Nano-scale fracture toughness of fly ash incorporated hydrating cementitious composites using experimental nanoindentation technique

Abstract

Instrumented nanoindentation technique has proven its strength by providing incredible insight into the material microstructure of homogeneous materials. Its success has led to its implementation in heterogenous materials such as cementitious composites. The implementation of instrumented nanoindentation on cement paste composites has proven to be very fruitful as it has enabled the evaluation of fundamental properties including Young’s modulus, hardness, creep at multiscales. However, there are more complex properties like fracture toughness which are crucial to characterize the material at macro scales. This characterisation would be benefitted by studies at micro-, nano- scales of the complex properties; but they are seldom reported. Cement-based materials have extremely complex, multiscale structures that are initiated from the chemical reactions of the hydration process and then continue to evolve with time. Also, preparing the surface of hydrating cementitious composites requires special consideration as it can greatly affect the indentation results. To explore the microstructural composition and its fracture toughness, an experimental grid nanoindentation protocol has been put forward. This experimental protocol has been implemented on three cement pastes, i.e., plain Ordinary Portland Cement (OPC), OPC + 20% fly ash, and OPC + 40% fly ash during their early stages as well as near complete stages of hydration. Since measuring the micro cracks around the indent is virtually near impossible, the load vs. displacement curves recorded by the indentations have been utilized in an energy-based method to evaluate the energy release rate and fracture toughness. The evaluated values of fracture toughness property of the individual microstructural phases will help in shedding a light on the development and spread of fracture toughness in cement-based materials. In this study the different methods for fracture toughness evaluation at the micro scales using nanoindentation technique are reported on and the method suitable for cementitious materials is implemented to evaluate fracture toughness of the microstructural phases.