Microstructural characteristics of bonding interfacial transition zone of concrete and magnesium ammonium phosphate cement

Microstructural characteristics of bonding interfacial transition zone (ITZ) are vital to the bond -ing performance of new-old concrete, which remains difficult to characterize accurately. In this paper, the microstructural characteristics of ITZ based on magnesium ammonium phosphate ce-ment (MPC) bonding to ordinary Portland concrete (OPC) were investigated by the combination of BSE-EDS and microhardness. It was found that ITZs were distinguished into MPC bonding to OPC paste (ITZ-P) and aggregate (ITZ-A). Meanwhile, Gaussian Kernel Smoothing was suitable to the statistical analysis of microhardness. The peak interval of microhardness in ITZ-P for 113.1 MPa was higher than that in ITZ-A for 87 MPa. It was reflected by the observation of denser hydration products and calculation of lower porosity in ITZ-P, especially the observation of enriched pore area in ITZ-A. Moreover, based on the interrelation of indentation lattice and corresponding distribution of element, the hydration products was quantitatively analyzed. The denser hydration products and lower porosity in ITZ-P are 41.4% and 4.3%, respectively, com-pared with the 29.3% and 12.5% in ITZ-A. The exposure of more aggregate is not conducive to the attachment of the hydration products of MPC.

Automated summary

The microstructural characteristics of the bonding interfacial transition zone (ITZ) between magnesium ammonium phosphate cement (MPC) and ordinary Portland concrete (OPC) were investigated using BSE-EDS and microhardness methods. The ITZ was divided into two types: ITZ-P (MPC bonding to OPC paste) and ITZ-A (MPC bonding to aggregate). The microhardness analysis revealed that the peak interval of microhardness in ITZ-P (113.1 MPa) was higher than that in ITZ-A (87 MPa). The observation of denser hydration products and lower porosity in ITZ-P, as well as the enriched pore area in ITZ-A, supported these findings. Quantitative analysis showed that the denser hydration products accounted for 41.4% and the porosity was 4.3% in ITZ-P, compared to 29.3% and 12.5% in ITZ-A. The exposure of more aggregate hindered the attachment of the hydration products of MPC.