Influence of silica fume on the setting time and mechanical properties of a new magnesium phosphate cement

Magnesium silicon potassium phosphate cement (MSPPC), a new magnesium phosphate cement of the MgO-SiO2-K2HPO4 system, can possess higher strength, longer setting time, and better fluidity than the traditional magnesium phosphate cement (MPC). In MSPPC, silica was an essential component, although it is not a primary material for preparing MPC. Four sets of specimens were designed and cast with varying percentages of silica fume (SF) to find out the influence of the silica fume content on the setting time, fluidity, compressive strength, and flexural strength of MSPPC. Experimental results showed that the fluidity of MSPPC was enhanced with the increased percentage of silica fume, reaching its peak at 15% SF. Silica fume significantly prolonged the initial setting time of MSPPC from 5.75 min to 16.50 min for 20% SF. The compressive strength and flexural strength of MSPPC at all ages initially increased, reaching their maximum when the SF was 15%, and then they decreased at 20% SF. Based on the pH value change, XRD analysis, MIP analysis, TG-DSC analysis, and SEM-EDS observations during the hydration process of MSPPC system, the effects of silica fume on the setting time, fluidity, compressive strength, and flexural strength of MSPPC were explained.