Degradation Mechanism of the Wood-Cell Wall Surface in a Cement Environment Measured by Atomic Force Microscopy

Abstract

Lignocellulosic materials in an alkaline cement environment can be subjected to degradation that always starts from the surface. Atomic force microscopy has been used to establish the changes of surface properties of wood-cell walls. We measured the topographies and adhesion force curves of wood-cell wall surfaces before and after treatments in cement alkaline solutions ranging from 0.5 to 1,104 h at 20°C and 50°C and compared them with untreated controls. We studied the degradation of the cell wall surfaces chemically (dissolution of extractives and degradation of the hemicelluloses-lignin matrix), structurally (changes in surface modulus and deformation), and topographically (looking at the change in cell wall surface). The adhesion forces and jump-off force ratios shifted along two sigmoidal curves and showed a transition phase based on the treatment times. In this article, we analyzed and explained how the cutting and surface heterogeneity affects the atomic force microscopy measurements, and how the temperature rise due to the cement hydration heat could accelerate the surface degradation. The article addresses the combination of concrete with plant-based materials, specifically wood. The alkaline environment and hydration temperatures appear to dissolve fundamental buildings blocks of wood used either as a natural aggregate (wood particles in concrete) or when solid wood structural elements are combined with concrete (such wood-concrete composite decks). This is fundamental research that has, in our view, significant implications for using any plant-based fibers or solid wood in combination with concrete in structures where chemical degradation that is accelerated by hydration process must be considered. Whether or not this degradation has an effect on the performance (say stiffness or capacity) of a structural element will require investigation at the element level. Our experiments at the nanoscale level showed, however, that a degradation of interface between concrete and wood is to be expected and that such investigations may be warranted.