Stearic acid-modified sodium methyl silicate for enhanced mechanical strength and hydrophobicity of 3D-printed mortar

The durability of cement-based materials is limited by inherent hydrophilicity and porosity, exacerbated in 3D-printed mortar by weak interlayer interfaces and interconnected pores, accelerating water ingress and performance degradation. This study developed a green aqueous modification method using stearic acid (SG) and sodium methyl silicate (SM) to synthesize a hydrophobic composite (MG). The coating and modification mechanism of SG on SM were characterized by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS) and dynamic light scattering (DLS), and scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM-EDS) elemental mapping. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) revealed that MG8, modified with 8% SG, exhibited optimal hydrophobicity through ion exchange between -COOH and Na + and condensation with Si-OH, resulting in the formation of stable Si-O-C(O)-R covalent structures. When incorporated into 3D-printed mortar, the effects of MG8 dosage were evaluated using SEM-EDS elemental mapping, XRD, FTIR, thermogravimetric–derivative thermogravimetric analysis (TG-DTG), and mercury intrusion porosimetry (MIP). Results showed that increasing MG8 dosage progressively reduced printability, accompanied by a decrease in dynamic yield stress and an increase in plastic viscosity, while the mechanical and hydrophobic properties initially improved and then declined. At 0.8% dosage, 28-d compressive, flexural, and interlayer bond strengths increased by 25%, 7%, and 9%, respectively; secondary water absorption and desorption rates decreased by 39.4% and 15.0%; and the contact angle reached 124.9°. Microscopic and phase analyses demonstrated that an appropriate MG8 dosage enhanced surface roughness, reduced the residual C 3 S content, strengthened the C-S-H-related features, and improved matrix strength; TG-DTG indicated accelerated decomposition of hydrophobic components within 300–450 ℃, enhancing hydrophobicity; and MIP confirmed pore refinement, with an 11.1% increase in gel and medium-fine capillary pores, reducing harmful porosity and improving compactness and hydrophobic stability of the mortar matrix. This work presents a green and efficient hydrophobic modification strategy for 3D-printed cement-based materials.