Potential of X-ray tomography for the exploration of vegetal concretes’ porous structure

Abstract

Vegetal concretes offer promising perspectives as building materials thanks to their low environmental impact and interesting hygrothermal behavior. Their insulating properties and moisture buffering capacity are largely controlled by their microstructure. In this context, X-ray tomography is a promising technique as it enables inner inspection of the microstructure in three dimensions in a non-destructive fashion. Geometrical characteristics, such as the overall porosity and the aggregate volume ratio, can be quantified within the limits of the accuracy of the technique. The present study focuses on a vegetal concrete obtained by combining a metakaolin-based pozzolanic binder with sunflower bark chips. Our measurements show that the vegetal concrete exhibits an open and interconnected pore space, with pore sizes varying over multiple orders of magnitude. The arrangement of the particles is clearly affected by their elongated shape in combination with the compacting force applied during settlement. The material also exhibits shrinkage-induced cracks at the interface between bio-aggregates and binder paste. These observations demonstrate the potential and limitations of X-ray tomography applied to bio-based concretes. The three-dimensional datasets yield more insight compared to typical two-dimensional digital imaging methods such as SEM. Moreover, the non-destructive nature of the method could allow to monitor ageing mechanisms of bio-based concretes by scanning the same sample at different points in time. Similar to other techniques such as SEM, high-resolution tomographic scans can only be obtained on relatively small samples, which raises the question about the representativeness of the specimens in view of the high heterogeneity of vegetal concretes. Nevertheless, the non-destructive nature of the method enables performing multiple scans of different locations within the same sample and to combine the data. X-ray tomography is thus a powerful tool, which can easily be combined with other techniques and contribute to characterize the microstructure of vegetal concretes.