Cell wall water induced dimensional changes of beech and pine wood

Hygroexpansion in timber construction is a phenomenon that occurs as a result of changes in the cell wall structure of wood caused by water. In this study, beech (Fagus sylvatica) and pine (Pinus taeda L.) wood were conditioned at different relative humidities with saturated salt solutions and saturated through vacuum pressure impregnation. The moisture content and dimensions of the samples were evaluated under various adsorption equilibrium states, as well as during drying from water saturation to an air-dried state. The results obtained using the two-dimensional time-domain nuclear magnetic resonance technique revealed two distinct types of cell wall water in both wood species, designated as B-water and C-water, as they displayed different mobilities. With increasing moisture content, the mobility of cell wall water increased; however, B-water maintained a constant level of mobility and formed water clusters when it accumulated in the high humidity region and at slightly above the fiber saturation point (FSP), which is evidence of the occurrence of free-bound water. The B-water, located in the amorphous region of microfibrils and their surrounding hemicellulose, exerted a predominant influence on the dimensional changes of the wood both under equilibrium and nonequilibrium conditions, in comparison to the C-water which located in the matrix of hemicellulose and lignin. •Distinguish two wood cell wall waters in hygroscopic range and during drying.•Specify the mobility changes of cell wall water changing with moisture content.•Clarify the impact of the two cell wall waters on wood dimensional changes.