Leaching Behavior and Mechanical Properties Deterioration Model of the Itz between Aggregate and Cement Paste Modified by Micro-/Nano-Sio2

Abstract

Calcium leaching will cause deterioration of mechanical properties and durability of cement-based materials. The interfacial transition zone (ITZ) is a weak link between aggregate and cement paste in concrete, and its leaching behavior under complex influencing factors needs further exploration. Furthermore, it is necessary to establish a leaching-induced deterioration model for the ITZ mechanical properties that can be used in multi-scale discrete medium model-based numerical simulation. In this study, accelerated calcium leaching tests are conducted on aggregate-cement paste composite specimens using a 3 mol/L NH4Cl solution. The water cement (w/c) ratio and the content of micro-SiO2 (mS) and nano-SiO2 (nS) are taken as influencing factors. Time-varying laws of leaching degree, shear bonding strength and micro-Vickers hardness of the ITZ are investigated, and a deterioration model is established using the former two. The microstructure and elements of the ITZ are analyzed by BSEM/EDS. The results indicate that the leaching process of the ITZ satisfies the Fick’s diffusion law. Mechanical properties and leaching resistance of the ITZ are significantly lower than those of cement paste. Before leaching, the shear bonding strength of the latter is approximately 3.6 times that of the former, and it increases to 5.9 times after complete leaching. The micro-Vickers hardness of cement paste is approximately 1.5 times that of the ITZ. An appropriate content of mS or nS can reduce the Ca/Si ratio and improve the microstructure at ITZ. Mechanical properties and leaching resistance of the ITZ gradually decrease with the increase of w/c ratio, while these show a trend of first increasing and then decreasing with the increase of mS and nS content, with an optimum content of 8% and 2%, respectively. The mS and nS are beneficial to improve the leaching resistance of cement-based materials under long and short immersion time, respectively. The leaching degree-based deterioration model for the shear bonding strength of the ITZ has high prediction accuracy.

Keywords: Interfacial transition zone (ITZ), Calcium leaching, bonding strength, Micro-Vickers hardness, Micro-/nano-SiO2, Mechanical properties deterioration model