Compensatory effects of CO2 mineralization on the mechanical integrity and hydrophobicity of PDMS-modified coal-based solid waste backfill

This study investigates the effects of polydimethylsiloxane (PDMS) treatment and CO 2 carbonation on the wettability, mechanical properties, pore structure, and CO 2 adsorption behavior of hydrophobic backfill (HBF) materials. The results indicate that PDMS significantly enhances hydrophobicity, with the contact angle exceeding 90° at 2.88% PDMS and reaching a peak of 136.5°. However, excessive PDMS (>2.88%) inhibited alkali activation, leading to microstructural loosening and reduced compressive strength. CO 2 carbonation effectively counteracted this strength loss, filling voids and densifying the matrix, resulting in up to a 43.5% strength increase at 8.64% PDMS. The CO 2 adsorption process followed a Langmuir-like model, with rapid adsorption in the initial phase, followed by saturation. CO 2 uptake increased from 42.3 mg/g (untreated) to 60.5 mg/g (8.64% PDMS-treated), attributed to the indirect role of PDMS in creating additional CO 2 interaction sites. Pore structure analysis revealed that PDMS initially reduced porosity but promoted pore coarsening at higher concentrations, while CO 2 carbonation refined the pore network, decreasing total porosity and threshold diameters. Micromorphological and elemental analyses confirmed the formation of carbonation products, which reinforced the hydrated gel structure. The results demonstrate a compensatory mechanism where CO 2 carbonation remediates the strength loss typically caused by hydrophobic modification, resulting in a material that maintains high mechanical integrity while achieving superior water resistance.