Understanding the Colloidal and Hydration Control in Rheological Evolution of 3D Printed MgO-SiO2-K2HPO4 Gel System

Monitoring the time-dependent rheological properties of 3D printed MgO-SiO2-K2HPO4 is critical for optimizing the dynamic structural reconstruction ability. The collaborative analysis for the contribution of colloidal force based on EDLVO theory and the volume fraction of K-struvite (MgKPO4·6H2O) was conducted. Results showed that 20% silica fume (SF) was identified as the optimal content to achieve balanced rheo-mechanical performance (28 d compressive strength = 113.63 MPa, dynamic yield stress = 359.98 Pa, thixotropic area = 2.14 × 104 Pa/s). The static yield stress development within 50 min exhibited two distinct stages: the initial rapid linear growth stage (Stage I, 5–30 min) dominated by colloidal forces (R2 = 0.81 at 20% SF), followed by the slow increased plateau (Stage II, 30–50 min) correlated with K-struvite volume fraction. Also, dual crystallization pathways of K-struvite included direct precipitation from supersaturated Mg2+, K+, PO43- ionic species and transformation from potassium-deficient phosphate phase. Quantitative results establish a predictive framework for microstructural construction, enabling precise control of structural build-up and 3D printability in MgO-SiO2-K2HPO4 cementitious composites.