Influence of different additives on the rheology and microstructural development of MgO–SiO₂ mixes

Development of alternative binder systems to be used in additive and automated manufacturing technologies is crucial for increasing the sustainability and productivity of the construction process. Advancements in this area facilitate shorter construction durations, enhanced resource efficiency and reduced construction waste; enabling complex, high-quality and functional designs that would not be possible with traditional methods. The novelty of this work involves the demonstration of the properties of a promising alternative binder, reactive MgO–SiO₂ (RMS) binder, that can highlight its feasibility to be used in large-scale applications and the identification of the effects of different phosphate additives during this process. RMS mixes containing three different phosphate additives (sodium hexametaphosphate, trimetaphosphate and orthophosphate (OP)) were analyzed for their pH, reaction kinetics, workability, mechanical performance and rheological properties to reveal the influence of these additives on the properties of RMS mixes. These findings were supported by FTIR, ²⁹Si MAS NMR and XRD measurements. The inclusion of OP in RMS binder systems increased the pH of solution, thereby improving the dissolution of silica and its reaction with Mg-phases and resulting in enhanced magnesium-silicate-hydrate (M-S-H) formation. These improvements in hydration mechanisms translated into better mechanical performance and rheological properties, which can correlate to desirable properties for 3D printing applications.