydration reactions, mechanical properties, water absorption, microstructure, and intermolecular interactions of the mortar were systematically investigated. The results showed that water-washed–carbonated fly (CWF) ash promoted early-age hydration, increased hydration heat release, enhanced the compressive strength of mortar by up to 6.8 MPa, and reduced water absorption by up to 17.8%. These improvements may result from the combined effects of desalination during water washing and subsequent carbonation mineralization, in which the CaCO3 formed during carbonation can provide heterogeneous nucleation sites and a micro-filling effect, thereby promoting pore-structure refinement. In contrast, hydrophobically modified fly ash retarded cement hydration and reduced the compressive strength by up to 7.7 MPa, but significantly decreased water absorption. Microstructural analysis revealed that CWF promoted the formation of hydration products, including CaCO3, C-S-H, and AFt, thereby refining the pore structure. Meanwhile, hydrophobic fly ash reduced the free water content in mortar by introducing hydrophobic groups, weakening continuous capillary water transport and limiting chloride ingress. Molecular dynamics simulations further demonstrated that carbonation modification enhanced the interfacial bonding between fly ash and C-S-H. The CWF–C-S-H system exhibited an interfacial binding energy per unit area of −2.00 J/m2, which was higher than those of WF–C-S-H (−1.60 J/m2) and HWF–C-S-H (−1.10 J/m2), mainly due to enhanced van der Waals and electrostatic interactions. Moreover, MSD analysis indicated that both carbonation and hydrophobic modifications reduced the diffusion capability of mobile species, such as Cl−, at the C-S-H interface, providing molecular-scale insights into the stabilization mechanisms of modified fly ash.