Thixotropic and Mechanical Behavior of MgO–SiO ₂ Pastes Incorporating Rice Husk Ash for Extrusion‐Based 3D Printing

Rice husk ash (RHA) is an agricultural by‐product that can replace microsilica (MS) as a suitable silica source in MgO–SiO 2 cement mixes, offering low‐carbon benefits. This study investigated the 3D printing properties, including thixotropy and hardened mechanical properties, of the MgO–SiO 2 system formed by partial and total replacement of MS with RHA. Furthermore, the hydration behavior and phase evolution of the samples were analyzed at different curing ages using isothermal calorimetry, X‐ray diffraction (XRD), thermogravimetric analysis (TGA), and field emission scanning electron microscope (FESEM). Among all mixtures, MgO–SiO 2 paste containing 10 wt.% RHA exhibited the most pronounced thixotropic behavior and superior buildability after extrusion, which is supported by the largest thixotropic hysteresis loop area (888.36 Pa·s −1 ) and the highest recovery rate (0.735), corresponding to the maximum attenuation coefficient of yield stress ( μ paste = 0.877). Increasing the RHA content substantially lowered the hydration rate after 6 h, and along with a decrease in the formation of M–S–H gel, resulting in a reduction in strength.