Thermal and Alkaline Aging of Wood as a Construction Material Measured by Atomic Force Microscopy

Wood has been used as a construction material by human beings since ancient times. Under dry conditions, wood structures can sustain hundreds or even thousands of years. The combination of lignocellulosic materials with concrete reduces the mass of structures, reduces the waste material, increases ductility of the structure, and reduces the construction cost. Natural fibers or wood particles are used as the reinforcements in concrete and to reduce weight; composite timber-concrete beams and slabs are used to control fire resistance and acoustic performance, for example. The combination of different materials raises the issues of compatibility and aging due to various technological (such as cement hydrations) and environmental factors. The aging of wood has attracted a lot of research interest in both natural aging and accelerated aging studies. However, the fundamental understanding of aging mechanisms is still lacking. This work focuses on the aging mechanism of wood surfaces in thermal and cement alkaline conditions in the nanoscale using atomic force microscopy (AFM) and other analytical techniques. We proposed two sigmoidal curves (using Boltzmann sigmoid equation) to describe the change of adhesion forces and jump-off force ratios during the aging of wood cell walls. During thermal aging, the 1st sigmoidal curve of the adhesion force – jump-off force ratio relationship described the transportation of extractives and their oxidation on the surface. The 2nd sigmoidal curve suggested the degradation of the hemicellulose-lignin matrix and the exposure of the cellulose aggregates. This interpretation was independently proven by the cell wall topography before and after treatments, and chemical analysis with Fourier-transform infrared spectroscopy (FTIR), and Headspace GS-MS. During alkaline aging, the gradual shift of data points could also be described by the two sigmoidal curves. We measured the immediate and short-term degradation process of the wood cell wall surfaces from 0.5 h to several days quantitatively in thermal loading and cement alkaline environment. We described the progressive degradation from the dissolution of extractives, through the degradation of the hemicelluloses-lignin matrix, to the exposure of cellulose aggregates using the two sigmoidal curves in terms of jump-off force ratio and adhesion force. Measuring the changes in surface deformation and modulus of wood in cement alkaline aging revealed that the cement hydration heat significantly accelerated the aging process: The 1-h treatment at 50 ℃ (cement hydration temperature) caused a 20% decrease in the surface moduli similar to 144–624 h at 20 ℃; a 50% decrease of the surface modulus occurred after 264 h at 50 ℃ similar to after 1104 h at 20 ℃.