Optimization of Magnesium Potassium Phosphate Cements Using Ultrafine Fly Ash and Fly Ash

The effect of ultrafine fly ash (UFA) and fly ash (FA) on the physical properties, phase assemblage, and microstructure of magnesium potassium phosphate cement (MKPC) was investigated. This study revealed that the UFA addition does not affect the calorimetry hydration peak associated with MKPC formation when normalized to the reactive components (MgO and KH₂PO₄). However, there is an indication that greater UFA additions lead to an increased reaction duration, suggesting the potential formation of secondary reaction products. The addition of a UFA:FA blend can delay the hydration and the setting time of MKPC, enhancing workability. MgKPO₄·6H₂O was the main crystalline phase observed in all systems; however, at low replacement levels in the UFA-only system (<30 wt %), Mg₂KH(PO₄)₂·15H₂O was also observed by XRD, SEM/EDS, TGA, and NMR (³¹P MAS, ¹H-³¹P CP MAS). Detailed SEM/EDS and MAS NMR investigations (²⁷Al, ²⁹Si, ³¹P) demonstrated that the role of UFA and UFA:FA was mainly as a filler and diluent. Overall, the optimized formulation was determined to contain 40 wt % fly ash (10 wt % UFA and 30 wt % FA (U10F30)), which achieved the highest compressive strength and fluidity and produced a dense microstructure.