Tuning the hydration and printability in nano-modified quaternary binder systems for 3D-printed mortar

This study investigates the combined influence of fly ash (FA), ultrafine fly ash (UFFA), and nano-silica (NS) on hydration characteristics and printability of quaternary binder-based 3D-printable mortars. Mixes were designed using the modified Andreasen and Andersen particle packing model to achieve optimized gradation. Printability parameters, including flowability, open time, extrudability, shape retention, and buildability, were systematically evaluated alongside mechanical performance across different printing orientations. The nano-modified mix (OFUNS) exhibited the most stable deposition, attaining a shape retention factor of 0.85 and sustaining more than 45 printed layers. It also achieved the highest compressive and flexural strengths with reduced anisotropy. Microstructural and phase analyses using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA) confirmed accelerated hydration, reduced portlandite content (5.6%), higher bound water (16.43), and formation of dense C–S–H/C–A–S–H gels (Ca/Si ratio-1.48). The results demonstrate that multi-scale binder modification significantly improves print quality and structural performance.