Effect of fly ash and metakaolin on the mechanical properties and microstructure of magnesium ammonium phosphate cement paste

To solve the current application problems of magnesium phosphate cement (MPC), this study investigates the working properties, hydration process, strength performance, bonding strength, and other characteristics of MPC with 10% to 40% substitution of fly ash (FA) and metakaolin (MK) using different mixing methods. Among them, FA has beneficial effects such as spherical effect, microaggregate effect, and volcanic ash effect in the MPC system, and in addition, the aluminum phase material contained in MK has been shown to react with phosphates. This experimental design was aimed at determining the intrinsic link between bond strength and the aforementioned properties of MPC. The findings suggest that the optimal strength performance was achieved with an FA content of 20%, while the best bonding performance was observed at an FA content of 10%. There was minimal difference in peak hydration temperature among different MK admixtures, and no significant deviation was observed in the strength properties. The poor fluidity of the slurry hindered its penetration into the matrix, resulting in weak early bonding strength within the MK group. In the MK=5% mixing ratio of FA and MK (FK), FK-31 exhibited excellent strength performance with distinct advantages in terms of hydration product quantity and morphology, as well as remarkable macroscopic properties. On the other hand, in the MK=10%, FK-22 demonstrated outstanding performance utilizing the FK groups with strength comparable to FA-1 and FA-2. However, its toughness index-C/F decreased to a minimum level, which implied that the toughness of MPC was improved. Simultaneously, it displayed superior bonding properties surpassing MPC by reaching 5 MPa at 7d. The experimental group showed that excellent bonding properties mainly relied on chemical penetration as the bounding mechanism while interfacial damage primarily manifested as matrix damage. The microscopic tests showed that aluminum-containing phase substances were detected in the MK group, which made the equal substitution amount of MK show superior strength to FA. In this study, MK and FA were mixed in two ways: individual and composite mixing to investigate the modification effect of compounding of dopes with different chemical and physical properties and their bonding properties were analyzed. The test results indicated that under appropriate mixing proportions, incorporating both MK and FA for modified MPC could compensate for shortcomings associated with single mixing methods and become a novel option for modified MPC.