Effects of porosity on the geometry of laser-cut notches in cement paste

Abstract

Laser machining is a powerful and flexible technique for research and commercial purposes but is not well studied for processing cementitious materials. Unlike most common substances, cementitious materials undergo time-dependent changes in mechanical and thermal properties as they harden through a series of temperature-dependent chemical reactions, but it is not known if the material response to laser machining is also dependent on this development. Using porosity as a proxy variable for development of the cement matrix, this thesis examined how porosity impacted geometry of laser-cut notches in cement paste. Samples were notched at different levels of development and the notch cross-sections were imaged and measured using an optical microscope. Evidence of significant correlation was found only for notch kerf width, especially at high porosity, while notch depth was found to be largely independent of porosity. One laser beam intersected an air bubble, creating a deeper impact and showing reentry behavior at the bottom. A higher incidence of thermal damage was found in more porous samples, and it is hypothesized that very high porosity affects the mechanisms for energy dispersion into the cement matrix. This study provides a baseline for predicting laser processing outcomes in cement of varying age and composition, improving precision and predictability of future research. Further study is needed to clarify behavior of kerf width and to investigate differing vulnerability to thermal damage.