Effect of fly ash on the rheological properties of potassium magnesium phosphate cement paste

Abstract

To ensure that the potassium magnesium phosphate cement (MKPC) paste is both easily pourable and compact when repairing narrow cracks, the paste must be capable of self-compacting. Herein, the effects of fly ash (FA) on the workability, rheological properties, and early strength of the MKPC paste were tested, and the physical phase composition and microstructure of the paste were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results suggested that the working properties of the resulting MKPC paste satisfy the index requirements of the European Design Code for Self-Compacting Concrete of European Federation of National Associations Representing for Concrete when 10 % and 20 % FA are used instead of magnesium oxide powder. The early plastic viscosity of the slurry mixed with 10 % and 20 % (M1, M2) increased by 0.114 and 0.174 Pa s, respectively, in relation to that of the MKPC paste without FA (M0). This result indicated that the FA admixture increases the plastic viscosity of the MKPC paste because FA can promote the early hydration of the MKPC paste and consume more free water. The static yield stress of M2 pastes blended with 20 % FA and the thixotropic ring area decreased by 31.89 Pa and 971.12 Pa s−1, respectively, in relation to those of M0. The addition of an appropriate amount of FA reduces the early static yield stress and improves the thixotropy of the MKPC pastes owing to the dispersing effect of spherical particles in FA and the tumbling effect. Owing to heterogeneous core-forming, active, and microaggregate effects of FA, the structure of the hydrated 1 d M2 sample was significantly denser than that of M0, which subsequently improved the 1 d strength of the hardened body.

Keywords

• MKPC paste
• Fly ash
• Plastic viscosity
• Yield stress
• Strength